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

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Message-ID: <ddb9d0cb-5f78-4242-b431-878d0372884b@sifive.com>
Date:   Tue, 21 Nov 2023 16:20:54 -0600
From:   Samuel Holland <samuel.holland@...ive.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-kernel@...r.kernel.org
Subject: Re: [PATCH] riscv: select ARCH_HAS_FAST_MULTIPLIER

On 2023-11-21 8:43 AM, Jisheng Zhang wrote:
> Currently, riscv linux requires at least IMA, so all platforms have a
> multiplier. And I assume the 'mul' efficiency is comparable or better
> than a sequence of five or so register-dependent arithmetic
> instructions. Select ARCH_HAS_FAST_MULTIPLIER to get slightly nicer
> codegen. Refer to commit f9b4192923fa ("[PATCH] bitops: hweight()
> speedup") for more details.
> 
> In a simple benchmark test calling hweight64() in a loop, it got:
> about 14% preformance improvement on JH7110, tested on Milkv Mars.

typo: performance

> about 23% performance improvement on TH1520 and SG2042, tested on
> Sipeed LPI4A and SG2042 platform.
> 
> a slight performance drop on CV1800B, tested on milkv duo. Among all
> riscv platforms in my hands, this is the only one which sees a slight
> performance drop. It means the 'mul' isn't quick enough. However, the
> situation exists on x86 too, for example, P4 doesn't have fast
> integer multiplies as said in the above commit, x86 also selects
> ARCH_HAS_FAST_MULTIPLIER. So let's select ARCH_HAS_FAST_MULTIPLIER
> which can benefit almost riscv platforms.

On Unmatched: 20% speedup for __sw_hweight32 and 30% speedup for __sw_hweight64.
On D1: 8% speedup for __sw_hweight32 and 8% slowdown for __sw_hweight64.

So overall still an improvement.

> Signed-off-by: Jisheng Zhang <jszhang@...nel.org>
> ---
>  arch/riscv/Kconfig | 1 +
>  1 file changed, 1 insertion(+)

Reviewed-by: Samuel Holland <samuel.holland@...ive.com>
Tested-by: Samuel Holland <samuel.holland@...ive.com>