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Message-ID: <cd9a4c97-b054-4cc0-afa2-ac2d420e65c7@ghiti.fr>
Date: Mon, 15 Jan 2024 10:54:48 +0100
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Subject: Re: [PATCH v2] riscv: select ARCH_HAS_FAST_MULTIPLIER
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To: Jisheng Zhang <jszhang@kernel.org>,
 Paul Walmsley <paul.walmsley@sifive.com>, Palmer Dabbelt
 <palmer@dabbelt.com>, Albert Ou <aou@eecs.berkeley.edu>
Cc: linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
 Samuel Holland <samuel.holland@sifive.com>
References: <20231202135202.4071-1-jszhang@kernel.org>
From: Alexandre Ghiti <alex@ghiti.fr>
In-Reply-To: <20231202135202.4071-1-jszhang@kernel.org>
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Hi Jisheng,

On 02/12/2023 14:52, 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% performance improvement on JH7110, tested on Milkv Mars.
>
> 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.
>
> Samuel also provided some performance numbers:
> 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.
>
> Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
> Reviewed-by: Samuel Holland <samuel.holland@sifive.com>
> Tested-by: Samuel Holland <samuel.holland@sifive.com>
> ---
>
> since v1:
>   - fix typo in commit msg
>   - add some performance numbers provided by Samuel
>   - collect Reviewed-by and Tested-by tag
>
>   arch/riscv/Kconfig | 1 +
>   1 file changed, 1 insertion(+)
>
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 95a2a06acc6a..e4834fa76417 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -23,6 +23,7 @@ config RISCV
>   	select ARCH_HAS_DEBUG_VIRTUAL if MMU
>   	select ARCH_HAS_DEBUG_VM_PGTABLE
>   	select ARCH_HAS_DEBUG_WX
> +	select ARCH_HAS_FAST_MULTIPLIER
>   	select ARCH_HAS_FORTIFY_SOURCE
>   	select ARCH_HAS_GCOV_PROFILE_ALL
>   	select ARCH_HAS_GIGANTIC_PAGE

You can add:

Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>

Thanks,

Alex