Arsthbt

I wanted to benchmark glibc's strlen function for some reason and found out it apparently performs much slower with optimizations enabled in GCC and I have no idea why.

Here's my code:

#include <time.h>
#include <string.h>
#include <stdlib.h>
#include <stdio.h>

int main() 
 char *s = calloc(1 << 20, 1);
 memset(s, 65, 1000000);
 clock_t start = clock();
 for (int i = 0; i < 128; ++i) 
 s[strlen(s)] = 'A';
 
 clock_t end = clock();
 printf("%lldn", (long long)(end-start));
 return 0;

On my machine it outputs:

$ gcc test.c && ./a.out
13336
$ gcc -O1 test.c && ./a.out
199004
$ gcc -O2 test.c && ./a.out
83415
$ gcc -O3 test.c && ./a.out
83415

Somehow, enabling optimizations causes it to execute longer.

Testing your code on Godbolt's Compiler Explorer provides this explanation:

• at -O0 or without optimisations, the generated code call the C library function strlen
  


• at -O1 the generated code uses a simple inline expansion using a rep scasb instruction.


• at -O2 and above, the generated code uses a more elaborate inline expansion.

Benchmarking your code repeatedly shows a substantial variation from one run to another, but increasing the number of iterations shows that:

• the -O1 code is much slower than the C library implementation: 32240 vs 3090
  


• the -O2 code is faster than the -O1 but still substantially slower than the C ibrary code: 8570 vs 3090.

This behavior is specific to gcc and the glibc. The same test on OS/X with clang and Apple's Libc does not show a significant difference, which is not a surprise as Godbolt shows that clang generates a call to the C library strlen at all optimisation levels.

This could be considered a bug in gcc/glibc but more extensive benchmarking might show that the overhead of calling strlen has a more important impact than the lack of performance of the inline code for small strings. The strings on which you benchmark are uncommonly large, so focusing the benchmark on ultra-long strings might not give meaningful results.

I updated the benchmark for smaller strings and it shows similar performance for string lengths varying from 0 to 100 at -O0 and -O2 but still a much worse performance at -O1, 3 times slower.

Here is the updated code:

#include <stdlib.h>
#include <string.h>
#include <time.h>

void benchmark(int repeat, int minlen, int maxlen) 
 char *s = malloc(maxlen + 1);
 memset(s, 'A', minlen);
 long long bytes = 0, calls = 0;
 clock_t clk = clock();
 for (int n = 0; n < repeat; n++) 
 for (int i = minlen; i < maxlen; ++i) 
 bytes += i + 1;
 calls += 1;
 s[i] = '';
 s[strlen(s)] = 'A';
 
 
 clk = clock() - clk;
 free(s);
 double avglen = (minlen + maxlen - 1) / 2.0;
 double ns = (double)clk * 1e9 / CLOCKS_PER_SEC;
 printf("average length %7.0f -> avg time: %7.3f ns/byte, %7.3f ns/calln",
 avglen, ns / bytes, ns / calls);


int main() 
 benchmark(10000000, 0, 1);
 benchmark(1000000, 0, 10);
 benchmark(1000000, 5, 15);
 benchmark(100000, 0, 100);
 benchmark(100000, 50, 150);
 benchmark(10000, 0, 1000);
 benchmark(10000, 500, 1500);
 benchmark(1000, 0, 10000);
 benchmark(1000, 5000, 15000);
 benchmark(100, 1000000 - 50, 1000000 + 50);
 return 0;

Here is the output:

chqrlie> gcc -std=c99 -O0 benchstrlen.c && ./a.out
average length 0 -> avg time: 14.000 ns/byte, 14.000 ns/call
average length 4 -> avg time: 2.364 ns/byte, 13.000 ns/call
average length 10 -> avg time: 1.238 ns/byte, 13.000 ns/call
average length 50 -> avg time: 0.317 ns/byte, 16.000 ns/call
average length 100 -> avg time: 0.169 ns/byte, 17.000 ns/call
average length 500 -> avg time: 0.074 ns/byte, 37.000 ns/call
average length 1000 -> avg time: 0.068 ns/byte, 68.000 ns/call
average length 5000 -> avg time: 0.064 ns/byte, 318.000 ns/call
average length 10000 -> avg time: 0.062 ns/byte, 622.000 ns/call
average length 1000000 -> avg time: 0.062 ns/byte, 62000.000 ns/call
chqrlie> gcc -std=c99 -O1 benchstrlen.c && ./a.out
average length 0 -> avg time: 20.000 ns/byte, 20.000 ns/call
average length 4 -> avg time: 3.818 ns/byte, 21.000 ns/call
average length 10 -> avg time: 2.190 ns/byte, 23.000 ns/call
average length 50 -> avg time: 0.990 ns/byte, 50.000 ns/call
average length 100 -> avg time: 0.816 ns/byte, 82.000 ns/call
average length 500 -> avg time: 0.679 ns/byte, 340.000 ns/call
average length 1000 -> avg time: 0.664 ns/byte, 664.000 ns/call
average length 5000 -> avg time: 0.651 ns/byte, 3254.000 ns/call
average length 10000 -> avg time: 0.649 ns/byte, 6491.000 ns/call
average length 1000000 -> avg time: 0.648 ns/byte, 648000.000 ns/call
chqrlie> gcc -std=c99 -O2 benchstrlen.c && ./a.out
average length 0 -> avg time: 10.000 ns/byte, 10.000 ns/call
average length 4 -> avg time: 2.000 ns/byte, 11.000 ns/call
average length 10 -> avg time: 1.048 ns/byte, 11.000 ns/call
average length 50 -> avg time: 0.337 ns/byte, 17.000 ns/call
average length 100 -> avg time: 0.299 ns/byte, 30.000 ns/call
average length 500 -> avg time: 0.202 ns/byte, 101.000 ns/call
average length 1000 -> avg time: 0.188 ns/byte, 188.000 ns/call
average length 5000 -> avg time: 0.174 ns/byte, 868.000 ns/call
average length 10000 -> avg time: 0.172 ns/byte, 1716.000 ns/call
average length 1000000 -> avg time: 0.172 ns/byte, 172000.000 ns/call