C++ performance challenge: integer to std::string conversion

Question

Can anyone beat the performance of my integer to std::string code, linked below?

There are already several questions that explain how to convert an integer into a std::string in C++, such as this one, but none of the solutions provided are efficient.

Here is compile-ready code for some common methods to compete against:

• The "C++ way", using stringstream: http://ideone.com/jh3Sa
• sprintf, which SO-ers usually recommend to the performance conscious: http://ideone.com/82kwR

Contrary to popular belief, boost::lexical_cast has its own implementation (white paper) and does not use stringstream and numeric insertion operators. I'd really like to see its performance compared, because this other question suggests that it's miserable.

And my own contribution, which is competitive on desktop computers, and demonstrates an approach that runs at full speed on embedded systems as well, unlike algorithms dependent on integer modulo:

• Ben's algorithms: http://ideone.com/SsEUW

If you want to use that code, I'll make it available under a simplified BSD license (commercial use allowed, attribution required). Just ask.

Finally, the function ltoa is non-standard but widely available.

• ltoa version, for anyone who has a compiler that provides it (ideone doesn't): http://ideone.com/T5Wim

I'll post my performance measurements as an answer shortly.

Rules for algorithms

• Provide code for a conversion of at least 32-bit signed and unsigned integers into decimal.
• Produce output as a std::string.
• No tricks that are incompatible with threading and signals (for example, static buffers).
• You may assume an ASCII character set.
• Make sure to test your code on INT_MIN on a two's complement machine where the absolute value is not representable.
• Ideally, the output should be character-for-character identical with the canonical C++ version using stringstream, http://ideone.com/jh3Sa, but anything that is clearly understandable as the correct number is ok, too.
• NEW: Although you can use whatever compiler and optimizer options (except completely disabled) you want for the comparison, the code needs to also compile and give correct results under at least VC++ 2010 and g++.

Hoped-for Discussion

Besides better algorithms, I'd also like to get some benchmarks on several different platforms and compilers (let's use MB/s throughput as our standard unit of measure). I believe that the code for my algorithm (I know the sprintf benchmark takes some shortcuts -- now fixed) is well-defined behavior by the standard, at least under the ASCII assumption, but if you see any undefined behavior or inputs for which the output is invalid, please point that out.

Conclusions:

Different algorithms perform for g++ and VC2010, likely due to the different implementations of std::string on each. VC2010 clearly does a better job with NRVO, getting rid of return-by-value helped only on gcc.

Code was found that outperforms sprintf by an order of magnitude. ostringstream falls behind by a factor of 50 and more.

The winner of the challenge is user434507 who produces code that runs 350% of the speed of my own on gcc. Further entries are closed due to the whims of the SO community.

The current (final?) speed champions are:

• For gcc: Eugene Smith, at 8 times faster than sprintf: http://ideone.com/0uhhX
• For Visual C++: Timo, at 15 times faster than sprintf: http://ideone.com/VpKO3

Answer 1

#include <string>

const char digit_pairs[201] = {
  "00010203040506070809"
  "10111213141516171819"
  "20212223242526272829"
  "30313233343536373839"
  "40414243444546474849"
  "50515253545556575859"
  "60616263646566676869"
  "70717273747576777879"
  "80818283848586878889"
  "90919293949596979899"
};


std::string& itostr(int n, std::string& s)
{
    if(n==0)
    {
        s="0";
        return s;
    }

    int sign = -(n<0);
    unsigned int val = (n^sign)-sign;

    int size;
    if(val>=10000)
    {
        if(val>=10000000)
        {
            if(val>=1000000000)
                size=10;
            else if(val>=100000000)
                size=9;
            else 
                size=8;
        }
        else
        {
            if(val>=1000000)
                size=7;
            else if(val>=100000)
                size=6;
            else
                size=5;
        }
    }
    else 
    {
        if(val>=100)
        {
            if(val>=1000)
                size=4;
            else
                size=3;
        }
        else
        {
            if(val>=10)
                size=2;
            else
                size=1;
        }
    }
    size -= sign;
    s.resize(size);
    char* c = &s[0];
    if(sign)
        *c='-';

    c += size-1;
    while(val>=100)
    {
       int pos = val % 100;
       val /= 100;
       *(short*)(c-1)=*(short*)(digit_pairs+2*pos); 
       c-=2;
    }
    while(val>0)
    {
        *c--='0' + (val % 10);
        val /= 10;
    }
    return s;
}

std::string& itostr(unsigned val, std::string& s)
{
    if(val==0)
    {
        s="0";
        return s;
    }

    int size;
    if(val>=10000)
    {
        if(val>=10000000)
        {
            if(val>=1000000000)
                size=10;
            else if(val>=100000000)
                size=9;
            else 
                size=8;
        }
        else
        {
            if(val>=1000000)
                size=7;
            else if(val>=100000)
                size=6;
            else
                size=5;
        }
    }
    else 
    {
        if(val>=100)
        {
            if(val>=1000)
                size=4;
            else
                size=3;
        }
        else
        {
            if(val>=10)
                size=2;
            else
                size=1;
        }
    }

    s.resize(size);
    char* c = &s[size-1];
    while(val>=100)
    {
       int pos = val % 100;
       val /= 100;
       *(short*)(c-1)=*(short*)(digit_pairs+2*pos); 
       c-=2;
    }
    while(val>0)
    {
        *c--='0' + (val % 10);
        val /= 10;
    }
    return s;
}

This will blow up on systems that disallow unaligned memory accesses (in which case, the first unaligned assignment via *(short*) would cause a segfault), but should work very nicely otherwise.

One important thing to do is to minimize the use of std::string. (Ironic, I know.) In Visual Studio, for example, most calls to methods of std::string are not inlined, even if you specify /Ob2 in compiler options. So even something as trivial as a call to std::string::clear(), which you might expect to be very fast, can take 100 clockticks when linking CRT as a static library, and as much as 300 clockticks when linking as a DLL.

For the same reason, returning by reference is better because it avoids an assignment, a constructor and a destructor.

Answer 2

Ah, awesome challenge by the way... I've had a lot of fun with this.

I have two algorithms to submit (code is at the bottom if you feel like skipping to it). In my comparisons I require that the function return a string and that it can handle int and unsigned int. Comparing things that don't construct a string to those that do doesn't really make sense.

The first one is a fun implementation that doesn't use any precomputed lookup tables or explicit division/modulo. This one is competitive with the others with gcc and with all but Timo's on msvc (for a good reason that I explain below). The second algorithm is my actual submission for highest performance. In my tests it beats all the others on both gcc and msvc.

I think I know why some of the results on MSVC are very good. std::string has two relevant constructors std::string(char* str, size_t n)
and
std::string(ForwardIterator b, ForwardIterator e)
gcc does the same thing for both of them... that is it uses the second to implement the first. The first constructor can be implemented significantly more efficiently than that and MSVC does so. The side benefit of this is that in some cases (like my fast code and Timo's code) the string constructor can be inlined. In fact, just switching between these constructors in MSVC is almost a 2x difference for my code.

My performance testing results:

Code Sources:

- Voigt
- Timo
- ergosys
- user434507
- user-voigt-timo
- hopman-fun
- hopman-fast

gcc 4.4.5 -O2 on Ubuntu 10.10 64-bit, Core i5

hopman_fun: 124.688  MB/sec --- 8.020 s
hopman_fast: 137.552  MB/sec --- 7.270 s
voigt: 120.192  MB/sec --- 8.320 s
user_voigt_timo: 97.9432  MB/sec --- 10.210 s
timo: 120.482  MB/sec --- 8.300 s
user: 97.7517  MB/sec --- 10.230 s
ergosys: 101.42  MB/sec --- 9.860 s

MSVC 2010 64-bit /Ox on Windows 7 64-bit, Core i5

hopman_fun: 127  MB/sec --- 7.874 s
hopman_fast: 259  MB/sec --- 3.861 s
voigt: 221.435  MB/sec --- 4.516 s
user_voigt_timo: 195.695  MB/sec --- 5.110 s
timo: 253.165  MB/sec --- 3.950 s
user: 212.63  MB/sec --- 4.703 s
ergosys: 78.0518  MB/sec --- 12.812 s

Here are some results and a testing/timing framework on ideone
http://ideone.com/XZRqp
Note that ideone is a 32-bit environment. Both of my algorithms suffer from that, but hopman_fast is at least still competetive.

Note that for those the two or so that don't construct a string I added the following function template:

template <typename T>
std::string itostr(T t) {
    std::string ret;
    itostr(t, ret);
    return ret;
}

Now for my code...first the fun one:

    // hopman_fun

template <typename T> 
T reduce2(T v) {
    T k = ((v * 410) >> 12) & 0x000F000F000F000Full;
    return (((v - k * 10) << 8) + k);
}

template <typename T>
T reduce4(T v) {
    T k = ((v * 10486) >> 20) & 0xFF000000FFull;
    return reduce2(((v - k * 100) << 16) + (k));
}

typedef unsigned long long ull;
inline ull reduce8(ull v) {
    ull k = ((v * 3518437209u) >> 45);
    return reduce4(((v - k * 10000) << 32) + (k));
}

template <typename T>
std::string itostr(T o) {
    union {
        char str[16];
        unsigned short u2[8];
        unsigned u4[4];
        unsigned long long u8[2];
    };

    unsigned v = o < 0 ? ~o + 1 : o;

    u8[0] = (ull(v) * 3518437209u) >> 45;
    u8[0] = (u8[0] * 28147497672ull);
    u8[1] = v - u2[3] * 100000000;

    u8[1] = reduce8(u8[1]);
    char* f;
    if (u2[3]) {
        u2[3] = reduce2(u2[3]);
        f = str + 6;
    } else {
        unsigned short* k = u4[2] ? u2 + 4 : u2 + 6;
        f = *k ? (char*)k : (char*)(k + 1);
    }
    if (!*f) f++;

    u4[1] |= 0x30303030;
    u4[2] |= 0x30303030;
    u4[3] |= 0x30303030;
    if (o < 0) *--f = '-';
    return std::string(f, (str + 16) - f);
}

And then the fast one:

    // hopman_fast

struct itostr_helper {
    static unsigned out[10000];

    itostr_helper() {
        for (int i = 0; i < 10000; i++) {
            unsigned v = i;
            char * o = (char*)(out + i);
            o[3] = v % 10 + '0';
            o[2] = (v % 100) / 10 + '0';
            o[1] = (v % 1000) / 100 + '0';
            o[0] = (v % 10000) / 1000;
            if (o[0]) o[0] |= 0x30;
            else if (o[1] != '0') o[0] |= 0x20;
            else if (o[2] != '0') o[0] |= 0x10;
            else o[0] |= 0x00;
        }
    }
};
unsigned itostr_helper::out[10000];

itostr_helper hlp_init;

template <typename T>
std::string itostr(T o) {
    typedef itostr_helper hlp;

    unsigned blocks[3], *b = blocks + 2;
    blocks[0] = o < 0 ? ~o + 1 : o;
    blocks[2] = blocks[0] % 10000; blocks[0] /= 10000;
    blocks[2] = hlp::out[blocks[2]];

    if (blocks[0]) {
        blocks[1] = blocks[0] % 10000; blocks[0] /= 10000;
        blocks[1] = hlp::out[blocks[1]];
        blocks[2] |= 0x30303030;
        b--;
    }

    if (blocks[0]) {
        blocks[0] = hlp::out[blocks[0] % 10000];
        blocks[1] |= 0x30303030;
        b--;
    }

    char* f = ((char*)b);
    f += 3 - (*f >> 4);

    char* str = (char*)blocks;
    if (o < 0) *--f = '-';
    return std::string(f, (str + 12) - f);
}

