linux-kernel - Re: [PATCH] riscv: avoid enabling vectorized code generation

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Date:   Fri, 16 Dec 2022 12:56:23 -0800
From:   Saleem Abdulrasool <abdulras@...gle.com>
To:     Palmer Dabbelt <palmer@...belt.com>
Cc:     ben.dooks@...ethink.co.uk, ndesaulniers@...gle.com,
        nathan@...nel.org, Paul Walmsley <paul.walmsley@...ive.com>,
        aou@...s.berkeley.edu, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] riscv: avoid enabling vectorized code generation

On Fri, Dec 16, 2022 at 11:54 AM Palmer Dabbelt <palmer@...belt.com> wrote:
>
> On Fri, 16 Dec 2022 11:45:21 PST (-0800), ben.dooks@...ethink.co.uk wrote:
> >
> >
> > On 2022-12-16 18:50, Saleem Abdulrasool wrote:
> >> The compiler is free to generate vectorized operations for zero'ing
> >> memory.  The kernel does not use the vector unit on RISCV, similar to
> >> architectures such as x86 where we use `-mno-mmx` et al to prevent the
> >> implicit vectorization.  Perform a similar check for
> >> `-mno-implicit-float` to avoid this on RISC-V targets.
> >
> > I'm not sure if we should be emitting either of the vector or floating
> > point instrucitons in the kernel without explicitly marking the section
> > of code which is using them such as specific accelerator blocks.
>
> Yep, we can't let the compiler just blindly enable V or F/D.  V would
> very much break things as we have no support, but even when that's in
> we'll we at roughly the same spot as F/D are now where we need to handle
> the lazy save/restore bits.
>
> This looks like an LLVM-only option, I see at least some handling here
>
> https://github.com/llvm/llvm-project/blob/a72883b7612f5c00b592da85ed2f1fd81258cc08/clang/lib/Driver/ToolChains/Clang.cpp#L2098
>
> but I don't really know LLVM enough to understand if there's some
> default for `-mimplicit-float` and I can't find anything in the docs.
> If it can be turned on by default and that results in F/D/V instructions
> then we'll need to explicitly turn it off, and that would need to be
> backported.

Yes, this is an LLVM option, but I think that the `cc-option` wrapping
should help ensure that we do not break the gcc build.  This only
recently was added to clang, so an older clang would also miss this
flag.  The `-mimplicit-float` is the default AFAIK, which is why we
needed to add this flag in the first place.  Enabling V exposed this,
which is why the commit message mentions vector.

>
> Maybe Nick or Nathan knows what's up here?