Re: [PATCH v4 1/2] arm: lib: xor-neon: remove unnecessary GCC < 4.6 warning

[Adrian Ratiu]

On Tue, 19 Jan 2021, Nick Desaulniers <ndesaulniers@xxxxxxxxxx> 
wrote:
> On Tue, Jan 19, 2021 at 5:17 AM Adrian Ratiu 
> <adrian.ratiu@xxxxxxxxxxxxx> wrote: 
> > From: Nathan Chancellor <natechancellor@xxxxxxxxx> 
> >
> > 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@xxxxxxxxxx> 
> > Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> 
>
> 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@xxxxxxxxxx> 

Sorry for not removing the reviewed-by tags from the previous 
versions in this v4. I guess the only way forward with this is to 
actually make clang vectorization work. Also thanks for the patch 
suggestion in the other e-mail!

> > Signed-off-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
> > Signed-off-by: Adrian Ratiu <adrian.ratiu@xxxxxxxxxxxxx>
> > ---
> >  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