Fast way to replace elements in array - C

Question

Let's say we have an array of ints like this:

const int size = 100000;
int array[size];
//set some items to 0 and other items to 1

I'd like to replace all items that have value of 1 with another value, for example 123456. This can be trivially implemented with:

for(int i = 0; i < size ; i++){
    if(array[i] != 0) 
        array[i] = 123456;
}

Out of curiosity, is there a faster way to do this, by some kind of x86 trickery, or is this the best code for the processor?

Answer 1

For your specific case where you initially have 0 and 1, the following might be faster. You'll have to bench mark it. You probably can't do much better with plain C though; you may need to dive into assembly if you want to take advantage of "x86 trickery" that may exist.

for(int i = 0; i < size ; i++){
  array[i] *= 123456;
}

EDIT:

Benchmark code:

#include <time.h>
#include <stdlib.h>
#include <stdio.h>

size_t diff(struct timespec *start, struct timespec *end)
{
  return (end->tv_sec - start->tv_sec)*1000000000 + end->tv_nsec - start->tv_nsec;
}

int main(void)
{
  const size_t size = 1000000;
  int array[size];

  for(size_t i=0; i<size; ++i) {
    array[i] = rand() & 1;
  }

  struct timespec start, stop;

  clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &start);
  for(size_t i=0; i<size; ++i) {
    array[i] *= 123456;
    //if(array[i]) array[i] = 123456;
  }
  clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &stop);

  printf("size: %zu\t nsec: %09zu\n", size, diff(&start, &stop));
}

---

my results:

Computer: quad core AMD Phenom @2.5GHz, Linux, GCC 4.7, compiled with

$ gcc arr.c -std=gnu99 -lrt -O3 -march=native

• if version: ~5-10ms
• *= version: ~1.3ms

Answer 2

For a small array such as your it's no use trying to find another algorithm, and if the values are not in a specific pattern a simple loop is the only way to do it anyway.

However, if you have a very large array (we're talking several million entries), then you can split the work into threads. Each separate thread handles a smaller portion of the whole data set.

Answer 3

You might want to benchmark this as well:

for(int i = 0; i < size ; i++){
  array[i] = (~(array[i]-1) & 123456);
}

I run it through same benchmark as SchighSchagh, with little or no difference on my set up. It may differ on yours, however.

EDIT: Stop the presses!

I just remembered that x86 can "unbranch" ternary operators if arguments between ":" are constants. Consider following code:

for(size_t i=0; i<size; ++i) {
    array[i] = array[i] ? 123456 : 0;
}

Looks almost like your original code, doesn't it? Well, disassembly shows that it has been compiled without any branches:

  for(size_t i=0; i<size; ++i) {
00E3104C  xor         eax,eax  
00E3104E  mov         edi,edi  
        array[i] = array[i] ? 123456 : 0;
00E31050  mov         edx,dword ptr [esi+eax*4]  
00E31053  neg         edx  
00E31055  sbb         edx,edx  
00E31057  and         edx,1E240h  
00E3105D  mov         dword ptr [esi+eax*4],edx  
00E31060  inc         eax  
00E31061  cmp         eax,5F5E100h  
00E31066  jb          wmain+50h (0E31050h)  
    }

Performance-wise it seems on par or little better than my original and SchighSchagh solution. It is more readable and more flexible, though. For instance, it can work with array[i] having values different than 0 and 1.

Bottom line, benchmark AND take a peek in the disassembly.

Answer 4

The array is small enough that it fits in cache, so it should be worthwhile to use SIMD: (not tested)

  mov ecx, size
  lea esi, [array + ecx * 4]
  neg ecx
  pxor xmm0, xmm0
  movdqa xmm1, [_vec4_123456]  ; value of { 123456, 123456, 123456, 123456 }
_replaceloop:
  movdqa xmm2, [esi + ecx * 4] ; assumes the array is 16 aligned, make that true
  add ecx, 4
  pcmpeqd xmm2, xmm0
  pandn xmm2, xmm1
  movdqa [esi + ecx * 4 - 16], xmm2
  jnz _replaceloop

Unrolling by 2 might help.

If you have SSE4.1, you can use SchighSchagh's multiplication trick with pmulld.

