C 计时地震III黑客只有在使用优化编译时才有效
所以我刚刚发现了非常有趣的Quake III平方根逆黑客。在了解了它的工作原理之后,我决定测试它。我发现,在启用优化的情况下编译时,hack的性能仅优于math.h1/sqrt(X) 黑客的实施:C 计时地震III黑客只有在使用优化编译时才有效,c,sqrt,quake,C,Sqrt,Quake,所以我刚刚发现了非常有趣的Quake III平方根逆黑客。在了解了它的工作原理之后,我决定测试它。我发现,在启用优化的情况下编译时,hack的性能仅优于math.h1/sqrt(X) 黑客的实施: float q_sqrt(float x) { float x2 = x * 0.5F; int i = *( int* )&x; // evil floating point bit hack i = 0x5f3759df - (i
float q_sqrt(float x) {
float x2 = x * 0.5F;
int i = *( int* )&x; // evil floating point bit hack
i = 0x5f3759df - (i >> 1); // what the fuck?
x = *( float* )&i;
x = x * ( 1.5F - ( (x2 * x * x) ) ); //1st iteration
//y = y * ( 1.5F - ( (x2 * y * y) ) ); //2nd iteration, can be removed
return x;
}
要测试1/sqrt(x)相对于q_sqrt(x)的运行速度:
我的期望是q_sqrt(x)将比开箱即用的1/sqrt(x)工作得更好。在阅读了更多内容后,我现在知道要么libm优化得更好,要么我的CPU配备了sqrt(X)的硬件解决方案。毕竟,自从快速反向根黑客的发展以来,CPU发生了突飞猛进的变化
我不明白的是,编译器将应用哪种类型的优化,以使其更快。当然,也许我的基准设计不周
谢谢你的帮助 正如您所说,大多数现代CPU都包含浮点单元,它通常提供硬件指令来计算平方根。FPU还提供除法指令,所以我希望您的处理器(尽管我不知道)能够在几个汇编指令中计算出一个逆sqrt。您的结果有点令人惊讶:您应该检查FPU是否真的被使用了。我不知道Ryzen,但在ARM处理器上,您可以编译软件以使用硬件浮点指令或软件库
现在回答您的问题:GCC优化是一个复杂的问题,通常不可能精确预测给定级别对性能的影响。因此,请像您所做的那样运行一些测试,或者看看理论。关于CLang/LLVM的具体区别在于这些 无优化(-O0): 使用优化(-Ofast): 您可以使用检查编译器的程序集输出,使用各种不同的标志,并检查它如何影响输出
//qtest.c
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <time.h>
/*
Implementation of 1/sqrt(x) used in tue quake III game
*/
float q_sqrt(float x) {
float x2 = x * 0.5F;
int i = *( int* )&x; // evil floating point bit hack
i = 0x5f3759df - (i >> 1); // what the fuck?
