On Tue, May 28, 2019 at 4:57 PM Nathan Chancellor
<natechancel...@gmail.com> wrote:
>
> Currently, when compiling this code with clang, the following warning is
> emitted:
>
> CC arch/arm/lib/xor-neon.o
> arch/arm/lib/xor-neon.c:33:2: warning: This code requires at least
> version 4.6 of GCC [-W#warnings]
>
> This is because clang poses as GCC 4.2.1 with its __GNUC__ conditionals
> for glibc compatibility[1]:
>
> $ echo | clang -dM -E -x c /dev/null | grep GNUC | awk '{print $2" "$3}'
> __GNUC_MINOR__ 2
> __GNUC_PATCHLEVEL__ 1
> __GNUC_STDC_INLINE__ 1
> __GNUC__ 4
>
> As pointed out by Ard Biesheuvel and Arnd Bergmann in an earlier
> thread[2], the oldest version of GCC that is currently supported is gcc
> 4.6 after commit cafa0010cd51 ("Raise the minimum required gcc version
> to 4.6") so we do not need to check for anything older anymore.
>
> However, just removing the version check is not enough to silence clang
> because it does not recognize '#pragma GCC optimize':
>
> arch/arm/lib/xor-neon.c:25:13: warning: unknown pragma ignored
> [-Wunknown-pragmas]
> #pragma GCC optimize "tree-vectorize"
>
> Looking into it further, -ftree-vectorize (which '#pragma GCC optimize
> "tree-vectorize"' enables) is an alias in clang for -fvectorize[3],
> which according to the documentation is on by default[4] (at least at
> -O2 or -Os).
>
> Just add the pragma when compiling with GCC so that clang does not
> unnecessarily warn.
>
> [1]: https://reviews.llvm.org/D51011#1206981
> [2]:
> https://lore.kernel.org/lkml/cak8p3a3njtcgfd2dq9kbhp8dpxf6s-ulfeu6acayc4sdi+2...@mail.gmail.com/
> [3]:
> https://github.com/llvm/llvm-project/blob/eafe8ef6f2b44ba/clang/include/clang/Driver/Options.td#L1729
> [4]: https://llvm.org/docs/Vectorizers.html#usage
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/496
> Reported-by: Nick Desaulniers <ndesaulni...@google.com>
> Signed-off-by: Nathan Chancellor <natechancel...@gmail.com>
LGTM, thanks Nathan.
Reviewed-by: Nick Desaulniers <ndesaulni...@google.com>
> ---
> arch/arm/lib/xor-neon.c | 9 +--------
> 1 file changed, 1 insertion(+), 8 deletions(-)
>
> diff --git a/arch/arm/lib/xor-neon.c b/arch/arm/lib/xor-neon.c
> index c691b901092f..d532bc072ee4 100644
> --- a/arch/arm/lib/xor-neon.c
> +++ b/arch/arm/lib/xor-neon.c
> @@ -22,15 +22,8 @@ MODULE_LICENSE("GPL");
> * -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"
> --
> 2.22.0.rc1
>
--
Thanks,
~Nick Desaulniers