Answer 3

Benchmark data for the code provided in the question:

On ideone (gcc 4.3.4):

• stringstreams: 4.4 MB/s
• sprintf: 25.0 MB/s
• mine (Ben Voigt): 55.8 MB/s
• Timo: 58.5 MB/s
• user434507: 199 MB/s
• user434507's Ben-Timo-507 hybrid: 263 MB/s

Core i7, Windows 7 64-bit, 8 GB RAM, Visual C++ 2010 32-bit:

cl /Ox /EHsc

• stringstreams: 3.39 MB/s, 3.67 MB/s
• sprintf: 16.8 MB/s, 16.2 MB/s
• mine: 194 MB/s, 207 MB/s (with PGO enabled: 250 MB/s)

Core i7, Windows 7 64-bit, 8 GB RAM, Visual C++ 2010 64-bit:

cl /Ox /EHsc

• stringstreams: 4.42 MB/s, 4.92 MB/s
• sprintf: 21.0 MB/s, 20.8 MB/s
• mine: 238 MB/s, 228 MB/s

Core i7, Windows 7 64-bit, 8 GB RAM, cygwin gcc 4.3.4:

g++ -O3

• stringstreams: 2.19 MB/s, 2.17 MB/s
• sprintf: 13.1 MB/s, 13.4 MB/s
• mine: 30.0 MB/s, 30.2 MB/s

edit: I was gonna add my own answer, but the question was was closed so I'm adding it here. :) I wrote my own algorithm and managed to get a decent improvement over Ben's code, though I only tested it in MSVC 2010. I also made a benchmark of all the implementations presented so far, using the same testing setup that was in Ben's original code. -- Timo

Intel Q9450, Win XP 32bit, MSVC 2010

cl /O2 /EHsc

• stringstream: 2.87 MB/s
• sprintf: 16.1 MB/s
• Ben: 202 MB/s
• Ben (unsigned buffer): 82.0 MB/s
• ergosys (updated version): 64.2 MB/s
• user434507: 172 MB/s
• Timo: 241 MB/s

-

const char digit_pairs[201] = {
  "00010203040506070809"
  "10111213141516171819"
  "20212223242526272829"
  "30313233343536373839"
  "40414243444546474849"
  "50515253545556575859"
  "60616263646566676869"
  "70717273747576777879"
  "80818283848586878889"
  "90919293949596979899"
};

static const int BUFFER_SIZE = 11;

std::string itostr(int val)
{
  char buf[BUFFER_SIZE];
  char *it = &buf[BUFFER_SIZE-2];

  if(val>=0) {
    int div = val/100;
    while(div) {
      memcpy(it,&digit_pairs[2*(val-div*100)],2);
      val = div;
      it-=2;
      div = val/100;
    }
    memcpy(it,&digit_pairs[2*val],2);
    if(val<10)
      it++;
  } else {
    int div = val/100;
    while(div) {
      memcpy(it,&digit_pairs[-2*(val-div*100)],2);
      val = div;
      it-=2;
      div = val/100;
    }
    memcpy(it,&digit_pairs[-2*val],2);
    if(val<=-10)
      it--;
    *it = '-';
  }

  return std::string(it,&buf[BUFFER_SIZE]-it);
}

std::string itostr(unsigned int val)
{
  char buf[BUFFER_SIZE];
  char *it = (char*)&buf[BUFFER_SIZE-2];

  int div = val/100;
  while(div) {
    memcpy(it,&digit_pairs[2*(val-div*100)],2);
    val = div;
    it-=2;
    div = val/100;
  }
  memcpy(it,&digit_pairs[2*val],2);
  if(val<10)
    it++;

  return std::string((char*)it,(char*)&buf[BUFFER_SIZE]-(char*)it);
}

Answer 4

While the info we get here for the algorithms is pretty nice, I think the question is "broken", and I'll explain why I think this:

The question asks to take the performance of int->std::string conversion, and this may be of interest when comparing a commonly available method, such as different stringstream implementations or boost::lexical_cast. It does not, however, make sense when asking for new code, a specialized algorithm, to do this. The reason is that int2string will always involve heap allocation from std::string and if we are trying to squeeze the last out of our conversion algorithm, I do not think it makes sense to mix these measurements up with the heap allocations done by std::string. If I want performant conversion I will always use a fixed size buffer and certainly never allocate anything on the heap!

To sum up, I think the timings should be split:

• First, fastest (int -> fixed buffer) conversion.
• Second, timing of (fixed buffer -> std::string) copy.
• Third, checking how the std::string allocation can directly be used as buffer, to save the copying.

These aspects should not be mixed up in one timing, IMHO.

Answer 5

Updated user2985907's answer... modp_ufast...

Integer To String Test (Type 1)
[modp_ufast]Numbers: 240000000  Total:   657777786      Time:  1.1633sec        Rate:206308473.0686nums/sec
[sprintf] Numbers: 240000000    Total:   657777786      Time: 24.3629sec        Rate:  9851045.8556nums/sec
[karma]   Numbers: 240000000    Total:   657777786      Time:  5.2389sec        Rate: 45810870.7171nums/sec
[strtk]   Numbers: 240000000    Total:   657777786      Time:  3.3126sec        Rate: 72450283.7492nums/sec
[so   ]   Numbers: 240000000    Total:   657777786      Time:  3.0828sec        Rate: 77852152.8820nums/sec
[timo ]   Numbers: 240000000    Total:   657777786      Time:  4.7349sec        Rate: 50687912.9889nums/sec
[voigt]   Numbers: 240000000    Total:   657777786      Time:  5.1689sec        Rate: 46431985.1142nums/sec
[hopman]  Numbers: 240000000    Total:   657777786      Time:  4.6169sec        Rate: 51982554.6497nums/sec
Press any key to continue . . .

Integer To String Test(Type 2)
[modp_ufast]Numbers: 240000000  Total:   660000000      Time:  0.5072sec        Rate:473162716.4618nums/sec
[sprintf] Numbers: 240000000    Total:   660000000      Time: 22.3483sec        Rate: 10739062.9383nums/sec
[karma]   Numbers: 240000000    Total:   660000000      Time:  4.2471sec        Rate: 56509024.3035nums/sec
[strtk]   Numbers: 240000000    Total:   660000000      Time:  2.1683sec        Rate:110683636.7123nums/sec
[so   ]   Numbers: 240000000    Total:   660000000      Time:  2.7133sec        Rate: 88454602.1423nums/sec
[timo ]   Numbers: 240000000    Total:   660000000      Time:  2.8030sec        Rate: 85623453.3872nums/sec
[voigt]   Numbers: 240000000    Total:   660000000      Time:  3.4019sec        Rate: 70549286.7776nums/sec
[hopman]  Numbers: 240000000    Total:   660000000      Time:  2.7849sec        Rate: 86178023.8743nums/sec
Press any key to continue . . .

Integer To String Test (type 3)
[modp_ufast]Numbers: 240000000  Total:   505625000      Time:  1.6482sec        Rate:145610315.7819nums/sec
[sprintf] Numbers: 240000000    Total:   505625000      Time: 20.7064sec        Rate: 11590618.6109nums/sec
[karma]   Numbers: 240000000    Total:   505625000      Time:  4.3036sec        Rate: 55767734.3570nums/sec
[strtk]   Numbers: 240000000    Total:   505625000      Time:  2.9297sec        Rate: 81919227.9275nums/sec
[so   ]   Numbers: 240000000    Total:   505625000      Time:  3.0278sec        Rate: 79266003.8158nums/sec
[timo ]   Numbers: 240000000    Total:   505625000      Time:  4.0631sec        Rate: 59068204.3266nums/sec
[voigt]   Numbers: 240000000    Total:   505625000      Time:  4.5616sec        Rate: 52613393.0285nums/sec
[hopman]  Numbers: 240000000    Total:   505625000      Time:  4.1248sec        Rate: 58184194.4569nums/sec
Press any key to continue . . .

int ufast_utoa10(unsigned int value, char* str)
{
#define JOIN(N) N "0", N "1", N "2", N "3", N "4", N "5", N "6", N "7", N "8", N "9"
#define JOIN2(N) JOIN(N "0"), JOIN(N "1"), JOIN(N "2"), JOIN(N "3"), JOIN(N "4"), \
                 JOIN(N "5"), JOIN(N "6"), JOIN(N "7"), JOIN(N "8"), JOIN(N "9")
#define JOIN3(N) JOIN2(N "0"), JOIN2(N "1"), JOIN2(N "2"), JOIN2(N "3"), JOIN2(N "4"), \
                 JOIN2(N "5"), JOIN2(N "6"), JOIN2(N "7"), JOIN2(N "8"), JOIN2(N "9")
#define JOIN4    JOIN3("0"), JOIN3("1"), JOIN3("2"), JOIN3("3"), JOIN3("4"), \
                 JOIN3("5"), JOIN3("6"), JOIN3("7"), JOIN3("8"), JOIN3("9")
#define JOIN5(N) JOIN(N), JOIN(N "1"), JOIN(N "2"), JOIN(N "3"), JOIN(N "4"), \
                 JOIN(N "5"), JOIN(N "6"), JOIN(N "7"), JOIN(N "8"), JOIN(N "9")
#define JOIN6    JOIN5(), JOIN5("1"), JOIN5("2"), JOIN5("3"), JOIN5("4"), \
                 JOIN5("5"), JOIN5("6"), JOIN5("7"), JOIN5("8"), JOIN5("9")
#define F(N)     ((N) >= 100 ? 3 : (N) >= 10 ? 2 : 1)
#define F10(N)   F(N),F(N+1),F(N+2),F(N+3),F(N+4),F(N+5),F(N+6),F(N+7),F(N+8),F(N+9)
#define F100(N)  F10(N),F10(N+10),F10(N+20),F10(N+30),F10(N+40),\
                 F10(N+50),F10(N+60),F10(N+70),F10(N+80),F10(N+90)
  static const short offsets[] = { F100(0), F100(100), F100(200), F100(300), F100(400),
                                  F100(500), F100(600), F100(700), F100(800), F100(900)};
  static const char table1[][4] = { JOIN("") }; 
  static const char table2[][4] = { JOIN2("") }; 
  static const char table3[][4] = { JOIN3("") };
  static const char table4[][5] = { JOIN4 }; 
  static const char table5[][4] = { JOIN6 };
#undef JOIN
#undef JOIN2
#undef JOIN3
#undef JOIN4
  char *wstr;
  int remains[2];
  unsigned int v2;
  if (value >= 100000000) {
    v2 = value / 10000;
    remains[0] = value - v2 * 10000;
    value = v2;
    v2 = value / 10000;
    remains[1] = value - v2 * 10000;
    value = v2;
    wstr = str;
    if (value >= 1000) {
      *(__int32 *) wstr = *(__int32 *) table4[value];
      wstr += 4;
    } else {
      *(__int32 *) wstr = *(__int32 *) table5[value];
      wstr += offsets[value];
    }
    *(__int32 *) wstr = *(__int32 *) table4[remains[1]];
    wstr += 4;
    *(__int32 *) wstr = *(__int32 *) table4[remains[0]];
    wstr += 4;
    *wstr = 0;
    return (wstr - str);
  }
  else if (value >= 10000) {
    v2 = value / 10000;
    remains[0] = value - v2 * 10000;
    value = v2;
    wstr = str;
    if (value >= 1000) {
      *(__int32 *) wstr = *(__int32 *) table4[value];
      wstr += 4;
      *(__int32 *) wstr = *(__int32 *) table4[remains[0]];
      wstr += 4;
      *wstr = 0;
      return 8;
    } else {
      *(__int32 *) wstr = *(__int32 *) table5[value];
      wstr += offsets[value];
      *(__int32 *) wstr = *(__int32 *) table4[remains[0]];
      wstr += 4;
      *wstr = 0;
      return (wstr - str);
    }
  }
  else {
    if (value >= 1000) {
      *(__int32 *) str = *(__int32 *) table4[value];
      str += 4;
      *str = 0;
      return 4;
    } else if (value >= 100) {
      *(__int32 *) str = *(__int32 *) table3[value];
      return 3;
    } else if (value >= 10) {
      *(__int16 *) str = *(__int16 *) table2[value];
      str += 2;
      *str = 0;
      return 2;
    } else {
      *(__int16 *) str = *(__int16 *) table1[value];
      return 1;
    }
  }
}

