linux-kernel - Re: [PATCH v4 3/3] x86/build: Specify stack alignment for clang

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Date:   Wed, 21 Jun 2017 09:18:58 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Matthias Kaehlcke <mka@...omium.org>
Cc:     hpa@...or.com, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . J . Lu" <hjl.tools@...il.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Michal Marek <mmarek@...e.com>, x86@...nel.org,
        linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        Michael Davidson <md@...gle.com>,
        Greg Hackmann <ghackmann@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Stephen Hines <srhines@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Arnd Bergmann <arnd@...db.de>,
        Bernhard.Rosenkranzer@...aro.org,
        Peter Foley <pefoley2@...oley.com>,
        Behan Webster <behanw@...verseincode.com>,
        Douglas Anderson <dianders@...omium.org>
Subject: Re: [PATCH v4 3/3] x86/build: Specify stack alignment for clang


* Matthias Kaehlcke <mka@...omium.org> wrote:

> El Tue, Jun 20, 2017 at 11:20:54AM +0200 Ingo Molnar ha dit:
> 
> > 
> > * Matthias Kaehlcke <mka@...omium.org> wrote:
> > 
> > > Ingo didn't like the duplication and suggested the use of a variable, which 
> > > kinda implies a check for the compiler name.
> > 
> > I don't think it implies that: why cannot cc_stack_align_opt probe for the 
> > compiler option and use whichever is available, without hard-coding the compiler 
> > name?
> 
> We could do this:
> 
> ifneq ($(call __cc-option, $(CC), -mno-sse, -mpreferred-stack-boundary=3,),)
>         cc_stack_align_opt := -mpreferred-stack-boundary
> endif
> ifneq ($(call cc-option, -mstack-alignment=3,),)
>         cc_stack_align_opt := -mstack-alignment
> endif

The principle Looks good to me - but I'd make the second probing an 'else' branch, 
i.e. probe for a suitable compiler option until we find one. That would also not 
burden the GCC build with probing for different compiler options.

Please also add a comment in the code that explains that the first option is a GCC 
option and the second one is a Clang-ism.

> Since this solution also won't win a beauty price please let me know
> if it is acceptable before respinning the patch or if you have other
> suggestions.

This one already looks a lot cleaner to me than any of the previous ones.

Thanks,

	Ingo