Messages in this thread | | | From | "Bob Pearson" <> | Subject | RE: [PATCH] add slice by 8 algorithm to crc32.c | Date | Fri, 5 Aug 2011 12:27:26 -0500 |
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> > > > > > > > > > > > > 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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