int ufast_itoa10(int value, char* str) {
  if (value < 0) { *(str++) = '-'; 
    return ufast_utoa10(-value, str) + 1; 
  }
  else return ufast_utoa10(value, str);
}


    void ufast_test() {

   print_mode("[modp_ufast]");

   std::string s;
   s.reserve(32);
   std::size_t total_length = 0;
   strtk::util::timer t;
   t.start();

   char buf[128];
   int len;
   for (int i = (-max_i2s / 2); i < (max_i2s / 2); ++i)
   {
      #ifdef enable_test_type01
      s.resize(ufast_itoa10(((i & 1) ? i : -i), const_cast<char*>(s.c_str())));
      total_length += s.size();
      #endif

      #ifdef enable_test_type02
      s.resize(ufast_itoa10(max_i2s + i, const_cast<char*>(s.c_str())));
      total_length += s.size();
      #endif

      #ifdef enable_test_type03
      s.resize(ufast_itoa10(randval[(max_i2s + i) & 1023], const_cast<char*>(s.c_str())));
      total_length += s.size();
      #endif
   }
   t.stop();
   printf("Numbers:%10lu\tTotal:%12lu\tTime:%8.4fsec\tRate:%14.4fnums/sec\n",
          static_cast<unsigned long>(3 * max_i2s),
          static_cast<unsigned long>(total_length),
          t.time(),
          (3.0 * max_i2s) / t.time());
}

Answer 6

I can't test under VS, but this seems to be faster than your code for g++, about 10%. It could probably be tuned, the decision values chosen are guesses. int only, sorry.

typedef unsigned buf_t; 

static buf_t * reduce(unsigned val, buf_t * stp) {
   unsigned above = val / 10000; 
   if (above != 0) {
      stp = reduce(above, stp); 
      val -= above * 10000; 
   }

   buf_t digit  = val / 1000; 
   *stp++ = digit + '0'; 
   val -= digit * 1000; 

   digit  = val / 100; 
   *stp++ = digit + '0'; 
   val -= digit * 100; 

   digit  = val / 10; 
   *stp++ = digit + '0'; 
   val -= digit * 10; 
   *stp++ = val + '0'; 
   return stp; 
}

std::string itostr(int input) {

   buf_t buf[16]; 


   if(input == INT_MIN) {  
      char buf2[16]; 
      std::sprintf(buf2, "%d", input); 
      return std::string(buf2); 
   }

   // handle negative
   unsigned val = input;
   if(input < 0) 
      val = -input;

   buf[0] = '0'; 
   buf_t* endp = reduce(val, buf+1); 
   *endp = 127; 

   buf_t * stp = buf+1; 
   while (*stp == '0') 
      stp++;
   if (stp == endp)
      stp--; 

   if (input < 0) { 
      stp--; 
      *stp = '-'; 
   }
   return std::string(stp, endp); 
}

Answer 7

I've had this sitting around for awhile and finally got around to posting it.

A few more methods compared to the double-word at a time hopman_fast. The results are for the GCC's short-string-optimized std::string as otherwise performance differences get obscured by the overhead of the copy-on-write string management code. Throughput is measured the same way as elsewhere in this topic, cycle counts are for the raw serialization parts of the code prior to copying the output buffer into a string.

HOPMAN_FAST - performance reference  
TM_CPP, TM_VEC - scalar and vector versions of Terje Mathisen algorithm  
WM_VEC - intrinsics implementation of Wojciech Mula's vector algorithm  
AK_BW - word-at-a-time routine with a jump table that fills a buffer in reverse  
AK_FW - forward-stepping word-at-a-time routine with a jump table in assembly  
AK_UNROLLED - generic word-at-a-time routine that uses an unrolled loop  

Compile-time switches:

-DVSTRING - enables SSO strings for older GCC setups
-DBSR1 - enables fast log10
-DRDTSC - enables cycle counters

#include <cstdio>
#include <iostream>
#include <climits>
#include <sstream>
#include <algorithm>
#include <cstring>
#include <limits>
#include <ctime>
#include <stdint.h>
#include <x86intrin.h>

/* Uncomment to run */
// #define HOPMAN_FAST
// #define TM_CPP
// #define TM_VEC
// #define WM_VEC
// #define AK_UNROLLED
// #define AK_BW
// #define AK_FW

using namespace std;
#ifdef VSTRING
#include <ext/vstring.h>
typedef __gnu_cxx::__vstring string_type;
#else
typedef string string_type;
#endif

namespace detail {

#ifdef __GNUC__
#define ALIGN(N) __attribute__ ((aligned(N)))
#define PACK __attribute__ ((packed))
  inline size_t num_digits(unsigned u) {
    struct {
      uint32_t count;
      uint32_t max;
    } static digits[32] ALIGN(64) = {
    { 1, 9 }, { 1, 9 }, { 1, 9 }, { 1, 9 },
    { 2, 99 }, { 2, 99 }, { 2, 99 },
    { 3, 999 }, { 3, 999 }, { 3, 999 },
    { 4, 9999 }, { 4, 9999 }, { 4, 9999 }, { 4, 9999 },
    { 5, 99999 }, { 5, 99999 }, { 5, 99999 },
    { 6, 999999 }, { 6, 999999 }, { 6, 999999 },
    { 7, 9999999 }, { 7, 9999999 }, { 7, 9999999 }, { 7, 9999999 },
    { 8, 99999999 }, { 8, 99999999 }, { 8, 99999999 },
    { 9, 999999999 }, { 9, 999999999 }, { 9, 999999999 },
    { 10, UINT_MAX }, { 10, UINT_MAX }
    };
#if (defined(i386) || defined(__x86_64__)) && (defined(BSR1) || defined(BSR2))
    size_t l = u;
#if defined(BSR1)
    __asm__ __volatile__ (
      "bsrl %k0, %k0    \n\t"
      "shlq $32, %q1    \n\t" 
      "movq %c2(,%0,8), %0\n\t" 
      "cmpq %0, %q1     \n\t"
      "seta %b1         \n\t"
      "addl %1, %k0     \n\t"
      : "+r" (l), "+r"(u)
      : "i"(digits)
      : "cc"
    );
    return l;
#else
    __asm__ __volatile__ ( "bsr %0, %0;"  : "+r" (l) );
    return digits[l].count + ( u > digits[l].max );
#endif
#else
    size_t l = (u != 0) ? 31 - __builtin_clz(u) : 0;
    return digits[l].count + ( u > digits[l].max );
#endif 
  }
#else 
  inline unsigned msb_u32(unsigned x) {
    static const unsigned bval[] = { 0,1,2,2,3,3,3,3,4,4,4,4,4,4,4,4 };
    unsigned base = 0;
    if (x & (unsigned) 0xFFFF0000) { base += 32/2; x >>= 32/2; }
    if (x & (unsigned) 0x0000FF00) { base += 32/4; x >>= 32/4; }
    if (x & (unsigned) 0x000000F0) { base += 32/8; x >>= 32/8; }
    return base + bval[x];
  }

  inline size_t num_digits(unsigned x) {
    static const unsigned powertable[] = {
  0,10,100,1000,10000,100000,1000000,10000000,100000000, 1000000000 };
    size_t lg_ten = msb_u32(x) * 1233 >> 12;
    size_t adjust = (x >= powertable[lg_ten]);
    return lg_ten + adjust;
  }
#endif /* __GNUC__ */

  struct CharBuffer {
    class reverse_iterator : public iterator<random_access_iterator_tag, char> {
        char* m_p;
      public:
        reverse_iterator(char* p) : m_p(p - 1) {}
        reverse_iterator operator++() { return --m_p; }
        reverse_iterator operator++(int) { return m_p--; }
        char operator*() const { return *m_p; }
        bool operator==( reverse_iterator it) const { return m_p == it.m_p; }
        bool operator!=( reverse_iterator it) const { return m_p != it.m_p; }
        difference_type operator-( reverse_iterator it) const { return it.m_p - m_p; }
    };
  };

  union PairTable {
    char c[2];
    unsigned short u;
  } PACK table[100] ALIGN(1024) = {
    {{'0','0'}},{{'0','1'}},{{'0','2'}},{{'0','3'}},{{'0','4'}},{{'0','5'}},{{'0','6'}},{{'0','7'}},{{'0','8'}},{{'0','9'}},
    {{'1','0'}},{{'1','1'}},{{'1','2'}},{{'1','3'}},{{'1','4'}},{{'1','5'}},{{'1','6'}},{{'1','7'}},{{'1','8'}},{{'1','9'}},
    {{'2','0'}},{{'2','1'}},{{'2','2'}},{{'2','3'}},{{'2','4'}},{{'2','5'}},{{'2','6'}},{{'2','7'}},{{'2','8'}},{{'2','9'}},
    {{'3','0'}},{{'3','1'}},{{'3','2'}},{{'3','3'}},{{'3','4'}},{{'3','5'}},{{'3','6'}},{{'3','7'}},{{'3','8'}},{{'3','9'}},
    {{'4','0'}},{{'4','1'}},{{'4','2'}},{{'4','3'}},{{'4','4'}},{{'4','5'}},{{'4','6'}},{{'4','7'}},{{'4','8'}},{{'4','9'}},
    {{'5','0'}},{{'5','1'}},{{'5','2'}},{{'5','3'}},{{'5','4'}},{{'5','5'}},{{'5','6'}},{{'5','7'}},{{'5','8'}},{{'5','9'}},
    {{'6','0'}},{{'6','1'}},{{'6','2'}},{{'6','3'}},{{'6','4'}},{{'6','5'}},{{'6','6'}},{{'6','7'}},{{'6','8'}},{{'6','9'}},
    {{'7','0'}},{{'7','1'}},{{'7','2'}},{{'7','3'}},{{'7','4'}},{{'7','5'}},{{'7','6'}},{{'7','7'}},{{'7','8'}},{{'7','9'}},
    {{'8','0'}},{{'8','1'}},{{'8','2'}},{{'8','3'}},{{'8','4'}},{{'8','5'}},{{'8','6'}},{{'8','7'}},{{'8','8'}},{{'8','9'}},
    {{'9','0'}},{{'9','1'}},{{'9','2'}},{{'9','3'}},{{'9','4'}},{{'9','5'}},{{'9','6'}},{{'9','7'}},{{'9','8'}},{{'9','9'}}
  };
} // namespace detail

struct progress_timer {
    clock_t c;
    progress_timer() : c(clock()) {}
    int elapsed() { return clock() - c; }
    ~progress_timer() {
        clock_t d = clock() - c;
        cout << d / CLOCKS_PER_SEC << "."
            << (((d * 1000) / CLOCKS_PER_SEC) % 1000 / 100)
            << (((d * 1000) / CLOCKS_PER_SEC) % 100 / 10)
            << (((d * 1000) / CLOCKS_PER_SEC) % 10)
            << " s" << endl;
    }
};

