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SubjectRe: [PATCH] riscv: avoid enabling vectorized code generation
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On Fri, 16 Dec 2022 10:50:12 PST (-0800), 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 += $(call cc-option,-mno-implicit-float)
> +
> ifeq ($(CONFIG_STACKPROTECTOR_PER_TASK),y)
> prepare: stack_protector_prepare
> stack_protector_prepare: prepare0

Sorry to just restart the thread, but there's been discussions on this
in a bunch of places. From my understanding, we don't actually need
this: we have this tricky line in the Makefile

KBUILD_CFLAGS += -march=$(subst fd,,$(riscv-march-y))

that removes the floating-point extenions from what the kernel is built
with, so adding `-mno-implicit-float` doesn't actually do anything (and
we'd end up with essentially the same thing for V when it gets added).

So unless I'm missing something, we don't need this.

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