Re: [PATCH 1/1] riscv: Enable compiler optimizations

[Ard Biesheuvel]

On Sun, 19 Jul 2020 at 07:45, Palmer Dabbelt <palmer@xxxxxxxxxxx> wrote:
>
> On Tue, 07 Jul 2020 22:43:54 PDT (-0700), maochenxi@xxxxxxxxx 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@xxxxxxxxx>
> > ---
> >  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?
>

That looks like copy/paste from the arm64 tree to me, where the names
of the libgcc routines were changed at some point, and we only support
the newer ones in the kernel.

> The fast multiplier one looks fine to me.  They're supposed to stay
> alphabetical, though.
>
> >       select CLONE_BACKWARDS
> >       select COMMON_CLK
> >       select EDAC_SUPPORT