为什么 memcmp 比 for 循环检查快这么多? [英] Why is memcmp so much faster than a for loop check?
问题描述
为什么 memcmp(a, b, size) 比:
for(i = 0; i < nelements; i++) {
if a[i] != b[i] return 0;
}
return 1;
memcmp 是 CPU 指令还是什么?它一定很深，因为我在循环中使用 memcmp 获得了巨大的加速.
Is memcmp a CPU instruction or something? It must be pretty deep because I got a massive speedup using memcmp over the loop.
推荐答案
memcmp 通常在汇编中实现以利用许多特定于体系结构的功能，这可以使它很多 比 C 中的简单循环快.
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.
GCC 支持 memcmp(以及大量其他功能)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 不会向memcmp 库函数发出call，而是会发出一些指令来充当该函数的优化内联版本.
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.
在 x86 上，这利用了 cmpsb 指令的使用，该指令将一个内存位置的字节串与另一个进行比较.这与 repe 前缀相结合，因此将比较字符串，直到它们不再相等，或者计数用完为止.(正是 memcmp 所做的).
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).
给定以下代码:
int test(const void* s1, const void* s2, int count)
{
return memcmp(s1, s2, count) == 0;
}
Cygwin 上的
gcc version 3.4.4 生成以下程序集:
gcc version 3.4.4 on Cygwin generates the following assembly:
; (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)
参考:
memcmp 的高度优化版本存在于许多 C 标准库中.这些通常会利用特定于架构的指令并行处理大量数据.
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.
在 Glibc 中，有多个版本的 memcmp for x86_64 可以利用以下指令集扩展:
In Glibc, there are versions of memcmp for x86_64 that can take advantage of the following instruction set extensions:
- SSE2 -
sysdeps/x86_64/memcmp.S - SSE4 -
sysdeps/x86_64/multiarch/memcmp-sse4.S - SSSE3 -
sysdeps/x86_64/multiarch/memcmp-ssse3.S一个>
很酷的部分是 glibc 将检测(在运行时)您的 CPU 具有的最新指令集，并执行为其优化的版本.请参阅 sysdeps/x86_64/multiarch/memcmp.S:
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:
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)
在 Linux 内核中
Linux 似乎没有针对 x86_64 的 memcmp 优化版本，但它有针对 memcpy 的优化版本，位于 arch/x86/lib/memcpy_64.S.请注意，它使用了alternatives 基础结构(arch/x86/kernel/alternative.c) 不仅在运行时决定使用哪个版本，而且实际上修补自己只做出这个决定启动时一次.
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.
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