On Tuesday 06 March 2007 18:19, Linus Torvalds wrote:
> On Tue, 6 Mar 2007, Eric Dumazet wrote:
> > Something like :
> >
> > [PATCH] : Use reciprocal divides in sprintf()
>
> Try this on Core 2, and I suspect that you'll find that the hardware is
> actually *faster* than doing the shift/test, function call and the
> two multiplies.
>
Where do you see a function call ?
448: 44 89 d0 mov %r10d,%eax
44b: 44 89 ea mov %r13d,%edx
44e: 48 0f af c1 imul %rcx,%rax
452: 48 c1 e8 20 shr $0x20,%rax
456: 0f af d0 imul %eax,%edx
459: 49 29 d2 sub %rdx,%r10
45c: 43 0f b6 14 16 movzbl (%r14,%r10,1),%edx
461: 41 89 c2 mov %eax,%r10d
464: 41 88 13 mov %dl,(%r11)
467: 49 ff c3 inc %r11
46a: 85 c0 test %eax,%eax
46c: 75 da jne 448 <number+0x138>
> > Using reciprocal divides permits to change each divide by two multiplies,
> > less expensive on current CPUS.
>
> Are you sure?
I am going to test this, but at least on Opterons, the reciprocal divide I
added into mm/slab.c gave me a nice speedup.
I am going to bench some stupid loop :
for (i = 0 ; i < 1000*1000 ; i++) {
pipe(fds); close(fds[0]); close(fds[1]);
}
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