kev*_*lbs 23 c++ string performance
我一直在尝试strcmp在某些条件下提高性能.但是,遗憾的是,我甚至无法实现普通的vanilla strcmp以及库实现.
我看到了一个类似的问题,但答案说差异来自编译器优化掉字符串文字的比较.我的测试不使用字符串文字.
这是实现(comparisons.cpp)
int strcmp_custom(const char* a, const char* b) {
while (*b == *a) {
if (*a == '\0') return 0;
a++;
b++;
}
return *b - *a;
}
这是测试驱动程序(driver.cpp):
#include "comparisons.h"
#include <array>
#include <chrono>
#include <iostream>
void init_string(char* str, int nChars) {
// 10% of strings will be equal, and 90% of strings will have one char different.
// This way, many strings will share long prefixes so strcmp has to exercise a bit.
// Using random strings still shows the custom implementation as slower (just less so).
str[nChars - 1] = '\0';
for (int i = 0; i < nChars - 1; i++)
str[i] = (i % 94) + 32;
if (rand() % 10 != 0)
str[rand() % (nChars - 1)] = 'x';
}
int main(int argc, char** argv) {
srand(1234);
// Pre-generate some strings to compare.
const int kSampleSize = 100;
std::array<char[1024], kSampleSize> strings;
for (int i = 0; i < kSampleSize; i++)
init_string(strings[i], kSampleSize);
auto start = std::chrono::high_resolution_clock::now();
for (int i = 0; i < kSampleSize; i++)
for (int j = 0; j < kSampleSize; j++)
strcmp(strings[i], strings[j]);
auto end = std::chrono::high_resolution_clock::now();
std::cout << "strcmp - " << (end - start).count() << std::endl;
start = std::chrono::high_resolution_clock::now();
for (int i = 0; i < kSampleSize; i++)
for (int j = 0; j < kSampleSize; j++)
strcmp_custom(strings[i], strings[j]);
end = std::chrono::high_resolution_clock::now();
std::cout << "strcmp_custom - " << (end - start).count() << std::endl;
}
我的makefile:
CC=clang++
test: driver.o comparisons.o
$(CC) -o test driver.o comparisons.o
# Compile the test driver with optimizations off.
driver.o: driver.cpp comparisons.h
$(CC) -c -o driver.o -std=c++11 -O0 driver.cpp
# Compile the code being tested separately with optimizations on.
comparisons.o: comparisons.cpp comparisons.h
$(CC) -c -o comparisons.o -std=c++11 -O3 comparisons.cpp
clean:
rm comparisons.o driver.o test
根据这个答案的建议,我在一个单独的编译单元中编译了我的比较函数并进行了优化,并在关闭优化的情况下编译了驱动程序,但我仍然得到了大约5倍的减速.
strcmp - 154519
strcmp_custom - 506282
我也试过复制FreeBSD实现但得到了类似的结果.
我想知道我的表现测量是否忽略了某些东西.或者标准库实现做得更漂亮？
zwo*_*wol 38
我不知道你有哪个标准库,只是为了让你了解C库维护者对优化字符串原语有多么认真,GNU libc在x86-64上使用的默认值 strcmp是两千行手工优化组装语言,版本2.24.当SSSE3和SSE4.2指令集扩展可用时,有单独的,也是手动优化的版本.(该文件的复杂程度似乎是因为相同的源代码用于生成其他几个函数;机器代码最终只是"1120"指令.)2.24大约一年前发布,甚至更多从那时起,工作就开始了.
他们去到这么多的麻烦,因为它很常见的字符串原语的一个是在配置文件中的一个最热门的功能.
