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

[Adrian Ratiu]

On Fri, 13 Nov 2020, Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
> On Fri, 13 Nov 2020 at 12:05, Adrian Ratiu 
> <adrian.ratiu@xxxxxxxxxxxxx> wrote: 
> > 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. 
>
> But you are adding the IS_GCC check here, no? Is that 
> equivalent? IOW, does Clang today identify as GCC <= 4.6? 

I see what you mean now. Thanks.

Clang identifies as GCC <= 4.6 yes, so the code is not strictly 
speaking equivalent. The warning to upgrade GCC doesn't make sense 
for Clang but I should mention removing it in the commit message 
as well.

> > >
> > > 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.
>
> Fair enough.
>
> > >> ---
> > >>  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
> > >>