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

[Adrian Ratiu]

On Tue, 19 Jan 2021, Nathan Chancellor <natechancellor@xxxxxxxxx> 
wrote:
> On Tue, Jan 19, 2021 at 03:17:23PM +0200, Adrian Ratiu 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> 
> > Signed-off-by: Nathan Chancellor <natechancellor@xxxxxxxxx> 
> > Signed-off-by: Adrian Ratiu <adrian.ratiu@xxxxxxxxxxxxx> 
>
> The commit message looks like it is written by me but I never 
> added a Clang specific warning. I appreciate wanting to give me 
> credit but when you change things about my original commit 
> message, please make it clear that you did the edits, something 
> like: 
>
> Signed-off-by: Nathan Chancellor <natechancellor@xxxxxxxxx> 
> [adrian: Add clang specific warning] Signed-off-by: Adrian Ratiu 
> <adrian.ratiu@xxxxxxxxxxxxx> 

Thanks for the suggestion. Makes sense. I contemplated adding 
another patch by me on top but thought it was too much 
churn. Sorry if my edits were unclear.

> > --- 
> >  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 
>
> I really do not like this. With the GCC specific warning, the 
> user could just upgrade their GCC. With this warning, it is 
> basically telling them don't use clang, in which case, it would 
> just be better to disable this code altogether. I would rather 
> see: 
>
> 1. Just don't build this file with clang altogether, which I 
> believe was 
>    v1's 2/2 patch. 
>
> OR 
>
> 2. Use the pragma: 
>
> #pragma clang loop vectorize(enable) 
>
> as Nick suggests in v1's 2/2 patch. 
>
> Alternatively, __restrict__ sounds like it might be beneficial 
> for both GCC and clang: 
>
> https://lore.kernel.org/lkml/20201112215033.GA438824@xxxxxxxxxxxxxxxxxx/ 

Option 1 from v1 got clearly NACKed by Nick a while back so the 
only option gonig forward is to also fix clang vectorization 
together with these changes so the warning becomes unnecessary.

> >  /*
> >   * 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