#ifdef HOPMAN_FAST
namespace hopman_fast {

    static unsigned long cpu_cycles = 0;

    struct itostr_helper {
        static ALIGN(1024) unsigned out[10000];

        itostr_helper() {
            for (int i = 0; i < 10000; i++) {
                unsigned v = i;
                char * o = (char*)(out + i);
                o[3] = v % 10 + '0';
                o[2] = (v % 100) / 10 + '0';
                o[1] = (v % 1000) / 100 + '0';
                o[0] = (v % 10000) / 1000;
                if (o[0]) o[0] |= 0x30;
                else if (o[1] != '0') o[0] |= 0x20;
                else if (o[2] != '0') o[0] |= 0x10;
                else o[0] |= 0x00;
            }
        }
    };
    unsigned itostr_helper::out[10000];

    itostr_helper hlp_init;

    template <typename T>
    string_type itostr(T o) {
        typedef itostr_helper hlp;
#ifdef RDTSC
        long first_clock = __rdtsc();
#endif
        unsigned blocks[3], *b = blocks + 2;
        blocks[0] = o < 0 ? ~o + 1 : o;
        blocks[2] = blocks[0] % 10000; blocks[0] /= 10000;
        blocks[2] = hlp::out[blocks[2]];

        if (blocks[0]) {
            blocks[1] = blocks[0] % 10000; blocks[0] /= 10000;
            blocks[1] = hlp::out[blocks[1]];
            blocks[2] |= 0x30303030;
            b--;
        }

        if (blocks[0]) {
            blocks[0] = hlp::out[blocks[0] % 10000];
            blocks[1] |= 0x30303030;
            b--;
        }

        char* f = ((char*)b);
        f += 3 - (*f >> 4);

        char* str = (char*)blocks;
        if (o < 0) *--f = '-';

        str += 12;
#ifdef RDTSC
        cpu_cycles += __rdtsc() - first_clock;
#endif
        return string_type(f, str);
    }
      unsigned long cycles() { return cpu_cycles; }
      void reset() { cpu_cycles = 0; }
}
#endif

namespace ak {
#ifdef AK_UNROLLED
  namespace unrolled {
    static unsigned long cpu_cycles = 0;

    template <typename value_type> class Proxy {
      static const size_t MaxValueSize = 16;

      static inline char* generate(int value, char* buffer) {
        union { char* pc; unsigned short* pu; } b = { buffer + MaxValueSize };
        unsigned u, v = value < 0 ? unsigned(~value) + 1 : value;
        *--b.pu = detail::table[v % 100].u; u = v;
        if ((v /= 100)) {
          *--b.pu = detail::table[v % 100].u; u = v;
          if ((v /= 100)) {
            *--b.pu = detail::table[v % 100].u; u = v;
            if ((v /= 100)) {
              *--b.pu = detail::table[v % 100].u; u = v;
              if ((v /= 100)) {
                *--b.pu = detail::table[v % 100].u; u = v;
        } } } }
        *(b.pc -= (u >= 10)) = '-';
        return b.pc + (value >= 0);
      }
      static inline char* generate(unsigned value, char* buffer) {
        union { char* pc; unsigned short* pu; } b = { buffer + MaxValueSize };
        unsigned u, v = value;
        *--b.pu = detail::table[v % 100].u; u = v;
        if ((v /= 100)) {
          *--b.pu = detail::table[v % 100].u; u = v;
          if ((v /= 100)) {
            *--b.pu = detail::table[v % 100].u; u = v;
            if ((v /= 100)) {
              *--b.pu = detail::table[v % 100].u; u = v;
              if ((v /= 100)) {
                *--b.pu = detail::table[v % 100].u; u = v;
        } } } }
        return b.pc + (u < 10);
      }
    public:
      static inline string_type convert(value_type v) {
        char buf[MaxValueSize];
#ifdef RDTSC
        long first_clock = __rdtsc();
#endif
        char* p = generate(v, buf);
        char* e = buf + MaxValueSize;
#ifdef RDTSC
        cpu_cycles += __rdtsc() - first_clock;
#endif
        return string_type(p, e);
      }
    };
    string_type itostr(int i) { return Proxy<int>::convert(i); }
    string_type itostr(unsigned i) { return Proxy<unsigned>::convert(i); }
    unsigned long cycles() { return cpu_cycles; }
    void reset() { cpu_cycles = 0; }
  }
#endif

#if defined(AK_BW)
  namespace bw {
    static unsigned long cpu_cycles = 0;
    typedef uint64_t u_type;

    template <typename value_type> class Proxy {

      static inline void generate(unsigned v, size_t len, char* buffer) {
        u_type u = v;
        switch(len) {
        default: u = (v * 1374389535ULL) >> 37; *(uint16_t*)(buffer + 8) = detail::table[v -= 100 * u].u; 
        case  8: v = (u * 1374389535ULL) >> 37; *(uint16_t*)(buffer + 6) = detail::table[u -= 100 * v].u; 
        case  6: u = (v * 1374389535ULL) >> 37; *(uint16_t*)(buffer + 4) = detail::table[v -= 100 * u].u;
        case  4: v = (u * 167773) >> 24; *(uint16_t*)(buffer + 2) = detail::table[u -= 100 * v].u;
        case  2: *(uint16_t*)buffer = detail::table[v].u;
        case  0: return;
        case  9: u = (v * 1374389535ULL) >> 37; *(uint16_t*)(buffer + 7) = detail::table[v -= 100 * u].u;
        case  7: v = (u * 1374389535ULL) >> 37; *(uint16_t*)(buffer + 5) = detail::table[u -= 100 * v].u;
        case  5: u = (v * 1374389535ULL) >> 37; *(uint16_t*)(buffer + 3) = detail::table[v -= 100 * u].u;
        case  3: v = (u * 167773) >> 24; *(uint16_t*)(buffer + 1) = detail::table[u -= 100 * v].u;
        case  1: *buffer = v + 0x30;
        }
      }
    public:
      static inline string_type convert(bool neg, unsigned val) {
        char buf[16];
#ifdef RDTSC
        long first_clock = __rdtsc();
#endif
        size_t len = detail::num_digits(val);
        buf[0] = '-';

        char* e = buf + neg;
        generate(val, len, e);
        e += len;
#ifdef RDTSC
        cpu_cycles += __rdtsc() - first_clock;
#endif
        return string_type(buf, e);
      }
    };
    string_type itostr(int i) { return Proxy<int>::convert(i < 0, i < 0 ? unsigned(~i) + 1 : i); }
    string_type itostr(unsigned i) { return Proxy<unsigned>::convert(false, i); }
    unsigned long cycles() { return cpu_cycles; }
    void reset() { cpu_cycles = 0; }
  }
#endif

#if defined(AK_FW)
  namespace fw {
        static unsigned long cpu_cycles = 0;
        typedef uint32_t u_type;
        template <typename value_type> class Proxy {

