Messages in this thread | | | Date | Sat, 14 May 2016 11:35:25 -0700 | Subject | Re: [patch 2/7] lib/hashmod: Add modulo based hash mechanism | From | Linus Torvalds <> |
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On Fri, May 13, 2016 at 8:54 PM, George Spelvin <linux@horizon.com> wrote: > > There are exactly three architectures which (some models) don't have > an efficient 32x32->32-bit multiply: > > - arch/m58k: MC68000 (and 68010 and 68328) no-mmu > - arch/h8300: Most (all?) of the H8 processor series > - arch/microblaze: Depending on Verilog compilation options
I wouldn't worry about it too much.
The architectures where performance really matters are x86, ARM and powerpc.
The rest need to *work* and not suck horribly, but we're not going to try to do cycle counting for them. It's not worth the pain.
If an architecture doesn't have a barrel shifter, it's not going to have fast hash functions.
So I'd be ok with just saying "32-bit architectures are going to use a multiply with non-sparse bits". Not a problem.
We do want to make sure that hash_64 isn't totally disgusting on 32-bit architectures (but that's a fairly rare case), so we probably do want to have that function fall back on something else than a 64x64 multiply on a 32-bit architecture. Presumably just "mix the two 32-bit words into one, then use hash_32() on that" is good enough.
Linus
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