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SubjectRe: [PATCH] memchr (trivial) optimization
On Thu, Aug 23, 2007 at 02:13:20AM +0200, Ingo Oeser wrote:
> On Wednesday 22 August 2007, lode leroy wrote:
> > While profiling something completely unrelated, I noticed
> > that on the workloads I used memchr for, I saw a 30%-40% improvement
> > in performance, with the following trivial changes...
> > (basically, it saves 3 operations for each call)
>
> Yes, but then you could be a bit more explicit to the compiler
> on what you are doing here:
>
> void *memchr(const void *s, int c, size_t n)
> {
> const unsigned char *p = s;
>
> for (; n != 0; n--, p++) {
> if ((unsigned char)c == *p) {
> return (void *)p;
> }
> return NULL;
> }
>
> Now the compiler should see the loop more clearly.

And you can do even better with this:

void *memchr(const void *s, int c, size_t n)
{
const unsigned char *p = s, *e = s + n;
const unsigned char *e = p + n;
for (; p < e ; p++)
if ((unsigned char)c == *p)
return (void *)p;
return NULL;
}
which changes the inner loop from:

50: 38 08 cmp %cl,(%eax)
52: 74 08 je 5c <memchr2+0x1a>
54: 4a dec %edx
55: 40 inc %eax
56: 85 d2 test %edx,%edx
58: 75 f6 jne 50 <memchr2+0xe>

to:

6e: 38 08 cmp %cl,(%eax)
70: 74 07 je 79 <memchr3+0x1b>
72: 40 inc %eax
73: 39 d0 cmp %edx,%eax
75: 72 f7 jb 6e <memchr3+0x10>


--
Mathematics is the supreme nostalgia of our time.
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