Re: [PATCH] bpf: don't rely on GCC __attribute__((optimize)) to disable GCSE

[Nick Desaulniers]

On Tue, Oct 27, 2020 at 3:23 PM Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
>
> On Tue, 27 Oct 2020 at 23:03, Nick Desaulniers <ndesaulniers@xxxxxxxxxx> wrote:
> >
> > On Tue, Oct 27, 2020 at 2:50 PM Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
> > >
> > > On Tue, 27 Oct 2020 at 22:20, Nick Desaulniers <ndesaulniers@xxxxxxxxxx> wrote:
> > > >
> > > > On Tue, Oct 27, 2020 at 1:57 PM Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
> > > > >
> > > > > diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> > > > > index 6e390d58a9f8..ac3fa37a84f9 100644
> > > > > --- a/include/linux/compiler_types.h
> > > > > +++ b/include/linux/compiler_types.h
> > > > > @@ -247,10 +247,6 @@ struct ftrace_likely_data {
> > > > >  #define asm_inline asm
> > > > >  #endif
> > > > >
> > > > > -#ifndef __no_fgcse
> > > > > -# define __no_fgcse
> > > > > -#endif
> > > > > -
> > > > Finally, this is going to disable GCSE for the whole translation unit,
> > > > which may be overkill.   Previously it was isolated to one function
> > > > definition.  You could lower the definition of the preprocessor define
> > > > into kernel/bpf/core.c to keep its use isolated as far as possible.
> > > >
> > >
> > > Which preprocessor define?
> >
> > __no_fgcse
> >
>
> But we can't use that, that is the whole point of this patch.

Ah, right because the attribute drops other command line flags...ok,
then -fno-gcse the whole translation unit it is then.

Still need to avoid new warnings with clang though.
-- 
Thanks,
~Nick Desaulniers