linux-kernel - Re: [PATCH 00/15] Add support for clang LTO

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Message-ID: <20171103184912.n4hoehrqqmf3ui44@lakrids.cambridge.arm.com>
Date:   Fri, 3 Nov 2017 18:49:12 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Sami Tolvanen <samitolvanen@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-arm-kernel@...ts.infradead.org,
        Matthias Kaehlcke <mka@...omium.org>,
        Kees Cook <keescook@...omium.org>,
        Greg Hackmann <ghackmann@...gle.com>,
        George Burgess <gbiv@...gle.com>,
        Stephen Hines <srhines@...gle.com>,
        Pirama Arumuga Nainar <pirama@...gle.com>,
        Manoj Gupta <manojgupta@...gle.com>
Subject: Re: [PATCH 00/15] Add support for clang LTO

On Fri, Nov 03, 2017 at 11:36:33AM -0700, Nick Desaulniers wrote:
> On Fri, Nov 3, 2017 at 11:29 AM, Mark Rutland <mark.rutland@....com> wrote:
> > On Fri, Nov 03, 2017 at 11:07:04AM -0700, Nick Desaulniers wrote:
> >> On Fri, Nov 3, 2017 at 10:51 AM, Mark Rutland <mark.rutland@....com> wrote:
> >> > I had to create an aarch64-linux-gnu-clang wrapper, too. I'm not sure if
> >> > there's build system help to avoid needing that?
> >>
> >> Gah!  So a BIG difference with Clang vs GCC for cross compiling is
> >> that Clang by default ships with all backends enabled, and uses a
> >> `-target <triple>` CFLAG to determine the arch to cross compile for,
> >> while GCC is configured at compile time to support one back end, IIUC.
> >
> > Yup.
> >
> > I initally tried passing CC=clang\ --target=aarch64-linux-gnu
> >
> > ... but since that conflicts with CROSS_COMPILE, you then have to
> > override AS, LD, OBJCOPY, etc, separately too.
> 
> make CC=clang CROSS_COMPILE=aarch64-linux-gnu-
> 
> I think you may need a trailing hyphen on your target as it gets
> prefixed onto as and ld:
> 
> Makefile:
> 
>  348 # Make variables (CC, etc...)
>  349 AS    = $(CROSS_COMPILE)as
>  350 LD    = $(CROSS_COMPILE)ld

I thought CC=clang CROSS_COMPILE=foo- would result in foo-clang, but
evidently this is not the case.

I must've meseed up something else; sorry for the noise.

Thanks for correcting me; now I can go delete my wrapper. :)

Thanks,
Mark.