linux-kernel - Re: [PATCH 1/1] riscv: Enable compiler optimizations

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Date:   Sat, 18 Jul 2020 21:45:23 -0700 (PDT)
From:   Palmer Dabbelt <palmer@...belt.com>
To:     maochenxi@...in.com, ardb@...nel.org
CC:     Paul Walmsley <paul.walmsley@...ive.com>, aou@...s.berkeley.edu,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        chenxi.mao2013@...il.com, maochenxi@...in.com
Subject:     Re: [PATCH 1/1] riscv: Enable compiler optimizations

On Tue, 07 Jul 2020 22:43:54 PDT (-0700), maochenxi@...in.com wrote:
> Enable ARCH_HAS_FAST_MULTIPLIER and ARCH_SUPPORTS_INT128 for better
> code generation.
> These 2 configurations works fine on GCC-9.3 and GCC-10.1
>
> Signed-off-by: Chenxi Mao <maochenxi@...in.com>
> ---
>  arch/riscv/Kconfig | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 128192e14ff2..f21b7e5188ca 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -26,6 +26,8 @@ config RISCV
>  	select ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT if MMU
>  	select ARCH_WANT_FRAME_POINTERS
>  	select ARCH_WANT_HUGE_PMD_SHARE if 64BIT
> +	select ARCH_HAS_FAST_MULTIPLIER
> +	select ARCH_SUPPORTS_INT128 if GCC_VERSION >= 50000 || CC_IS_CLANG

IIRC int128 is only supported on 64-bit targets, and we already have this under
RV64I but with a different set of requirements.  It looks like everyone has a
different set of requirements, but I feel like CC_HAS_INT128 is the right way
to go.  I'm not sure why we have the version check there.  Ard: do you know why
we would need both?

The fast multiplier one looks fine to me.  They're supposed to stay
alphabetical, though.

>  	select CLONE_BACKWARDS
>  	select COMMON_CLK
>  	select EDAC_SUPPORT