Re: [CORE (21.4)] "clever" compilers sometimes opt away too much.

Darryl Okahata <darrylo(a)soco.agilent.com&gt; writes: Yes, but what we have is what we have. Here is my investigations: ---- file m.c ---- void cptr1(void *ptr, unsigned int len) { if (len > 1) /* If we can, try to avoid problems with super-optimizing compilers that might decide that memcmp (ptr, ptr, len) can be optimized away since its result is always 1. */ memcmp (ptr, (char *) ptr + 1, len - 1); else memcmp (ptr, ptr, len); } void cptr2(void *ptr, unsigned int len) { while (len-- > 0) { char a = ((char*)ptr)[len]; if (a == '1') /* pass */; } } int main() { void *ptr = (void*)10; cptr1(ptr, 10); printf("cptr1\n"); cptr2(ptr, 10); printf("cptr2\n"); return 0; } ---- m.c ends here --- [0][~]$ gcc342 -o m m.c [0][~]$ ./m cptr1 zsh: segmentation fault (core dumped) ./m [139][~]$ gcc342 -O -o m m.c [0][~]$ ./m cptr1 cptr2 [0][~]$ gcc295 -o m m.c [0][~]$ ./m zsh: segmentation fault (core dumped) ./m [139][~]$ gcc295 -O -o m m.c [0][~]$ ./m cptr1 cptr2 [0][~]$ CPTR1 works only with gcc295 without any optimisation flags. CPTR2 works on gcc295 and gcc342 without optimisation flags. So even CPTR2 is does not cheats gcc optimisations fully, finally i was able to get good results with next CPTR2: void cptr2(void *ptr, unsigned int len) { while (len-- > 0) { char a = ((char*)ptr)[len]; if (a == '1') ((char*)ptr)[len] = a; } } [0][~]$ gcc342 -o m m.c [0][~]$ ./m cptr1 zsh: segmentation fault (core dumped) ./m [139][~]$ gcc342 -O -o m m.c [0][~]$ ./m cptr1 zsh: segmentation fault (core dumped) ./m [139][~]$ gcc342 -O3 -o m m.c [0][~]$ ./m cptr1 zsh: segmentation fault (core dumped) ./m [139][~]$ gcc295 -o m m.c [0][~]$ ./m zsh: segmentation fault (core dumped) ./m [139][~]$ gcc295 -O -o m m.c [0][~]$ ./m cptr1 zsh: segmentation fault (core dumped) ./m [139][~]$ gcc295 -O3 -o m m.c [0][~]$ ./m cptr1 zsh: segmentation fault (core dumped) ./m [139][~]$ As you see new CPTR2 does the job with both gcc342 and gcc 295 and any optimisation flags. Thanks! -- lg