为什么memcmp比for循环检查快得多？

Why is memcmp(a, b, size) so much faster than:

为什么memcmp（a，b，size）比以下快得多：

for(i = 0; i < nelements; i++) {
    if a[i] != b[i] return 0;
}
return 1;

Is memcmp a CPU instruction or something? It must be pretty deep because I got a massive speedup using memcmp over the loop.

memcmp是CPU指令还是什么？它必须非常深，因为我在循环中使用memcmp获得了大量的加速。

4 个解决方案

#1

memcmp is often implemented in assembly to take advantage of a number of architecture-specific features, which can make it much faster than a simple loop in C.

memcmp通常在汇编中实现，以利用许多特定于体系结构的特性，这可以使它比C中的简单循环快得多。

As a "builtin"

GCC supports memcmp (as well as a ton of other functions) as builtins. In some versions / configurations of GCC, a call to memcmp will be recognized as __builtin_memcmp. Instead of emitting a call to the memcmp library function, GCC will emit a handful of instructions to act as an optimized inline version of the function.

GCC支持memcmp（以及许多其他功能）作为内置。在GCC的某些版本/配置中，对memcmp的调用将被识别为__builtin_memcmp。 GCC不会发出对memcmp库函数的调用，而是发出一些指令作为函数的优化内联版本。

On x86, this leverages the use of the cmpsb instruction, which compares a string of bytes at one memory location to another. This is coupled with the repe prefix, so the strings are compared until they are no longer equal, or a count is exhausted. (Exactly what memcmp does).

在x86上，这利用了cmpsb指令，该指令将一个内存位置的字节串与另一个内存位置进行比较。这与重复前缀相结合，因此比较字符串直到它们不再相等，或者计数耗尽。 （正是memcmp的作用）。

Given the following code:

给出以下代码：

int test(const void* s1, const void* s2, int count)
{
    return memcmp(s1, s2, count) == 0;
}

gcc version 3.4.4 on Cygwin generates the following assembly:

Cygwin上的gcc版本3.4.4生成以下程序集：

; (prologue)
mov     esi, [ebp+arg_0]    ; Move first pointer to esi
mov     edi, [ebp+arg_4]    ; Move second pointer to edi
mov     ecx, [ebp+arg_8]    ; Move length to ecx

cld                         ; Clear DF, the direction flag, so comparisons happen
                            ; at increasing addresses
cmp     ecx, ecx            ; Special case: If length parameter to memcmp is
                            ; zero, don't compare any bytes.
repe cmpsb                  ; Compare bytes at DS:ESI and ES:EDI, setting flags
                            ; Repeat this while equal ZF is set
setz    al                  ; Set al (return value) to 1 if ZF is still set
                            ; (all bytes were equal).
; (epilogue) 

Reference:

参考：

• cmpsb instruction
• cmpsb指令

As a library function

Highly-optimized versions of memcmp exist in many C standard libraries. These will usually take advantage of architecture-specific instructions to work with lots of data in parallel.

许多C标准库中都存在高度优化的memcmp版本。这些通常会利用特定于体系结构的指令来并行处理大量数据。

In Glibc, there are versions of memcmp for x86_64 that can take advantage of the following instruction set extensions:

在Glibc中，有x86_64的memcmp版本可以利用以下指令集扩展：

• SSE2 - sysdeps/x86_64/memcmp.S
• SSE2 - sysdeps / x86_64 / memcmp.S
• SSE4 - sysdeps/x86_64/multiarch/memcmp-sse4.S
• SSE4 - sysdeps / x86_64 / multiarch / memcmp-sse4.S
• SSSE3 - sysdeps/x86_64/multiarch/memcmp-ssse3.S
• SSSE3 - sysdeps / x86_64 / multiarch / memcmp-ssse3.S

The cool part is that glibc will detect (at run-time) the newest instruction set your CPU has, and execute the version optimized for it. See this snippet from sysdeps/x86_64/multiarch/memcmp.S:

很酷的部分是glibc将检测（在运行时）CPU的最新指令集，并执行为其优化的版本。请参阅sysdeps / x86_64 / multiarch / memcmp.S中的以下代码段：

ENTRY(memcmp)
    .type   memcmp, @gnu_indirect_function
    LOAD_RTLD_GLOBAL_RO_RDX
    HAS_CPU_FEATURE (SSSE3)
    jnz 2f
    leaq    __memcmp_sse2(%rip), %rax
    ret 

2:  HAS_CPU_FEATURE (SSE4_1)
    jz  3f  
    leaq    __memcmp_sse4_1(%rip), %rax
    ret 

3:  leaq    __memcmp_ssse3(%rip), %rax
    ret 

END(memcmp)

In the Linux kernel

Linux does not seem to have an optimized version of memcmp for x86_64, but it does for memcpy, in arch/x86/lib/memcpy_64.S. Note that is uses the alternatives infrastructure (arch/x86/kernel/alternative.c) for not only deciding at runtime which version to use, but actually patching itself to only make this decision once at boot-up.

Linux似乎没有针对x86_64的memcmp的优化版本，但是对于memcpy，它在arch / x86 / lib / memcpy_64.S中。请注意，它使用备用基础结构（arch / x86 / kernel / alternative.c），不仅在运行时决定使用哪个版本，而且实际上修补自身仅在启动时做出此决定。

#2

It's usually a compiler intrinsic that is translated into fast assembly with specialized instructions for comparing blocks of memory.

它通常是一个编译器内在函数，它被转换为快速汇编，并带有用于比较内存块的专用指令。

intrinsic memcmp

内在的memcmp

#3

Is memcmp a CPU instruction or something?

memcmp是CPU指令还是什么？

It is at least a very highly optimized compiler-provided intrinsic function. Possibly a single machine instruction, or two, depending on the platform, which you haven't specified.

它至少是一个非常高度优化的编译器提供的内部函数。可能是一台或两台机器指令，具体取决于您未指定的平台。

#4

Yes, on intel hardware, there's a single assembly instruction for such a loop. The runtime will use that. (I don't exactly remember, it was something like rep cmps[b|w], depending also on the datasize)

是的，在intel硬件上，有一个用于这种循环的汇编指令。运行时将使用它。 （我不记得，它有点像rep cmps [b | w]，还取决于数据量）