linux-kernel - RE: [PATCH] riscv: select ARCH_HAS_FAST

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Message-ID: <47995b4daafb43348fc839ffb486f56b@AcuMS.aculab.com>
Date:   Wed, 22 Nov 2023 11:16:23 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Jisheng Zhang' <jszhang@...nel.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>
CC:     "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] riscv: select ARCH_HAS_FAST_MULTIPLIER

...
> However, the
> situation exists on x86 too, for example, P4 doesn't have fast
> integer multiplies ...

P4 doesn't have fast anything :-)

More interestingly does anything modern not have fast multiplies?
(that you'd consider running Linux on).

The silicon required isn't that big.
IIRC an 'n' bit multiplier requires n^2 full adders and has
the latency of 2n adders.

	David

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