6

According to the gcc docs, memcmp is not an intrinsic function of GCC. If you wanted to speed up glibc's memcmp under gcc, you would need to use the lower level intrinsics defined in the docs. However, when searching around the internet, it seems that many people have the impression that memcmp is a builtin function. Is it for some compilers and not for others?

Justin
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    Check out memcmp from glibc too. The relationship between GCC and glibc is quite complicated, with both providing different versions of the same functions and then sometimes fighting it out in header files whose definition will get in fact used in user programs. – Laurynas Biveinis May 13 '09 at 19:31

3 Answers3

9

Note that the repz cmpsb routine might not be faster than glibc's memcmp. In my tests, in fact, it's never faster, even when comparing just a few bytes.

See http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43052

Justin L.
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    +1 for a great link! I was just searching for an answer myself as to why the libc memcmp() performed orders of magnitude faster than a simple `repz cmpsb`. I guessed it had something to do with alignment, now I know:) – Unsigned Sep 06 '11 at 22:34
  • Truly, on 4.8 `memcmp` compiles to a `jmp memcmp` with `-O3`, not straight assembly. Can anyone summarize how can the glibc be faster? Something to do with alignment? – Ciro Santilli OurBigBook.com Apr 24 '15 at 20:35
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    @CiroSantilli六四事件法轮功包卓轩 A typical "fast `memcmp`" compares byte-for-byte until an alignment boundary is reached, then compares word-for-word (typically the native word size, 64-bit on x86-64), then if any less-than-word-size bytes remain they are compared byte-for-byte until finished. Special-case code for quickly checking small sizes are also common. See `glibc` source: https://sourceware.org/git/?p=glibc.git;a=blob;f=string/memcmp.c – Jody Bruchon Jan 03 '16 at 05:48
5

Your link appears to be for the x86 architecture-specific built-in functions, according to this memcmp is implemented as an architecture-independent built-in by gcc.

Edit:

Compiling the following code with Cygwin gcc version 3.3.1 for i686, -O2:

#include <stdlib.h>

struct foo {
    int a;
    int b;
} ;

int func(struct foo *x, struct foo *y)
{
    return memcmp(x, y, sizeof (struct foo));
}

Produces the following output (note that the call to memcmp() is converted to an 8-byte "repz cmpsb"):

   0:   55                      push   %ebp
   1:   b9 08 00 00 00          mov    $0x8,%ecx
   6:   89 e5                   mov    %esp,%ebp
   8:   fc                      cld    
   9:   83 ec 08                sub    $0x8,%esp
   c:   89 34 24                mov    %esi,(%esp)
   f:   8b 75 08                mov    0x8(%ebp),%esi
  12:   89 7c 24 04             mov    %edi,0x4(%esp)
  16:   8b 7d 0c                mov    0xc(%ebp),%edi
  19:   f3 a6                   repz cmpsb %es:(%edi),%ds:(%esi)
  1b:   0f 92 c0                setb   %al
  1e:   8b 34 24                mov    (%esp),%esi
  21:   8b 7c 24 04             mov    0x4(%esp),%edi
  25:   0f 97 c2                seta   %dl
  28:   89 ec                   mov    %ebp,%esp
  2a:   5d                      pop    %ebp
  2b:   28 c2                   sub    %al,%dl
  2d:   0f be c2                movsbl %dl,%eax
  30:   c3                      ret    
  31:   90                      nop
Lance Richardson
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0

Now in 2017, GCC and Clang seems to have some optimizations for buffers of sizes 1, 2, 4, 8 and some others, for example 3, 5 and multiple of 8.

Nick
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