Messages in this thread | | | Date | Wed, 13 Aug 2003 07:18:19 +0200 | From | Willy Tarreau <> | Subject | Re: generic strncpy - off-by-one error |
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On Tue, Aug 12, 2003 at 11:38:31PM -0400, Albert Cahalan wrote: > That's excellent. On ppc I count 12 instructions, > 4 of which would go away for typical usage if inlined. > Annoyingly, gcc doesn't get the same assembly from my > attempt at that general idea: > > char * strncpy_5(char *dest, const char *src, size_t count){ > char *tmp = dest; > while (count--){ > if(( *tmp++ = *src )) src++; > } > return dest; > } > > I suppose that gcc could use a bug report.
I often noticed that using '++' and '--' within or just before assignments and/or comparisons often break the code and make it suboptimal. C provides enough flexibility to code what you think nearly at the instruction level. Since 'while' loops often start with a jump to the end, you can sometimes help the compiler by enclosing them within an 'if' statement such as below. BTW, in your case, count ends with -1.
I've absolutely not tried this one, but it could produce different code on your PPC, and can trivially be derived to cleaner constructs. I proceeded the same way when I wrote my own optimized strlcpy() implementation which is 45 bytes long and copies 1 char per CPU cycle on i686.
char *strncpy(char *dest, const char *src, size_t count) { if (count) { char *tmp = dest; while (1) { *tmp = *src; if (*src) src++; tmp++; if (!count--) break; } } return dest; }
Cheers, Willy
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