Re: [PATCH] riscv: avoid enabling vectorized code generation

[Bin Meng]

Hi,

On Thu, Dec 22, 2022 at 1:39 AM Saleem Abdulrasool <abdulras@xxxxxxxxxx> wrote:
>
> On Wed, Dec 21, 2022 at 8:17 AM Bin Meng <bmeng.cn@xxxxxxxxx> wrote:
> >
> > Hi,
> >
> > On Sat, Dec 17, 2022 at 3:12 AM Saleem Abdulrasool <abdulras@xxxxxxxxxx> 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.
> > >
> > > Signed-off-by: Saleem Abdulrasool <abdulras@xxxxxxxxxx>
> > > ---
> > >  arch/riscv/Makefile | 4 ++++
> > >  1 file changed, 4 insertions(+)
> > >
> > > diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
> > > index 0d13b597cb55..68433476a96e 100644
> > > --- a/arch/riscv/Makefile
> > > +++ b/arch/riscv/Makefile
> > > @@ -89,6 +89,10 @@ KBUILD_AFLAGS_MODULE += $(call as-option,-Wa$(comma)-mno-relax)
> > >  # architectures.  It's faster to have GCC emit only aligned accesses.
> > >  KBUILD_CFLAGS += $(call cc-option,-mstrict-align)
> > >
> > > +# Ensure that we do not vectorize the kernel code when the `v` extension is
> > > +# enabled.  This mirrors the `-mno-mmx` et al on x86.
> > > +KBUILD_CFLAGS += $(call cc-option,-mno-implicit-float)
> >
> > This looks like an LLVM flag, but not GCC.
>
> Correct, this is a clang flag, though I imagine that GCC will need a
> similar flag once it receives support for the V extension.
>
> > Can you elaborate what exact combination (compiler flag and source)
> > would cause an issue?
>
> The particular case that I was using was simply `clang -target
> riscv64-unknown-linux-musl -march=rv64gcv` off of main.
>
> > From your description, I guess it's that when enabling V extension in
> > LLVM, the compiler tries to use vector instructions to zero memory,
> > correct?
>
> Correct.

Thanks for the confirmation.

>
> > Can you confirm LLVM does not emit any float instructions (like F/D
> > extensions) because the flag name suggests something like "float"?
>
> The `-mno-implicit-float` should disable any such emission.  I assume
> that you are worried about the case without the flag?  I'm not 100%
> certain without this flag, but the RISCV build with this flag has been
> running smoothly locally for a while.
>
>

I still have some questions about the `-mno-implicit-float` option's behavior.

- If this option is not on, does the compiler emit any F/D extension
instruction for zero'ing memory when -march=rv64g? I want to know
whether the `-mno-implicit-float` option only takes effect when "v"
appears on the -march string.
- If the answer to the above question is no, I wonder why the option
is called `-mno-implicit-float` as float suggests the FPU usage, but
actually it is about vectorization. The Clang documentation says
almost nothing about this option.

> > > +
> > >  ifeq ($(CONFIG_STACKPROTECTOR_PER_TASK),y)
> > >  prepare: stack_protector_prepare
> > >  stack_protector_prepare: prepare0
> > > --

Regards,
Bin