[cfe-dev] clang generates way more code than - Optimizer bug?

[Dennis Luehring via cfe-dev]

Am 03.12.2021 um 14:58 schrieb Sjoerd Meijer:
> I guess the only way to tell is to run and measure it.


yeah, the clang code is faster than gcc - even with the much larger code
- as you told :)

thank you


gcc O1: 9.782262 seconds
gcc O2: 8.115871 seconds
gcc O3: 3.092142 seconds

clang O1: 9.905967 seconds
clang O2: 1.629295 seconds
clang O3: 1.629502 seconds


benchmark-code:


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

void decipher(unsigned char* text_, int text_len_)
{
   for (int i = text_len_ - 1; i >= 0; --i)
   {
     text_[i] ^= 0xff - (i << 2);
   }
}

int benchmark()
{
   unsigned char text[] = { 0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77,
0x88, 0x99, 0xAA, 0xBB, 0xCC, 0xDD, 0xEE, 0xFF };

   int anti_optimizer = 0;
   for (int i = 0; i < 1000000000; ++i)
   {
     decipher(text, sizeof(text));

     for (int x = 0; x < sizeof(text); ++x)
     {
       anti_optimizer += text[x];
     }
   }
   return anti_optimizer;
}

int main()
{
   clock_t start_time = clock();
   int result = benchmark();
   double elapsed_time = (double)(clock() - start_time) / CLOCKS_PER_SEC;
   printf("Done in %f seconds\n", elapsed_time);
   return result;
}