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Date: Sat, 14 May 2016 11:35:25 -0700 From: Linus Torvalds <torvalds@...ux-foundation.org> To: George Spelvin <linux@...izon.com> Cc: Eric Dumazet <eric.dumazet@...il.com>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Rik van Riel <riel@...hat.com>, Thomas Gleixner <tglx@...utronix.de> Subject: Re: [patch 2/7] lib/hashmod: Add modulo based hash mechanism On Fri, May 13, 2016 at 8:54 PM, George Spelvin <linux@...izon.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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