        static inline void generate(unsigned v, size_t len, char* buffer) {
#if defined(__GNUC__) && defined(__x86_64__)
          uint16_t w;
          uint32_t u;
          __asm__ __volatile__ (
        "jmp %*T%=(,%3,8)       \n\t"
        "T%=: .quad L0%=        \n\t"
        "     .quad L1%=        \n\t"
        "     .quad L2%=        \n\t"
        "     .quad L3%=        \n\t"
        "     .quad L4%=        \n\t"
        "     .quad L5%=        \n\t"
        "     .quad L6%=        \n\t"
        "     .quad L7%=        \n\t"
        "     .quad L8%=        \n\t"
        "     .quad L9%=        \n\t"
        "     .quad L10%=       \n\t"
        "L10%=:         \n\t"
        " imulq $1441151881, %q0, %q1\n\t"
        " shrq $57, %q1     \n\t"
        " movw %c5(,%q1,2), %w2 \n\t"
        " imull $100000000, %1, %1  \n\t"
        " subl %1, %0       \n\t"
        " movw %w2, (%4)        \n\t"
        "L8%=:          \n\t"
        " imulq $1125899907, %q0, %q1\n\t"
        " shrq $50, %q1     \n\t"
        " movw %c5(,%q1,2), %w2 \n\t"
        " imull $1000000, %1, %1    \n\t"
        " subl %1, %0       \n\t"
        " movw %w2, -8(%4,%3)   \n\t"
        "L6%=:          \n\t"
        " imulq $429497, %q0, %q1   \n\t"
        " shrq $32, %q1     \n\t"
        " movw %c5(,%q1,2), %w2 \n\t"
        " imull $10000, %1, %1  \n\t"
        " subl %1, %0       \n\t"
        " movw %w2, -6(%4,%3)   \n\t"
        "L4%=:          \n\t"
        " imull $167773, %0, %1 \n\t"
        " shrl $24, %1      \n\t"
        " movw %c5(,%q1,2), %w2 \n\t"
        " imull $100, %1, %1    \n\t"
        " subl %1, %0       \n\t"
        " movw %w2, -4(%4,%3)   \n\t"
        "L2%=:          \n\t"
        " movw %c5(,%q0,2), %w2 \n\t"
        " movw %w2, -2(%4,%3)   \n\t"
        "L0%=: jmp 1f       \n\t"
        "L9%=:          \n\t"
        " imulq $1801439851, %q0, %q1\n\t"
        " shrq $54, %q1     \n\t"
        " movw %c5(,%q1,2), %w2 \n\t"
        " imull $10000000, %1, %1   \n\t"
        " subl %1, %0       \n\t"
        " movw %w2, (%4)        \n\t"
        "L7%=:          \n\t"
        " imulq $43980466, %q0, %q1 \n\t"
        " shrq $42, %q1     \n\t"
        " movw %c5(,%q1,2), %w2 \n\t"
        " imull $100000, %1, %1 \n\t"
        " subl %1, %0       \n\t"
        " movw %w2, -7(%4,%3)   \n\t"
        "L5%=:          \n\t"
        " imulq $268436, %q0, %q1   \n\t"
        " shrq $28, %q1     \n\t"
        " movw %c5(,%q1,2), %w2 \n\t"
        " imull $1000, %1, %1   \n\t"
        " subl %1, %0       \n\t"
        " movw %w2, -5(%4,%3)   \n\t"
        "L3%=:          \n\t"
        " imull $6554, %0, %1   \n\t"
        " shrl $15, %1      \n\t"
        " andb $254, %b1        \n\t"
        " movw %c5(,%q1), %w2   \n\t"
        " leal (%1,%1,4), %1    \n\t"
        " subl %1, %0       \n\t"
        " movw %w2, -3(%4,%3)   \n\t"
        "L1%=:          \n\t"
        " addl $48, %0      \n\t"
        " movb %b0, -1(%4,%3)   \n\t"
        "1:             \n\t"
        : "+r"(v), "=&q"(u), "=&r"(w)
        : "r"(len), "r"(buffer), "i"(detail::table)
        : "memory", "cc"
          ); 
#else
          u_type u;
          switch(len) {
        default: u = (v * 1441151881ULL) >> 57; *(uint16_t*)(buffer) = detail::table[u].u; v -= u * 100000000;
        case  8: u = (v * 1125899907ULL) >> 50; *(uint16_t*)(buffer + len - 8) = detail::table[u].u; v -= u * 1000000;
        case  6: u = (v * 429497ULL) >> 32; *(uint16_t*)(buffer + len - 6) = detail::table[u].u; v -= u * 10000;
        case  4: u = (v * 167773) >> 24; *(uint16_t*)(buffer + len - 4) = detail::table[u].u; v -= u * 100;
        case  2: *(uint16_t*)(buffer + len - 2) = detail::table[v].u;
        case  0: return;
        case  9: u = (v * 1801439851ULL) >> 54; *(uint16_t*)(buffer) = detail::table[u].u; v -= u * 10000000; 
        case  7: u = (v * 43980466ULL) >> 42; *(uint16_t*)(buffer + len - 7) = detail::table[u].u; v -= u * 100000; 
        case  5: u = (v * 268436ULL) >> 28;  *(uint16_t*)(buffer + len - 5) = detail::table[u].u; v -= u * 1000;
        case  3: u = (v * 6554) >> 16; *(uint16_t*)(buffer + len - 3) = detail::table[u].u; v -= u * 10;
        case  1: *(buffer + len - 1) = v + 0x30;
          }
#endif
        }
      public:
        static inline string_type convert(bool neg, unsigned val) {
        char buf[16];
#ifdef RDTSC
        long first_clock = __rdtsc();
#endif
        size_t len = detail::num_digits(val);
        if (neg) buf[0] = '-';
        char* e = buf + len + neg;
        generate(val, len, buf + neg);
#ifdef RDTSC
        cpu_cycles += __rdtsc() - first_clock;
#endif
        return string_type(buf, e);
        }
      };
      string_type itostr(int i) { return Proxy<int>::convert(i < 0, i < 0 ? unsigned(~i) + 1 : i); }
      string_type itostr(unsigned i) { return Proxy<unsigned>::convert(false, i); }
      unsigned long cycles() { return cpu_cycles; }
      void reset() { cpu_cycles = 0; }
  }
#endif
} // ak

namespace wm {
#ifdef WM_VEC
#if defined(__GNUC__) && defined(__x86_64__)
  namespace vec {
      static unsigned long cpu_cycles = 0;

      template <typename value_type> class Proxy {

      static inline unsigned generate(unsigned v, char* buf) {
        static struct {
          unsigned short mul_10[8];
          unsigned short div_const[8];
          unsigned short shl_const[8];
          unsigned char  to_ascii[16];
        } ALIGN(64) bits = 
        {
          { // mul_10
           10, 10, 10, 10, 10, 10, 10, 10
          },
          { // div_const
            8389, 5243, 13108, 0x8000, 8389, 5243, 13108, 0x8000
          },
          { // shl_const
            1 << (16 - (23 + 2 - 16)),
            1 << (16 - (19 + 2 - 16)),
            1 << (16 - 1 - 2),
            1 << (15),
            1 << (16 - (23 + 2 - 16)),
            1 << (16 - (19 + 2 - 16)),
            1 << (16 - 1 - 2),
            1 << (15)
          },
          { // to_ascii 
            '0', '0', '0', '0', '0', '0', '0', '0',
            '0', '0', '0', '0', '0', '0', '0', '0'
          }
        };
        unsigned x, y, l;
        x = (v * 1374389535ULL) >> 37;
        y = v;
        l = 0;
        if (x) {
          unsigned div = 0xd1b71759;
          unsigned mul = 55536;
          __m128i z, m, a, o;
          y -= 100 * x;
          z = _mm_cvtsi32_si128(x);
          m = _mm_load_si128((__m128i*)bits.mul_10);
          o = _mm_mul_epu32( z, _mm_cvtsi32_si128(div));
          z = _mm_add_epi32( z, _mm_mul_epu32( _mm_cvtsi32_si128(mul), _mm_srli_epi64( o, 45) ) );
          z = _mm_slli_epi64( _mm_shuffle_epi32( _mm_unpacklo_epi16(z, z), 5 ), 2 );
          a = _mm_load_si128((__m128i*)bits.to_ascii);
          z = _mm_mulhi_epu16( _mm_mulhi_epu16( z, *(__m128i*)bits.div_const ), *(__m128i*)bits.shl_const );
          z = _mm_sub_epi16( z, _mm_slli_epi64( _mm_mullo_epi16( m, z ), 16 ) );
          z = _mm_add_epi8( _mm_packus_epi16( z, _mm_xor_si128(o, o) ), a );
          x = __builtin_ctz( ~_mm_movemask_epi8( _mm_cmpeq_epi8( a, z ) ) );
          l = 8 - x;
          uint64_t q = _mm_cvtsi128_si64(z) >> (x * 8);
          *(uint64_t*)buf = q;
          buf += l;
          x = 1;
        }
        v = (y * 6554) >> 16;
        l += 1 + (x | (v != 0));
            *(unsigned short*)buf = 0x30 + ((l > 1) ? ((0x30 + y - v * 10) << 8) + v : y);
            return l;
        }
      public:
        static inline string_type convert(bool neg, unsigned val) {
        char buf[16];
#ifdef RDTSC
        long first_clock = __rdtsc();
#endif
        buf[0] = '-';
        unsigned len = generate(val, buf + neg);
        char* e = buf + len + neg;
#ifdef RDTSC
        cpu_cycles += __rdtsc() - first_clock;
#endif
        return string_type(buf, e);
        }
      };
      inline string_type itostr(int i) { return Proxy<int>::convert(i < 0, i < 0 ? unsigned(~i) + 1 : i); }
      inline string_type itostr(unsigned i) { return Proxy<unsigned>::convert(false, i); }
      unsigned long cycles() { return cpu_cycles; }
      void reset() { cpu_cycles = 0; }
  }
#endif
#endif
} // wm

namespace tmn {

#ifdef TM_CPP
  namespace cpp {
      static unsigned long cpu_cycles = 0;

      template <typename value_type> class Proxy {

        static inline void generate(unsigned v, char* buffer) {
          unsigned const f1_10000 = (1 << 28) / 10000;
          unsigned tmplo, tmphi;

          unsigned lo = v % 100000;
          unsigned hi = v / 100000;

          tmplo = lo * (f1_10000 + 1) - (lo >> 2);
          tmphi = hi * (f1_10000 + 1) - (hi >> 2);

          unsigned mask = 0x0fffffff;
          unsigned shift = 28;

          for(size_t i = 0; i < 5; i++)
          {
            buffer[i + 0] = '0' + (char)(tmphi >> shift);
            buffer[i + 5] = '0' + (char)(tmplo >> shift);
            tmphi = (tmphi & mask) * 5;
            tmplo = (tmplo & mask) * 5;
            mask >>= 1;
            shift--;
          }
        }
      public:
        static inline string_type convert(bool neg, unsigned val) {
#ifdef RDTSC
        long first_clock = __rdtsc();
#endif
        char buf[16];
        size_t len = detail::num_digits(val);
        char* e = buf + 11;
        generate(val, buf + 1);
        buf[10 - len] = '-';
        len += neg;
        char* b = e - len;
#ifdef RDTSC
        cpu_cycles += __rdtsc() - first_clock;
#endif
        return string_type(b, e);
        }
      };
      string_type itostr(int i) { return Proxy<int>::convert(i < 0, i < 0 ? unsigned(~i) + 1 : i); }
      string_type itostr(unsigned i) { return Proxy<unsigned>::convert(false, i); }
      unsigned long cycles() { return cpu_cycles; }
      void reset() { cpu_cycles = 0; }
  }
#endif

#ifdef TM_VEC
  namespace vec {
      static unsigned long cpu_cycles = 0;

      template <typename value_type> class Proxy {

        static inline unsigned generate(unsigned val, char* buffer) {
        static struct {
            unsigned char mul_10[16];
            unsigned char to_ascii[16];
            unsigned char gather[16];
            unsigned char shift[16];
        } ALIGN(64) bits = {
            { 10,0,0,0,10,0,0,0,10,0,0,0,10,0,0,0 },
            { '0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0' },
            { 3,5,6,7,9,10,11,13,14,15,0,0,0,0,0,0 },
            { 0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15 }
        };

        unsigned u = val / 1000000;
        unsigned l = val - u * 1000000;

        __m128i x, h, f, m, n;

        n = _mm_load_si128((__m128i*)bits.mul_10);
        x = _mm_set_epi64x( l, u );
        h = _mm_mul_epu32( x, _mm_set1_epi32(4294968) );
        x = _mm_sub_epi64( x, _mm_srli_epi64( _mm_mullo_epi32( h, _mm_set1_epi32(1000) ), 32 ) );
        f = _mm_set1_epi32((1 << 28) / 1000 + 1);
        m = _mm_srli_epi32( _mm_cmpeq_epi32(m, m), 4 );
        x = _mm_shuffle_epi32( _mm_blend_epi16( x, h, 204 ), 177 );
        f = _mm_sub_epi32( _mm_mullo_epi32(f, x), _mm_srli_epi32(x, 2) );

        h = _mm_load_si128((__m128i*)bits.to_ascii);

        x = _mm_srli_epi32(f, 28);
        f = _mm_mullo_epi32( _mm_and_si128( f, m ), n );

        x = _mm_or_si128( x, _mm_slli_epi32(_mm_srli_epi32(f, 28), 8) );
        f = _mm_mullo_epi32( _mm_and_si128( f, m ), n );

        x = _mm_or_si128( x, _mm_slli_epi32(_mm_srli_epi32(f, 28), 16) );
        f = _mm_mullo_epi32( _mm_and_si128( f, m ), n );

        x = _mm_or_si128( x, _mm_slli_epi32(_mm_srli_epi32(f, 28), 24) );

        x = _mm_add_epi8( _mm_shuffle_epi8(x, *(__m128i*)bits.gather), h );
        l = __builtin_ctz( ~_mm_movemask_epi8( _mm_cmpeq_epi8( h, x ) ) | (1 << 9) );

        x = _mm_shuffle_epi8( x, _mm_add_epi8(*(__m128i*)bits.shift, _mm_set1_epi8(l) ) );

