pmeerw's blog

Sun, 28 Mar 2010

Floating-point performance: Compiler comparison and speedup

Trying different compilers for double precision math...

Conclusion: icc is tremendously faster than gcc on log, exp, pow, gcc is doing better on erfc, erf, gamma -- the accuracy might be different though. For comparison, the 64bit machine is a bit faster (3 GHz) than the 32bit one (2.67 GHz).

The Intel compiler is always using SSE instructions and has reasonable good SSE implementations for log, exp, pow; the GNU compiler struggles with FPU instructions and does not have double-precision SSE code, some operations get slower with SSE, e.g. floor, tan, log. It is hard to get good overall performance with gcc for 32bit, 64bit fares better.

To speed up log and exp, one can use approximate math functions (at single precision). Shigeo MITSUNARI offers fmath which implements log and exp in SSE with table lookups. It also can benefit from the Xbyak JIT compiler.

José Fonseca has a blog entry Fast SSE2 pow: tables or polynomials? which also looks interesting.


icc gcc-4.2 gcc-4.2 (gcc-4.4) gcc-4.4
float->double 150 120 120 110 70
double->float 110 90 90 90 100
erf 5020 1960 1990 1960 1080
erfc 9600 2310 2220 2220 1270
gamma 12040 6340 6340 6340 6570
tgamma 9050 12660 12570 12890 9920
log 1900 2340 4390 4430 4440
fmath::log 560 1170 490 590 450
exp 1290 5290 6280 6380 2870
fmath::exp 640 600 650 660 620
pow 3660 11980 11820 11680 9210
fmath::pow 2170 2720 1650 1800 1580
sqrt 2140 2550 2140 2140 640
cos 1660 3380 3870 3860 1690
tan 2520 4840 5930 5930 3480
atan 1600 5330 5880 5630 1660
mul 91 91 86 98 99
div 1190 1400 1170 1180 680
add 92 96 87 93 99
floor 356 788 2177 319 262
fabs 88 90 91 89 96

Here's the updated code using the fmath library. 

#include <cmath>
#include <cstdio>
#include <ctime>
#include <cstdlib>

#include "fmath.hpp"

int main() {
    volatile double a;
    volatile double b = 0.8234234;
    volatile double c = 1.43234;
    volatile float d;
    clock_t t;

    const int N = 100000000;

    t = clock();
    for (int i=0; i < N; i++)
        a = d;
    printf("float -> double %.3f\n", (clock()-t) / (float) 
CLOCKS_PER_SEC);    

    t = clock();
    for (int i=0; i < N; i++)
        d = a;
    printf("double -> float %.3f\n", (clock()-t) / (float) 
CLOCKS_PER_SEC);    

    t = clock();
    for (int i=0; i < N; i++)
        a = erf(b);
    printf("erf %.3f\n", (clock()-t) / (float) CLOCKS_PER_SEC);    

    t = clock();
    for (int i=0; i < N; i++)
        a = erfc(b);
    printf("erfc %.3f\n", (clock()-t) / (float) CLOCKS_PER_SEC);    

    t = clock();
    for (int i=0; i < N; i++)
        a = gamma(b);
    printf("gamma %.3f\n", (clock()-t) / (float) CLOCKS_PER_SEC);    

    t = clock();
    for (int i=0; i < N; i++)
        a = tgamma(b);
    printf("tgamma %.3f\n", (clock()-t) / (float) CLOCKS_PER_SEC);     

    t = clock();
    for (int i=0; i < N; i++)
        a = log(b);
    printf("log %.3f\n", (clock()-t) / (float) CLOCKS_PER_SEC);      

    t = clock();
    for (int i=0; i < N; i++)
        a = fmath::log(b);
    printf("fmath::log %.3f\n", (clock()-t) / (float) CLOCKS_PER_SEC);      

    t = clock();
    for (int i=0; i < N; i++)
        a = exp(b);
    printf("exp %.3f\n", (clock()-t) / (float) CLOCKS_PER_SEC);     

    t = clock();
    for (int i=0; i < N; i++)
        a = fmath::exp(b);
    printf("fmath::exp %.3f\n", (clock()-t) / (float) CLOCKS_PER_SEC);     

    t = clock();
    for (int i=0; i < N; i++)
        a = pow(b, c);
    printf("pow %.3f\n", (clock()-t) / (float) CLOCKS_PER_SEC);     

    t = clock();
    for (int i=0; i < N; i++)
        a = fmath::exp(fmath::log(b) * c);
    printf("fmath::pow %.3f\n", (clock()-t) / (float) CLOCKS_PER_SEC);     

    t = clock();
    for (int i=0; i < N; i++)
        a = sqrt(b);
    printf("sqrt %.3f\n", (clock()-t) / (float) CLOCKS_PER_SEC);     

    t = clock();
    for (int i=0; i < N; i++)
        a = cos(b);
    printf("cos %.3f\n", (clock()-t) / (float) CLOCKS_PER_SEC);     

    t = clock();
    for (int i=0; i < N; i++)
        a = tan(b);
    printf("tan %.3f\n", (clock()-t) / (float) CLOCKS_PER_SEC);     

    t = clock();
    for (int i=0; i < N; i++)
        a = atan(b);
    printf("atan %.3f\n", (clock()-t) / (float) CLOCKS_PER_SEC);     

        
    t = clock();
    for (int i=0; i < 10*N; i++)
        a = b*c;
    printf("mul %.3f\n", (clock()-t) / 10 / (float) CLOCKS_PER_SEC);    

    t = clock();
    for (int i=0; i < N; i++)
        a = b/c;
    printf("div %.3f\n", (clock()-t) / (float) CLOCKS_PER_SEC);  

    t = clock();
    for (int i=0; i < 10*N; i++)
        a = b+c;
    printf("add %.3f\n", (clock()-t) / 10 / (float) CLOCKS_PER_SEC);  

    t = clock();
    for (int i=0; i < 10*N; i++)
        a = floor(b);
    printf("floor %.3f\n", (clock()-t) / 10 / (float) CLOCKS_PER_SEC);  

    t = clock();
    for (int i=0; i < 10*N; i++)
        a = fabs(b);
    printf("fabs %.3f\n", (clock()-t) / 10 / (float) CLOCKS_PER_SEC);  

    return EXIT_SUCCESS;
}

posted at: 15:34 | path: /programming | permanent link

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