Re: [PATCH v2 1/2] arm: lib: xor-neon: remove unnecessary GCC < 4.6 warning

[Adrian Ratiu]

Hi Ard,

On Fri, 13 Nov 2020, Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
> On Thu, 12 Nov 2020 at 22:23, 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"). 
> >
> > Reported-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> 
> > Signed-off-by: Nathan Chancellor <natechancellor@xxxxxxxxx> 
> > Signed-off-by: Adrian Ratiu <adrian.ratiu@xxxxxxxxxxxxx> 
>
> Again, this does not do what it says on the tin. 
>
> If you want to disable the pragma for Clang, call that out in 
> the commit log, and don't hide it under a GCC version change.

I am not doing anything for Clang in this series.

The option to auto-vectorize in Clang is enabled by default but 
doesn't work for some reason (likely to do with how it computes 
the cost model, so maybe not even a bug at all) and if we enable 
it explicitely (eg via a Clang specific pragma) we get some 
warnings we currently do not understand, so I am not changing the 
Clang behaviour at the recommendation of Nick.

So this is only for GCC as the "tin" says :) We can fix clang 
separately as the Clang bug has always been present and is 
unrelated.

> Without the pragma, the generated code is the same as the 
> generic code, so it makes no sense to build xor-neon.ko at all, 
> right? 

Yes that is correct and that is the reason why in v1 I opted to 
not build xor-neon.ko for Clang anymore, but that got NACKed, so 
here I'm fixing the low hanging fruit: the very obvious & clear 
GCC problems.


> > ---
> >  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 b99dd8e1c93f..e1e76186ec23 100644
> > --- a/arch/arm/lib/xor-neon.c
> > +++ b/arch/arm/lib/xor-neon.c
> > @@ -19,15 +19,8 @@ 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"
> > --
> > 2.29.2