        _mm_store_si128( (__m128i*)buffer, x );
        return 10 - l;
        }

      public:
        static inline string_type convert(bool neg, unsigned val) {
#ifdef RDTSC
        long first_clock = __rdtsc();
#endif
        char arena[32];
        char* buf = (char*)((uintptr_t)(arena + 16) & ~(uintptr_t)0xf);
        *(buf - 1)= '-';
        unsigned len = generate(val, buf) + neg;
        buf -= neg;
        char* end = buf + len;
#ifdef RDTSC
        cpu_cycles += __rdtsc() - first_clock;
#endif
        return string_type(buf, end);
        }
      };
      string_type itostr(int i) { return Proxy<int>::convert(i < 0, i < 0 ? unsigned(~i) + 1 : i); }
      string_type itostr(unsigned i) { return Proxy<unsigned>::convert(false, i); }
      unsigned long cycles() { return cpu_cycles; }
      void reset() { cpu_cycles = 0; }
  }
#endif
}

bool fail(string in, string_type out) {
    cout << "failure: " << in << " => " << out << endl;
    return false;
}

#define TEST(x, n) \
    stringstream ss; \
    string_type s = n::itostr(x); \
    ss << (long long)x; \
    if (::strcmp(ss.str().c_str(), s.c_str())) { \
        passed = fail(ss.str(), s); \
        break; \
    }

#define test(x) { \
    passed = true; \
    if (0 && passed) { \
        char c = CHAR_MIN; \
        do { \
            TEST(c, x); \
        } while (c++ != CHAR_MAX); \
        if (!passed) cout << #x << " failed char!!!" << endl; \
    } \
    if (0 && passed) { \
        short c = numeric_limits<short>::min(); \
        do { \
            TEST(c, x); \
        } while (c++ != numeric_limits<short>::max()); \
        if (!passed) cout << #x << " failed short!!!" << endl; \
    } \
    if (passed) { \
        int c = numeric_limits<int>::min(); \
        do { \
            TEST(c, x); \
        } while ((c += 100000) < numeric_limits<int>::max() - 100000); \
        if (!passed) cout << #x << " failed int!!!" << endl; \
    } \
    if (passed) { \
        unsigned c = numeric_limits<unsigned>::max(); \
        do { \
            TEST(c, x); \
        } while ((c -= 100000) > 100000); \
        if (!passed) cout << #x << " failed unsigned int!!!" << endl; \
    } \
}

#define time(x, N) \
if (passed) { \
    static const int64_t limits[] = \
        {0, 10, 100, 1000, 10000, 100000, \
         1000000, 10000000, 100000000, 1000000000, 10000000000ULL }; \
    long passes = 0; \
    cout << #x << ": "; \
    progress_timer t; \
    uint64_t s = 0; \
    if (do_time) { \
        for (int n = 0; n < N1; n++) { \
            int i = 0; \
            while (i < N2) { \
                int v = ((NM - i) % limits[N]) | (limits[N] / 10); \
                int w = x::itostr(v).size() + \
                    x::itostr(-v).size(); \
                i += w * mult; \
                                passes++; \
            } \
            s += i / mult; \
        } \
    } \
    k += s; \
    cout << N << " digits: " \
          << s / double(t.elapsed()) * CLOCKS_PER_SEC/1000000 << " MB/sec, " << (x::cycles() / passes >> 1) << " clocks per pass "; \
    x::reset(); \
}

#define series(n) \
    { if (do_test) test(n);    if (do_time) time(n, 1); if (do_time) time(n, 2); \
      if (do_time) time(n, 3); if (do_time) time(n, 4); if (do_time) time(n, 5); \
      if (do_time) time(n, 6); if (do_time) time(n, 7); if (do_time) time(n, 8); \
      if (do_time) time(n, 9); if (do_time) time(n, 10); }

int N1 = 1, N2 = 500000000, NM = INT_MAX;
int mult = 1; //  used to stay under timelimit on ideone
unsigned long long k = 0;

int main(int argc, char** argv) {
    bool do_time = 1, do_test = 1;
    bool passed = true;
#ifdef HOPMAN_FAST
    series(hopman_fast)
#endif
#ifdef WM_VEC
    series(wm::vec)
#endif
#ifdef TM_CPP
    series(tmn::cpp)
#endif
#ifdef TM_VEC
    series(tmn::vec)
#endif
#ifdef AK_UNROLLED
    series(ak::unrolled)
#endif
#if defined(AK_BW)
    series(ak::bw)
#endif
#if defined(AK_FW)
    series(ak::fw)
#endif
    return k;
}

Answer 8

I believe I have created the fastest integer-to-string algorithm. It's a variation of the Modulo 100 algorithm that is about 33% faster, and most importantly it's faster for both smaller and large numbers. It's called the Script ItoS Algorithm. To read the paper that explains how I engineered the algorithm @see https://github.com/kabuki-starship/kabuki-toolkit/wiki/Engineering-a-Faster-Integer-to-String-Algorithm. You may use the algorithm but please think about contributing back to the Kabuki VM and check out Script; especially if you're interested in AMIL-NLP and/or software-defined networking protocols.

/** Kabuki Toolkit
    @version 0.x
    @file    ~/source/crabs/print_itos.cc
    @author  Cale McCollough <cale.mccollough@gmail.com>
    @license Copyright (C) 2017-2018 Cale McCollough <calemccollough@gmail.com>;
             All right reserved (R). Licensed under the Apache License, Version 
             2.0 (the "License"); you may not use this file except in 
             compliance with the License. You may obtain a copy of the License 
             [here](http://www.apache.org/licenses/LICENSE-2.0). Unless 
             required by applicable law or agreed to in writing, software 
             distributed under the License is distributed on an "AS IS" BASIS, 
             WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or 
             implied. See the License for the specific language governing 
             permissions and limitations under the License.
*/

#include <stdafx.h>
#include "print_itos.h"

#if MAJOR_SEAM >= 1 && MINOR_SEAM >= 1

#if MAJOR_SEAM == 1 && MINOR_SEAM == 1
#define DEBUG 1

#define PRINTF(format, ...) printf(format, __VA_ARGS__);
#define PUTCHAR(c) putchar(c);
#define PRINT_PRINTED\
    sprintf_s (buffer, 24, "%u", value); *text_end = 0;\
    printf ("\n    Printed \"%s\" leaving value:\"%s\":%u",\
            begin, buffer, (uint)strlen (buffer));
#define PRINT_BINARY PrintBinary (value);
#define PRINT_BINARY_TABLE PrintBinaryTable (value);
#else
#define PRINTF(x, ...)
#define PUTCHAR(c)
#define PRINT_PRINTED
#define PRINT_BINARY
#define PRINT_BINARY_TABLE
#endif

namespace _ {

void PrintLine (char c) {
    std::cout << '\n';
    for (int i = 80; i > 0; --i) 
        std::cout << c;
}

char* Print (uint32_t value, char* text, char* text_end) {

    // Lookup table for powers of 10.
    static const uint32_t k10ToThe[]{
        1, 10, 100, 1000, 10000, 100000, 1000000, 10000000, 100000000,
        1000000000, ~(uint32_t)0 };

    /** Lookup table of ASCII char pairs for 00, 01, ..., 99.
        To convert this algorithm to big-endian, flip the digit pair bytes. */
    static const uint16_t kDigits00To99[100] = {
        0x3030, 0x3130, 0x3230, 0x3330, 0x3430, 0x3530, 0x3630, 0x3730, 0x3830,
        0x3930, 0x3031, 0x3131, 0x3231, 0x3331, 0x3431, 0x3531, 0x3631, 0x3731,
        0x3831, 0x3931, 0x3032, 0x3132, 0x3232, 0x3332, 0x3432, 0x3532, 0x3632,
        0x3732, 0x3832, 0x3932, 0x3033, 0x3133, 0x3233, 0x3333, 0x3433, 0x3533,
        0x3633, 0x3733, 0x3833, 0x3933, 0x3034, 0x3134, 0x3234, 0x3334, 0x3434,
        0x3534, 0x3634, 0x3734, 0x3834, 0x3934, 0x3035, 0x3135, 0x3235, 0x3335,
        0x3435, 0x3535, 0x3635, 0x3735, 0x3835, 0x3935, 0x3036, 0x3136, 0x3236,
        0x3336, 0x3436, 0x3536, 0x3636, 0x3736, 0x3836, 0x3936, 0x3037, 0x3137,
        0x3237, 0x3337, 0x3437, 0x3537, 0x3637, 0x3737, 0x3837, 0x3937, 0x3038,
        0x3138, 0x3238, 0x3338, 0x3438, 0x3538, 0x3638, 0x3738, 0x3838, 0x3938,
        0x3039, 0x3139, 0x3239, 0x3339, 0x3439, 0x3539, 0x3639, 0x3739, 0x3839,
        0x3939, };

    static const char kMsbShift[] = { 4, 7, 11, 14, 17, 21, 24, 27, 30, };

    if (!text) {
        return nullptr;
    }
    if (text >= text_end) {
        return nullptr;
    }

    uint16_t* text16;
    char      digit;
    uint32_t  scalar;
    uint16_t  digits1and2,
              digits3and4,
              digits5and6,
              digits7and8;
    uint32_t  comparator;

    #if MAJOR_SEAM == 1 && MINOR_SEAM == 1
    // Write a bunches of xxxxxx to the buffer for debug purposes.
    for (int i = 0; i <= 21; ++i) {
        *(text + i) = 'x';
    }
    *(text + 21) = 0;
    char* begin = text;
    char buffer[256];
    #endif

