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SubjectRE: [PATCH] add slice by 8 algorithm to crc32.c
Date
> >
> > >
> > > >
> > > > Modify all 'i' loops from for (i = 0; i < foo; i++) { ... } to for
(i =
> > foo
> > > > - 1; i >= 0; i--) { ... }
> > >
> > > That should be (i = foo; i ; --i) { ... }
> >
> > Shouldn't make much difference, branch on zero bit or branch on sign
bit.
> > But at the end of the day didn't help on Nehalem.

I figured out why "for (i = 0; i < len; i++) {...}" is faster than "for (;
len; len--) {...}" on my system.
The current code is
for (; Ien; len--) {
load *++p
...
}
Which turns into (in fake assembly)

top:
dec len
inc p
load p
...
test len
branch neq top

But when I replace that with

for(i = 0; i < len; i++) {
load *++p
...
}
Gcc turns it into

top:
load p[i]
i++
...
compare i, len
branch lt top

which is fewer instructions and i++ is well scheduled. Incrementing the
pointer has been moved out of the loop.



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