x = *( float* )&i;
x = x * ( 1.5F - ( (x2 * x * x) ) ); //1st iteration
//y = y * ( 1.5F - ( (x2 * y * y) ) ); //2nd iteration, can be removed
return x;
}
int main(int argc, char *argv[]) {
struct timespec start, stop;
//Will work on floats in the range [0,100]
float maxn = 100;
//Work on 10000 random floats or as many as user provides
size_t num = 10000;
//Bogus
float ans = 0;
//Measure nanoseconds
size_t ns = 0;
if (argc > 1)
num = atoll(argv[1]);
if (num <= 0) return -1;
//Compute "num" random floats
float *vecs = malloc(num * sizeof(float));
if (!vecs) return -1;
for (int i = 0; i < num; i++)
vecs[i] = maxn * ( (float)rand() / (float)RAND_MAX );
fprintf(stderr, "Measuring 1/sqrt(x)\n");
clock_gettime( CLOCK_REALTIME, &start);
for (size_t i = 0; i < num; i++)
ans += 1 / sqrt(vecs[i]);
clock_gettime( CLOCK_REALTIME, &stop);
ns = ( stop.tv_sec - start.tv_sec ) * 1E9 + ( stop.tv_nsec - start.tv_nsec );
fprintf(stderr, "1/sqrt(x) took %.6f nanosecods\n", (double)ns/num );
fprintf(stderr, "Measuring q_sqrt(x)\n");
clock_gettime( CLOCK_REALTIME, &start);
for (size_t i = 0; i < num; i++)
ans += q_sqrt(vecs[i]);
clock_gettime( CLOCK_REALTIME, &stop);
ns = ( stop.tv_sec - start.tv_sec ) * 1E9 + ( stop.tv_nsec - start.tv_nsec );
fprintf(stderr, "q_sqrt(x) took %.6f nanosecods\n", (double)ns/num );
//Side by side
//for (size_t i = 0; i < num; i++)
// fprintf(stdout, "%.6f\t%.6f\n", 1/sqrt(vecs[i]),q_sqrt(vecs[i]));
free(vecs);
}
gcc -Wall -pedantic -o qtest qtest.c -lm
./qtest
Measuring 1/sqrt(x)
1/sqrt(x) took 4.470000 nanosecods
Measuring q_sqrt(x)
q_sqrt(x) took 4.859000 nanosecods
gcc -Wall -pedantic -O1 -o qtest qtest.c -lm
./qtest
Measuring 1/sqrt(x)
1/sqrt(x) took 0.378000 nanosecods
Measuring q_sqrt(x)
q_sqrt(x) took 0.497000 nanosecods
gcc -Wall -pedantic -O2 -o qtest qtest.c -lm
qtest.c: In function ‘q_sqrt’:
qtest.c:11:14: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
11 | int i = *( int* )&x; // evil floating point bit hack
|
qtest.c:13:10: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
13 | x = *( float* )&i;
|
./qtest
Measuring 1/sqrt(x)
1/sqrt(x) took 0.500000 nanosecods
Measuring q_sqrt(x)
q_sqrt(x) took 0.002000 nanosecods
q_sqrt(float): # @q_sqrt(float)
push rbp
mov rbp, rsp
movss dword ptr [rbp - 4], xmm0
movss xmm0, dword ptr [rip + .LCPI0_1] # xmm0 = mem[0],zero,zero,zero
mulss xmm0, dword ptr [rbp - 4]
movss dword ptr [rbp - 8], xmm0
mov eax, dword ptr [rbp - 4]
mov dword ptr [rbp - 12], eax
mov ecx, dword ptr [rbp - 12]
sar ecx, 1
mov eax, 1597463007
sub eax, ecx
mov dword ptr [rbp - 12], eax
movss xmm0, dword ptr [rbp - 12] # xmm0 = mem[0],zero,zero,zero
movss dword ptr [rbp - 4], xmm0
movss xmm0, dword ptr [rbp - 4] # xmm0 = mem[0],zero,zero,zero
movss xmm2, dword ptr [rbp - 8] # xmm2 = mem[0],zero,zero,zero
mulss xmm2, dword ptr [rbp - 4]
mulss xmm2, dword ptr [rbp - 4]
movss xmm1, dword ptr [rip + .LCPI0_0] # xmm1 = mem[0],zero,zero,zero
subss xmm1, xmm2
mulss xmm0, xmm1
movss dword ptr [rbp - 4], xmm0
movss xmm0, dword ptr [rbp - 4] # xmm0 = mem[0],zero,zero,zero
pop rbp
ret
q_sqrt(float): # @q_sqrt(float)
movd eax, xmm0
sar eax
mov ecx, 1597463007
sub ecx, eax
movd xmm1, ecx
mulss xmm0, dword ptr [rip + .LCPI0_0]
movdqa xmm2, xmm1
mulss xmm2, xmm1
mulss xmm0, xmm2
addss xmm0, dword ptr [rip + .LCPI0_1]
mulss xmm0, xmm1
ret