Benchmark pow() vs explicit multiplication

main.c

#if __STDC_VERSION__ >= 199901L /* POSIX 2008 SUS v4 */
#define _XOPEN_SOURCE 700 
#else
#define _XOPEN_SOURCE 500
#endif /* __STDC_VERSION__ */

#include <math.h> // pow
#include <time.h> // clock_gettime
#include <stdio.h> // printf
#include <stdlib.h> // rand

#define SAMPLE_N 10000000

int main(int argc, char **argv){
    if (argc != 2)
        return 1;
    int idx = atoi(argv[1]);
    double in[SAMPLE_N], out[SAMPLE_N];
    struct timespec start, finish;
    for (int i=0; i<SAMPLE_N; ++i){
        in[i] = ((double)rand()) / RAND_MAX;
    }

    for (int j=0; j<10; ++j){
        clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &start);
        for (int i=0; i<SAMPLE_N; ++i){
            out[i] = pow(in[i], 5);
        }
        clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &finish);
        printf("optimization guard: %5.3f'", out[idx]);
        printf("pow     : %12.8f\n", finish.tv_sec-start.tv_sec + 1e-9*(finish.tv_nsec-start.tv_nsec));

        clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &start);
        for (int i=0; i<SAMPLE_N; ++i){
            out[i] = in[i]*in[i]*in[i]*in[i]*in[i];
        }
        clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &finish);
        printf("optimization guard: %5.3f'", out[idx]);
        printf("explicit: %12.8f\n", finish.tv_sec-start.tv_sec + 1e-9*(finish.tv_nsec-start.tv_nsec));
    }
    return 0;
}

Makefile

# Enabling -ffast-math makes the two versions competetive, without it explicit is much faster...
# (~20x for exponent of 5)
CFLAGS ?= -std=c99 -O3 #-ffast-math 
LIBS ?= -lrt -lm

.PHONY: all

output.txt: main
	$(CC) --version > $@
	./$< 42 >> $@

main: main.c
	$(CC) $(CFLAGS) -o $@ $^ $(LIBS)

output.txt

gcc (Ubuntu 4.8.2-19ubuntu1) 4.8.2
Copyright (C) 2013 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

optimization guard: 0.010'pow     :   1.90185076
optimization guard: 0.010'explicit:   0.08865624
optimization guard: 0.010'pow     :   1.88004106
optimization guard: 0.010'explicit:   0.09030047
optimization guard: 0.010'pow     :   1.87685020
optimization guard: 0.010'explicit:   0.08904781
optimization guard: 0.010'pow     :   1.88149224
optimization guard: 0.010'explicit:   0.08866682
optimization guard: 0.010'pow     :   1.87501423
optimization guard: 0.010'explicit:   0.08858129
optimization guard: 0.010'pow     :   1.87789195
optimization guard: 0.010'explicit:   0.08893130
optimization guard: 0.010'pow     :   1.87002738
optimization guard: 0.010'explicit:   0.08871757
optimization guard: 0.010'pow     :   1.87417454
optimization guard: 0.010'explicit:   0.08991346
optimization guard: 0.010'pow     :   1.88419079
optimization guard: 0.010'explicit:   0.08897720
optimization guard: 0.010'pow     :   1.87802256
optimization guard: 0.010'explicit:   0.08939641