Answer 5

Here's some Win32 code to profile various versions of the algorithm (compiled using VS2010 Express using default release build):-

#include <windows.h>
#include <stdlib.h>
#include <stdio.h>

const size_t
  size = 0x1D4C00;

_declspec(align(16)) int
  g_array [size];

_declspec(align(16)) int
  _vec4_123456 [] = { 123456, 123456, 123456, 123456 };

void Test (void (*fn) (size_t, int *), char *test)
{
  printf ("Executing test: %s\t", test);

  for(size_t i=0; i<size; ++i) {
    g_array[i] = rand() & 1;
  }

  LARGE_INTEGER
    start,
    end;

  QueryPerformanceCounter (&start);

  fn (size, g_array);

  QueryPerformanceCounter (&end);

  printf("size: %u\t count: %09u\n", size, (int) (end.QuadPart - start.QuadPart));
}

void Test1 (size_t size, int *array)
{
  for(size_t i=0; i<size; ++i) {
    array[i] *= 123456;
  }
}

void Test2 (size_t size, int *array)
{
  for(size_t i=0; i<size; ++i) {
    if(array[i]) array[i] = 123456;
  }
}

void Test3 (size_t array_size, int *array)
{
  __asm
  {
    mov edi,array
    mov ecx, array_size 
    lea esi, [edi + ecx * 4]
    neg ecx
    pxor xmm0, xmm0
    movdqa xmm1, [_vec4_123456]  ; value of { 123456, 123456, 123456, 123456 }
_replaceloop:
    movdqa xmm2, [esi + ecx * 4] ; assumes the array is 16 aligned, make that true
    add ecx, 4
    pcmpeqd xmm2, xmm0
    pandn xmm2, xmm1
    movdqa [esi + ecx * 4 - 16], xmm2
    jnz _replaceloop
  }
}

void Test4 (size_t array_size, int *array)
{
  array_size = array_size * 8 / 12;

  __asm
  {
        mov edi,array
        mov ecx,array_size
        lea esi,[edi+ecx*4]
                                      lea edi,[edi+ecx*4]
        neg ecx
                                      mov edx,[_vec4_123456]
        pxor xmm0,xmm0
        movdqa xmm1,[_vec4_123456]
replaceloop:
        movdqa xmm2,[esi+ecx*4]
                                      mov eax,[edi]
                                      mov ebx,[edi+4]
        movdqa xmm3,[esi+ecx*4+16]
                                      add edi,16
        add ecx,9
                                      imul eax,edx    
        pcmpeqd xmm2,xmm0
                                      imul ebx,edx
        pcmpeqd xmm3,xmm0
                                      mov [edi-16],eax
                                      mov [edi-12],ebx
        pandn xmm2,xmm1
                                      mov eax,[edi-8]
                                      mov ebx,[edi-4]
        pandn xmm3,xmm1
                                      imul eax,edx    
        movdqa [esi+ecx*4-36],xmm2
                                      imul ebx,edx
        movdqa [esi+ecx*4-20],xmm3
                                      mov [edi-8],eax
                                      mov [edi-4],ebx
        loop replaceloop
  }
}

int main()
{
    Test (Test1, "Test1 - mul");
    Test (Test2, "Test2 - branch");
    Test (Test3, "Test3 - simd");
    Test (Test4, "Test4 - simdv2");
}

It's got for tests: C using an if()..., C using a multiply, harold's simd version and my simd version.

Running it many times (remember, when profiling you should average the results over several runs) there's little difference between all the versions except the branching one which is significantly slower.

This is not very surprising as the algortihm is doing very little work for each memory item. What this means is that the real limiting factor is the bandwidth between the CPU and the memory, the CPU is constantly waiting for the memory to catch up, even with the cpu helping with prefetching the data (ia32's detect and prefetch data linearly).

Answer 6

This might prove faster.

for(int i = 0; i < size ; i++){
  array[i] = ((123456 << array[i]) - 123456);
}

EDIT: Changed bitwise operation to left shift.

Answer 7

You could use another array or some other data structure to keep track of the indices of the elements you set to one and then only visit those elements. This will work best if there are only few elements that are set to one

Answer 8

one more way to speed up the array assignment you can use the c inline assembly. Like below,

#include<stdio.h>
#include<string.h>
#include<stdlib.h>

const int size = 100000; 
void main(void) {
  int array[size];
  int value = 1000;

  __asm__ __volatile__("cld\n\t"
          "rep\n\t"
          "stosl\n\t"
          :
          :"c"(size*4), "a"(value), "D"(array)
          :
         );

  printf("Array[0] : %d \n", array[0]);
}

This should be speed when we compared to plain c program to assign the array values. And also the stosl instruction take 4 clock cycle.