Messages in this thread | | | From | Bin Meng <> | Date | Thu, 22 Dec 2022 17:40:50 +0800 | Subject | Re: [PATCH] riscv: avoid enabling vectorized code generation |
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Hi,
On Thu, Dec 22, 2022 at 1:39 AM Saleem Abdulrasool <abdulras@google.com> wrote: > > On Wed, Dec 21, 2022 at 8:17 AM Bin Meng <bmeng.cn@gmail.com> wrote: > > > > Hi, > > > > On Sat, Dec 17, 2022 at 3:12 AM Saleem Abdulrasool <abdulras@google.com> 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@google.com> > > > --- > > > 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
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