Linus Torvalds wrote:
>
> Yeah, verified. Google for
>
> northwood "barrel shifter"
>
> and you'll find a lot of it.
>
> Basically, older P4's will I think shift one bit at a time. So while even
> Prescott is relatively weak in the shifter department, pre-prescott
> (Willamette and Northwood) are _really_ weak. If your P4 is one of those,
> you really shouldn't use it to decide on optimizations.
Actually that's even more of a reason to make sure the code doesn't suck :)
The difference on less perverse cpus will usually be small, but on P4 it
can be huge.
A few years back I found my old ip checksum microbenchmark, and when I ran
it on a P4 (prescott iirc) i didn't believe my eyes. The straightforward
32-bit C implementation was running circles around the in-kernel one...
And a few tweaks to the assembler version got me another ~100% speedup.[1]
After that the P4 became the very first cpu to test any code on... :)
artur
[1] just reran the benchmark on this p4; true on northwood too:
IACCK 0.9.30 Artur Skawina <...>
[ exec time; lower is better ] [speed ] [ time ] [ok?]
TIME-N+S TIME32 TIME33 TIME1480 MBYTES/S TIMEXXXX CSUM FUNCTION ( rdtsc_overhead=0 null=0 )
17901 510 557 3010 393.36 59772 56dd csum_partial_cdumb16
3019 154 156 431 2747.10 43106 56dd csum_partial_c32
2413 170 177 328 3609.76 37501 56dd csum_partial_c32l
2437 170 170 328 3609.76 37488 56dd csum_partial_c32i
5078 205 254 767 1543.68 48117 56dd csum_partial_std
5612 299 291 851 1391.30 53673 56dd csum_partial_686
1584 99 127 227 5215.86 14495 56dd csum_partial_586f
1738 107 121 229 5170.31 14785 56dd csum_partial_586fs
4893 175 171 759 1559.95 52347 56dd csum_partial_copy_generic_std
4949 151 189 756 1566.14 67847 56dd csum_partial_copy_generic_686
2072 110 134 302 3920.53 39061 56dd csum_partial_copy_generic_p4as1
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