    if (value < 10) {
        PRINTF ("\n    Range:[0, 9] length:1 ")
        if (text + 1 >= text_end) {
            return nullptr;
        }
        *text++ = '0' + (char)value;
        PRINT_PRINTED
        return text;
    }
    if (value < 100) {
        PRINTF ("\n    Range:[10, 99] length:2 ")
        if (text + 2 >= text_end) {
            return nullptr;
        }
        *reinterpret_cast<uint16_t*> (text) = kDigits00To99[value];
        PRINT_PRINTED
        return text + 2;
    }
    if (value >> 14) {
        if (value >> 27) {
            if (value >> 30) {
                PRINTF ("\n    Range:[1073741824, 4294967295] length:10")
                Print10:
                if (text + 10 >= text_end) {
                    return nullptr;
                }
                comparator = 100000000;
                digits1and2 = (uint16_t)(value / comparator);
                PRINTF ("\n    digits1and2:%u", digits1and2)
                value -= digits1and2 * comparator;
                *reinterpret_cast<uint16_t*> (text) = kDigits00To99[digits1and2];
                PRINT_PRINTED
                text += 2;
                goto Print8;
            }
            else {
                comparator = 1000000000;
                if (value >= comparator) {
                    PRINTF ("\n    Range:[100000000, 1073741823] length:10")
                    goto Print10;
                }
                PRINTF ("\n    Range:[134217727, 999999999] length:9")
                if (text + 9 >= text_end) {
                    return nullptr;
                }
                comparator = 100000000;
                digit = (char)(value / comparator);
                *text++ = digit + '0';
                PRINT_PRINTED
                value -= comparator * digit;
                goto Print8;
            }
        }
        else if (value >> 24) {
            comparator = k10ToThe[8];
            if (value >= comparator) {
                PRINTF ("\n    Range:[100000000, 134217728] length:9")
                if (text + 9 >= text_end) {
                    return nullptr;
                }
                *text++ = '1';
                PRINT_PRINTED
                value -= comparator;
            }
            PRINTF ("\n    Range:[16777216, 9999999] length:8")
            if (text + 8 >= text_end) {
                return nullptr;
            }
            Print8:
            PRINTF ("\n    Print8:")
            scalar = 10000;
            digits5and6 = (uint16_t)(value / scalar);
            digits1and2 = value - scalar * digits5and6;
            digits7and8 = digits5and6 / 100;
            digits3and4 = digits1and2 / 100;
            digits5and6 -= 100 * digits7and8;
            digits1and2 -= 100 * digits3and4;
            *reinterpret_cast<uint16_t*> (text + 6) = 
                kDigits00To99[digits1and2];
            PRINT_PRINTED
            *reinterpret_cast<uint16_t*> (text + 4) = 
                kDigits00To99[digits3and4];
            PRINT_PRINTED
            *reinterpret_cast<uint16_t*> (text + 2) = 
                kDigits00To99[digits5and6];
            PRINT_PRINTED
            *reinterpret_cast<uint16_t*> (text) = 
                kDigits00To99[digits7and8];
            PRINT_PRINTED
            return text + 8;
        }
        else if (value >> 20) {
            comparator = 10000000;
            if (value >= comparator) {
                PRINTF ("\n    Range:[10000000, 16777215] length:8")
                if (text + 8 >= text_end) {
                    return nullptr;
                }
                *text++ = '1';
                PRINT_PRINTED
                value -= comparator;
            }
            else {
                PRINTF ("\n    Range:[1048576, 9999999] length:7")
                if (text + 7 >= text_end) {
                    return nullptr;
                }
            }
            scalar = 10000;
            digits5and6 = (uint16_t)(value / scalar);
            digits1and2 = value - scalar * digits5and6;
            digits7and8 = digits5and6 / 100;
            digits3and4 = digits1and2 / 100;
            digits5and6 -= 100 * digits7and8;
            digits1and2 -= 100 * digits3and4;;
            *reinterpret_cast<uint16_t*> (text + 5) = 
                kDigits00To99[digits1and2];
            PRINT_PRINTED
            *reinterpret_cast<uint16_t*> (text + 3) = 
                kDigits00To99[digits3and4];
            PRINT_PRINTED
            *reinterpret_cast<uint16_t*> (text + 1) = 
                kDigits00To99[digits5and6];
            PRINT_PRINTED
            *text = (char)digits7and8 + '0';
            return text + 7;
        }
        else if (value >> 17) {
            comparator = 1000000;
            if (value >= comparator) {
                PRINTF ("\n    Range:[100000, 1048575] length:7")
                if (text + 7 >= text_end) {
                    return nullptr;
                }
                *text++ = '1';
                PRINT_PRINTED
                value -= comparator;
            }
            else {
                PRINTF ("\n    Range:[131072, 999999] length:6")
                if (text + 6 >= text_end) {
                    return nullptr;
                }
            }
            Print6:
            scalar = 10000;
            digits5and6 = (uint16_t)(value / scalar);
            digits1and2 = value - scalar * digits5and6;
            digits7and8 = digits5and6 / 100;
            digits3and4 = digits1and2 / 100;
            digits5and6 -= 100 * digits7and8;
            digits1and2 -= 100 * digits3and4;
            text16 = reinterpret_cast<uint16_t*> (text + 6);
            *reinterpret_cast<uint16_t*> (text + 4) = kDigits00To99[digits1and2];
            PRINT_PRINTED
            *reinterpret_cast<uint16_t*> (text + 2) = kDigits00To99[digits3and4];
            PRINT_PRINTED
            *reinterpret_cast<uint16_t*> (text    ) = kDigits00To99[digits5and6];
            PRINT_PRINTED
            return text + 6;
        }
        else { // (value >> 14)
            if (value >= 100000) {
                PRINTF ("\n    Range:[65536, 131071] length:6")
                goto Print6;
            }
            PRINTF ("\n    Range:[10000, 65535] length:5")
            if (text + 5 >= text_end) {
                return nullptr;
            }
            digits5and6 = 10000;
            digit = (uint8_t)(value / digits5and6);
            value -= digits5and6 * digit;
            *text = digit + '0';
            PRINT_PRINTED
            digits1and2 = (uint16_t)value;
            digits5and6 = 100;
            digits3and4 = digits1and2 / digits5and6;
            digits1and2 -= digits3and4 * digits5and6;
            *reinterpret_cast<uint16_t*> (text + 1) = 
                kDigits00To99[digits3and4];
            PRINT_PRINTED
                PRINTF ("\n    digits1and2:%u", digits1and2)
            *reinterpret_cast<uint16_t*> (text + 3) = 
                kDigits00To99[digits1and2];
            PRINT_PRINTED
            return text + 5;
        }
    }
    digits1and2 = (uint16_t)value;
    if (value >> 10) {
        digits5and6 = 10000;
        if (digits1and2 >= digits5and6) {
            if (text + 5 >= text_end) {
                return nullptr;
            }
            PRINTF ("\n    Range:[10000, 16383] length:5")
            *text++ = '1';
            PRINT_PRINTED
            digits1and2 -= digits5and6;

        }
        else {
            PRINTF ("\n    Range:[1024, 9999] length:4")
            if (text + 4 >= text_end) {
                return nullptr;
            }
        }
        digits5and6 = 100;
        digits3and4 = digits1and2 / digits5and6;
        digits1and2 -= digits3and4 * digits5and6;
        *reinterpret_cast<uint16_t*> (text    ) = kDigits00To99[digits3and4];
        PRINT_PRINTED
        *reinterpret_cast<uint16_t*> (text + 2) = kDigits00To99[digits1and2];
        PRINT_PRINTED
        return text + 4;
    }
    else {
        if (text + 4 >= text_end) {
            return nullptr;
        }
        digits3and4 = 1000;
        if (digits1and2 >= digits3and4) {
            PRINTF ("\n    Range:[1000, 1023] length:4")
            digits1and2 -= digits3and4;
            text16 = reinterpret_cast<uint16_t*> (text + 2);
            *text16-- = kDigits00To99[digits1and2];
            PRINT_PRINTED
            *text16 = (((uint16_t)'1') | (((uint16_t)'0') << 8));
            PRINT_PRINTED
            return text + 4;
        }
        PRINTF ("\n    Range:[100, 999] length:3")
        digits1and2 = (uint16_t)value;
        digits3and4 = 100;
        digit = (char)(digits1and2 / digits3and4);
        digits1and2 -= digit * digits3and4;
        *text = digit + '0';
        PRINT_PRINTED
        *reinterpret_cast<uint16_t*> (text + 1) = kDigits00To99[digits1and2];
        PRINT_PRINTED
        return text + 3;
    }
}

}       //< namespace _
#undef  PRINTF
#undef  PRINT_PRINTED
#endif  //< MAJOR_SEAM >= 1 && MINOR_SEAM >= 1

Author

• Cale McCollough <cale.mccollough@gmail.com>

Answer 9

Here's my little attempt of this fun puzzle.

Instead of using lookup tables, I wanted the compiler to figure it all out. In this case in particular - if you read Hackers' Delight, you see how divide and modulo work -- which makes it very possible to optimize that using SSE/AVX instructions.

Performance benchmark

As for speed, my benchmark here tells me it's 1,5 times faster than the work of Timo (on my Intel Haswell it runs on approximately 1 GB/s).

Things you could consider a cheat

As for the not-making-a-std-string cheat that I use -- of course I took that into consideration for my benchmark of Timo's method as well.

I do use an intrinsic: BSR. If you like, you can also use DeBruijn tables instead - which is one of the things I wrote quite a bit about on my 'fastest 2log' post. Of course, this does have a performance penalty (*well... if you're doing a lot of itoa operations you can actually make a faster BSR but I guess that's not fair...).

The way it works

First thing to do is figure out how much memory we need. This is basically a 10log, which can be implemented in a number of smart ways. See the frequently quoted "Bit Twiddling Hacks" for details.

Next thing to do is to execute the numeric output. I use template recursion for this, so the compiler will figure it out.

I use 'modulo' and 'div' right next to each other. If you read Hacker's Delight, you will notice that the two are closely related, so if you have one answer, you probably have the other as well. I figured the compiler can figure out the details... :-)

The code

Getting the number of digits using a (modified) log10:

struct logarithm
{
    static inline int log2(unsigned int value)
    {
        unsigned long index;
        if (!_BitScanReverse(&index, value))
        {
            return 0;
        }

        // add 1 if x is NOT a power of 2 (to do the ceil)
        return index + (value&(value - 1) ? 1 : 0);
    }

    static inline int numberDigits(unsigned int v)
    {
        static unsigned int const PowersOf10[] =
        { 0, 10, 100, 1000, 10000, 100000, 1000000, 10000000, 100000000, 1000000000 };

        int t = (logarithm::log2(v) + 1) * 1233 >> 12; // (use a lg2 method from above)
        return 1 + t - (v < PowersOf10[t]);
    }
};

Getting yourself the string:

template <int count>
struct WriteHelper
{
    inline static void WriteChar(char* buf, unsigned int value)
    {
        unsigned int div = value / 10;
        unsigned int rem = value % 10;
        buf[count - 1] = rem + '0';

        WriteHelper<count - 1>::WriteChar(buf, div);
    }
};

template <>
struct WriteHelper<1>
{
    inline static void WriteChar(char* buf, unsigned int value) 
    {
        buf[0] = '0' + value;
    }
};

// Boring code that converts a length into a switch.
// TODO: Test if recursion with an 'if' is faster.
static inline void WriteNumber(char* data, int len, unsigned int val) 
{
    switch (len) {
    case 1:
        WriteHelper<1>::WriteChar(data, static_cast<unsigned int>(val));
        break;
    case 2:
        WriteHelper<2>::WriteChar(data, static_cast<unsigned int>(val));
        break;
    case 3:
        WriteHelper<3>::WriteChar(data, static_cast<unsigned int>(val));
        break;
    case 4:
        WriteHelper<4>::WriteChar(data, static_cast<unsigned int>(val));
        break;
    case 5:
        WriteHelper<5>::WriteChar(data, static_cast<unsigned int>(val));
        break;
    case 6:
        WriteHelper<6>::WriteChar(data, static_cast<unsigned int>(val));
        break;
    case 7:
        WriteHelper<7>::WriteChar(data, static_cast<unsigned int>(val));
        break;
    case 8:
        WriteHelper<8>::WriteChar(data, static_cast<unsigned int>(val));
        break;
    case 9:
        WriteHelper<9>::WriteChar(data, static_cast<unsigned int>(val));
        break;
    case 10:
        WriteHelper<10>::WriteChar(data, static_cast<unsigned int>(val));
        break;
    }
}

// The main method you want to call...
static int Write(char* data, int val) 
{
    int len;
    if (val >= 0) 
    {
        len = logarithm::numberDigits(val);
        WriteNumber(data, len, unsigned int(val));
        return len;
    }
    else 
    {
        unsigned int v(-val);
        len = logarithm::numberDigits(v);
        WriteNumber(data+1, len, v);
        data[0] = '-';
        return len + 1;
    }
}

Answer 10

Modification to user434507's solution. Modified to use character array instead of C++ string. Runs a bit faster. Also moved the check for 0 lower in the code...as this never happens for my particular case. Move it back if it's more common for your case.

