[PATCH v2] riscv: select ARCH_HAS_FAST_MULTIPLIER

[Jisheng Zhang]

On Sat, Dec 02, 2023 at 09:52:02PM +0800, 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 at kernel.org>
> Reviewed-by: Samuel Holland <samuel.holland at sifive.com>
> Tested-by: Samuel Holland <samuel.holland at sifive.com>

Hi @Palmer,

I saw this simple patch is missed in your for-next tree, could you
please pick it up?

Thanks in advance

> ---
> 
> 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
> -- 
> 2.42.0
> 
> 
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