摘自我的glibcv2.2.5,x86_64 linux的反汇编:
0000000000089cd0 <strcmp@@GLIBC_2.2.5>:
89cd0: 48 8b 15 99 a1 33 00 mov 0x33a199(%rip),%rdx # 3c3e70 <_IO_file_jumps@@GLIBC_2.2.5+0x790>
89cd7: 48 8d 05 92 58 01 00 lea 0x15892(%rip),%rax # 9f570 <strerror_l@@GLIBC_2.6+0x200>
89cde: f7 82 b0 00 00 00 10 testl $0x10,0xb0(%rdx)
89ce5: 00 00 00
89ce8: 75 1a jne 89d04 <strcmp@@GLIBC_2.2.5+0x34>
89cea: 48 8d 05 9f 48 0c 00 lea 0xc489f(%rip),%rax # 14e590 <__nss_passwd_lookup@@GLIBC_2.2.5+0x9c30>
89cf1: f7 82 80 00 00 00 00 testl $0x200,0x80(%rdx)
89cf8: 02 00 00
89cfb: 75 07 jne 89d04 <strcmp@@GLIBC_2.2.5+0x34>
89cfd: 48 8d 05 0c 00 00 00 lea 0xc(%rip),%rax # 89d10 <strcmp@@GLIBC_2.2.5+0x40>
89d04: c3 retq
89d05: 90 nop
89d06: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1)
89d0d: 00 00 00
89d10: 89 f1 mov %esi,%ecx
89d12: 89 f8 mov %edi,%eax
89d14: 48 83 e1 3f and $0x3f,%rcx
89d18: 48 83 e0 3f and $0x3f,%rax
89d1c: 83 f9 30 cmp $0x30,%ecx
89d1f: 77 3f ja 89d60 <strcmp@@GLIBC_2.2.5+0x90>
89d21: 83 f8 30 cmp $0x30,%eax
89d24: 77 3a ja 89d60 <strcmp@@GLIBC_2.2.5+0x90>
89d26: 66 0f 12 0f movlpd (%rdi),%xmm1
89d2a: 66 0f 12 16 movlpd (%rsi),%xmm2
89d2e: 66 0f 16 4f 08 movhpd 0x8(%rdi),%xmm1
89d33: 66 0f 16 56 08 movhpd 0x8(%rsi),%xmm2
89d38: 66 0f ef c0 pxor %xmm0,%xmm0
89d3c: 66 0f 74 c1 pcmpeqb %xmm1,%xmm0
89d40: 66 0f 74 ca pcmpeqb %xmm2,%xmm1
89d44: 66 0f f8 c8 psubb %xmm0,%xmm1
89d48: 66 0f d7 d1 pmovmskb %xmm1,%edx
89d4c: 81 ea ff ff 00 00 sub $0xffff,%edx
...
真实的是1183行汇编,在检测系统功能和矢量化指令方面有很多潜在的聪明才智.libc维护者知道他们可以通过优化应用程序数千次调用的一些函数来获得优势.
为了比较,您的版本在-O3:
comparisons.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <_Z13strcmp_customPKcS0_>:
int strcmp_custom(const char* a, const char* b) {
while (*b == *a) {
0: 8a 0e mov (%rsi),%cl
2: 8a 07 mov (%rdi),%al
4: 38 c1 cmp %al,%cl
6: 75 1e jne 26 <_Z13strcmp_customPKcS0_+0x26>
if (*a == '\0') return 0;
8: 48 ff c6 inc %rsi
b: 48 ff c7 inc %rdi
e: 66 90 xchg %ax,%ax
10: 31 c0 xor %eax,%eax
12: 84 c9 test %cl,%cl
14: 74 18 je 2e <_Z13strcmp_customPKcS0_+0x2e>
int strcmp_custom(const char* a, const char* b) {
while (*b == *a) {
16: 0f b6 0e movzbl (%rsi),%ecx
19: 0f b6 07 movzbl (%rdi),%eax
1c: 48 ff c6 inc %rsi
1f: 48 ff c7 inc %rdi
22: 38 c1 cmp %al,%cl
24: 74 ea je 10 <_Z13strcmp_customPKcS0_+0x10>
26: 0f be d0 movsbl %al,%edx
29: 0f be c1 movsbl %cl,%eax
if (*a == '\0') return 0;
a++;
b++;
}
return *b - *a;
2c: 29 d0 sub %edx,%eax
}
2e: c3 retq