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    SubjectRe: [PATCH v4 1/2] arm: lib: xor-neon: remove unnecessary GCC < 4.6 warning
    On Tue, Jan 19, 2021 at 5:17 AM Adrian Ratiu <adrian.ratiu@collabora.com> wrote:
    >
    > From: Nathan Chancellor <natechancellor@gmail.com>
    >
    > Drop warning because kernel now requires GCC >= v4.9 after
    > commit 6ec4476ac825 ("Raise gcc version requirement to 4.9")
    > and clarify that -ftree-vectorize now always needs enabling
    > for GCC by directly testing the presence of CONFIG_CC_IS_GCC.
    >
    > Another reason to remove the warning is that Clang exposes
    > itself as GCC < 4.6 so it triggers the warning about GCC
    > which doesn't make much sense and misleads Clang users by
    > telling them to update GCC.
    >
    > Because Clang is now supported by the kernel print a clear
    > Clang-specific warning.
    >
    > Link: https://github.com/ClangBuiltLinux/linux/issues/496
    > Link: https://github.com/ClangBuiltLinux/linux/issues/503
    > Reported-by: Nick Desaulniers <ndesaulniers@google.com>
    > Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

    This is not the version of the patch I had reviewed; please drop my
    reviewed-by tag when you change a patch significantly, as otherwise it
    looks like I approved this patch.

    Nacked-by: Nick Desaulniers <ndesaulniers@google.com>

    > Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
    > Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
    > ---
    > arch/arm/lib/xor-neon.c | 18 ++++++++++--------
    > 1 file changed, 10 insertions(+), 8 deletions(-)
    >
    > diff --git a/arch/arm/lib/xor-neon.c b/arch/arm/lib/xor-neon.c
    > index b99dd8e1c93f..f9f3601cc2d1 100644
    > --- a/arch/arm/lib/xor-neon.c
    > +++ b/arch/arm/lib/xor-neon.c
    > @@ -14,20 +14,22 @@ MODULE_LICENSE("GPL");
    > #error You should compile this file with '-march=armv7-a -mfloat-abi=softfp -mfpu=neon'
    > #endif
    >
    > +/*
    > + * TODO: Even though -ftree-vectorize is enabled by default in Clang, the
    > + * compiler does not produce vectorized code due to its cost model.
    > + * See: https://github.com/ClangBuiltLinux/linux/issues/503
    > + */
    > +#ifdef CONFIG_CC_IS_CLANG
    > +#warning Clang does not vectorize code in this file.
    > +#endif

    Arnd, remind me again why it's a bug that the compiler's cost model
    says it's faster to not produce a vectorized version of these loops?
    I stand by my previous comment: https://bugs.llvm.org/show_bug.cgi?id=40976#c8

    > +
    > /*
    > * Pull in the reference implementations while instructing GCC (through
    > * -ftree-vectorize) to attempt to exploit implicit parallelism and emit
    > * NEON instructions.
    > */
    > -#if __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 6)
    > +#ifdef CONFIG_CC_IS_GCC
    > #pragma GCC optimize "tree-vectorize"
    > -#else
    > -/*
    > - * While older versions of GCC do not generate incorrect code, they fail to
    > - * recognize the parallel nature of these functions, and emit plain ARM code,
    > - * which is known to be slower than the optimized ARM code in asm-arm/xor.h.
    > - */
    > -#warning This code requires at least version 4.6 of GCC
    > #endif
    >
    > #pragma GCC diagnostic ignored "-Wunused-variable"
    > --
    > 2.30.0
    >


    --
    Thanks,
    ~Nick Desaulniers

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