Hi Linus,
> I guess you could try to just remove the "if (left)" test entirely, if
> it is at least partly the mispredict. It should do the right thing
> even with a zero count, and it might schedule the code better. Code
> size _should_ be better with the byte mask model (which won't matter
> in the hot loop example, since it will all be cached, possibly even in
> the uop cache for really tight benchmark loops).
Originally I had just forgotten the `if (left)`, and had the same
sub-par benchmarks. In the v3 revision that I'm working on at the
moment, I'm using your dcache trick for cases 3,5,6,7 and
short-circuiting cases 1,2,4 to just directly access those bytes as
integers. For the 32-bit case, I do something similar, but built
inside of the duff's device. This should give optimal performance for
the most popular use cases, which involve hashing "some stuff" plus a
leftover u16 (port number?) or u32 (ipv4 addr?).
#if defined(CONFIG_DCACHE_WORD_ACCESS) && BITS_PER_LONG == 64
switch (left) {
case 0: break;
case 1: b |= data[0]; break;
case 2: b |= get_unaligned_le16(data); break;
case 4: b |= get_unaligned_le32(data); break;
default:
b |= le64_to_cpu(load_unaligned_zeropad(data) &
bytemask_from_count(left));
break;
}
#else
switch (left) {
case 7: b |= ((u64)data[6]) << 48;
case 6: b |= ((u64)data[5]) << 40;
case 5: b |= ((u64)data[4]) << 32;
case 4: b |= get_unaligned_le32(data); break;
case 3: b |= ((u64)data[2]) << 16;
case 2: b |= get_unaligned_le16(data); break;
case 1: b |= data[0];
}
#endif
It seems like this might be best of all worlds?
Jason
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