// Int2Str.cpp : Defines the entry point for the console application.
//
#include <stdio.h>
#include <iostream>
#include "StopWatch.h"

using namespace std;

const char digit_pairs[201] = {
  "00010203040506070809"
  "10111213141516171819"
  "20212223242526272829"
  "30313233343536373839"
  "40414243444546474849"
  "50515253545556575859"
  "60616263646566676869"
  "70717273747576777879"
  "80818283848586878889"
  "90919293949596979899"
};

void itostr(int n, char* c) {
    int sign = -(n<0);
    unsigned int val = (n^sign)-sign;

    int size;
    if(val>=10000) {
        if(val>=10000000) {
            if(val>=1000000000) {
                size=10;
            }
            else if(val>=100000000) {
                size=9;
            }
            else size=8;
        }
        else {
            if(val>=1000000) {
                size=7;
            }
            else if(val>=100000) {
                size=6;
            }
            else size=5;
        }
    }
    else {
        if(val>=100) {
            if(val>=1000) {
                size=4;
            }
            else size=3;
        }
        else {
            if(val>=10) {
                size=2;
            }
            else if(n==0) {
                c[0]='0';
                c[1] = '\0';
                return;
            }
            else size=1;
        }
    }
    size -= sign;
    if(sign)
    *c='-';

    c += size-1;
    while(val>=100) {
        int pos = val % 100;
        val /= 100;
        *(short*)(c-1)=*(short*)(digit_pairs+2*pos); 
        c-=2;
    }
    while(val>0) {
        *c--='0' + (val % 10);
        val /= 10;
    }
    c[size+1] = '\0';
}

void itostr(unsigned val, char* c)
{
    int size;
    if(val>=10000)
    {
        if(val>=10000000)
        {
            if(val>=1000000000)
                size=10;
            else if(val>=100000000)
                size=9;
            else 
                size=8;
        }
        else
        {
            if(val>=1000000)
                size=7;
            else if(val>=100000)
                size=6;
            else
                size=5;
        }
    }
    else 
    {
        if(val>=100)
        {
            if(val>=1000)
                size=4;
            else
                size=3;
        }
        else
        {
            if(val>=10)
                size=2;
            else if (val==0) {
                c[0]='0';
                c[1] = '\0';
                return;
            }
            else
                size=1;
        }
    }

    c += size-1;
    while(val>=100)
    {
       int pos = val % 100;
       val /= 100;
       *(short*)(c-1)=*(short*)(digit_pairs+2*pos); 
       c-=2;
    }
    while(val>0)
    {
        *c--='0' + (val % 10);
        val /= 10;
    }
    c[size+1] = '\0';
}

void test() {
    bool foundmismatch = false;
    char str[16];
    char compare[16];
    for(int i = -1000000; i < 1000000; i++) {
        int random = rand();
        itostr(random, str);
        itoa(random, compare, 10);
        if(strcmp(str, compare) != 0) {
            cout << "Mismatch found: " << endl;
            cout << "Generated: " << str << endl;
            cout << "Reference: " << compare << endl;
            foundmismatch = true;
        }
    }
    if(!foundmismatch) {
        cout << "No mismatch found!" << endl;
    }
    cin.get();
}

void benchmark() {
    StopWatch stopwatch;
    stopwatch.setup("Timer");
    stopwatch.reset();
    stopwatch.start();
    char str[16];
    for(unsigned int i = 0; i < 2000000; i++) {
        itostr(i, str);
    }
    stopwatch.stop();
    cin.get();
}

int main( int argc, const char* argv[]) {
    benchmark();
}

Answer 11

We use the following code (for MSVC):

Templated tBitScanReverse:

#include <intrin.h>

namespace intrin {

#pragma intrinsic(_BitScanReverse)
#pragma intrinsic(_BitScanReverse64)

template<typename TIntegerValue>
__forceinline auto tBitScanReverse(DWORD * out_index, TIntegerValue mask)
    -> std::enable_if_t<(std::is_integral<TIntegerValue>::value && sizeof(TIntegerValue) == 4), unsigned char>
{
    return _BitScanReverse(out_index, mask);
}
template<typename TIntegerValue>
__forceinline auto tBitScanReverse(DWORD * out_index, TIntegerValue mask)
    -> std::enable_if_t<(std::is_integral<TIntegerValue>::value && sizeof(TIntegerValue) == 8), unsigned char>
{
#if !(_M_IA64 || _M_AMD64)
    auto res = _BitScanReverse(out_index, (unsigned long)(mask >> 32));
    if (res) {
        out_index += 32;
        return res;
    }
    return _BitScanReverse(out_index, (unsigned long)mask);
#else
    return _BitScanReverse64(out_index, mask);
#endif
}

}

char/wchar_t helpers:

template<typename TChar> inline constexpr TChar   ascii_0();
template<>               inline constexpr char    ascii_0() { return  '0'; }
template<>               inline constexpr wchar_t ascii_0() { return L'0'; }

template<typename TChar, typename TInt> inline constexpr TChar ascii_DEC(TInt d) { return (TChar)(ascii_0<TChar>() + d); }

Powers of 10 tables:

static uint32 uint32_powers10[] = {
    1,
    10,
    100,
    1000,
    10000,
    100000,
    1000000,
    10000000,
    100000000,
    1000000000
//   123456789
};
static uint64 uint64_powers10[] = {
    1ULL,
    10ULL,
    100ULL,
    1000ULL,
    10000ULL,
    100000ULL,
    1000000ULL,
    10000000ULL,
    100000000ULL,
    1000000000ULL,
    10000000000ULL,
    100000000000ULL,
    1000000000000ULL,
    10000000000000ULL,
    100000000000000ULL,
    1000000000000000ULL,
    10000000000000000ULL,
    100000000000000000ULL,
    1000000000000000000ULL,
    10000000000000000000ULL
//   1234567890123456789
};

template<typename TUint> inline constexpr const TUint  * powers10();
template<>               inline constexpr const uint32 * powers10() { return uint32_powers10; }
template<>               inline constexpr const uint64 * powers10() { return uint64_powers10; }

Actual print:

template<typename TChar, typename TUInt>
__forceinline auto
print_dec(
    TUInt u,
    TChar * & buffer) -> typename std::enable_if_t<std::is_unsigned<TUInt>::value>
{
    if (u < 10) {                                                   // 1-digit, including 0  
        *buffer++ = ascii_DEC<TChar>(u);
    }
    else {
        DWORD log2u;
        intrin::tBitScanReverse(&log2u, u);                         //  log2u [3,31]  (u >= 10)
        DWORD log10u = ((log2u + 1) * 77) >> 8;                     //  log10u [1,9]   77/256 = ln(2) / ln(10)
        DWORD digits = log10u + (u >= powers10<TUInt>()[log10u]);   //  digits [2,10]

        buffer += digits;
        auto p = buffer;

        for (--digits; digits; --digits) {
            auto x = u / 10, d = u - x * 10;
            *--p = ascii_DEC<TChar>(d);
            u = x;
        }
        *--p = ascii_DEC<TChar>(u);
    }
}

Last loop can be unrolled:

switch (digits) {
case 10: { auto x = u / 10, d = u - x * 10; *--p = ascii_DEC<TChar>(d); u = x; }
case  9: { auto x = u / 10, d = u - x * 10; *--p = ascii_DEC<TChar>(d); u = x; }
case  8: { auto x = u / 10, d = u - x * 10; *--p = ascii_DEC<TChar>(d); u = x; }
case  7: { auto x = u / 10, d = u - x * 10; *--p = ascii_DEC<TChar>(d); u = x; }
case  6: { auto x = u / 10, d = u - x * 10; *--p = ascii_DEC<TChar>(d); u = x; }
case  5: { auto x = u / 10, d = u - x * 10; *--p = ascii_DEC<TChar>(d); u = x; }
case  4: { auto x = u / 10, d = u - x * 10; *--p = ascii_DEC<TChar>(d); u = x; }
case  3: { auto x = u / 10, d = u - x * 10; *--p = ascii_DEC<TChar>(d); u = x; }
case  2: { auto x = u / 10, d = u - x * 10; *--p = ascii_DEC<TChar>(d); u = x; *--p = ascii_DEC<TChar>(u); break; }
default: __assume(0);
}

The main idea is the same as @atlaste suggested before: https://stackoverflow.com/a/29039967/2204001

Answer 12

Just came across this because of recent activity; I don't really have time to add benchmarks, but I wanted to add what I wrote in the past for when I need fast integer to string conversion...

https://github.com/CarloWood/ai-utils/blob/master/itoa.h
https://github.com/CarloWood/ai-utils/blob/master/itoa.cxx

The trick used here is the user must provide a std::array that is large enough (on their stack) and that this code writes the string into that backwards, starting at the units, and then returning a pointer into the array with an offset to where the result actually starts.

This therefore doesn't allocate or move memory, but it still requires a division and modulo per result digit (which I believe to be fast enough as that is merely code run internally on the CPU; memory access is usually the problem imho).

Answer 13

Why is nobody using the div function from stdlib when both, quotient and remainder are needed?
Using Timo's source code, I ended up with something like this:

if(val >= 0)
{
    div_t   d2 = div(val,100);
    while(d2.quot)
    {
        COPYPAIR(it,2 * d2.rem);
        it-=2;
        d2 = div(d2.quot,100);
    }
    COPYPAIR(it,2*d2.rem);
    if(d2.quot<10)
        it++;
}
else
{
    div_t   d2 = div(val,100);
    while(d2.quot)
    {
        COPYPAIR(it,-2 * d2.rem);
        it-=2;
        d2 = div(d2.quot,100);
    }
    COPYPAIR(it,-2*d2.rem);
    if(d2.quot<=-10)
        it--;
    *it = '-';
}

Ok, for unsigned int's, the div function can't be used but unsigned's can be handled separate.
I've defined the COPYPAIR macro as follows to test variations how to copy the 2 characters from the digit_pairs (found no obvious advantage of any of these methods):

#define COPYPAIR0(_p,_i) { memcpy((_p), &digit_pairs[(_i)], 2); }
#define COPYPAIR1(_p,_i) { (_p)[0] = digit_pairs[(_i)]; (_p)[1] = digit_pairs[(_i)+1]; }
#define COPYPAIR2(_p,_i) { unsigned short * d = (unsigned short *)(_p); unsigned short * s = (unsigned short *)&digit_pairs[(_i)]; *d = *s; }

#define COPYPAIR COPYPAIR2