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

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Date:   Fri, 3 Nov 2017 11:36:33 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Mark Rutland <mark.rutland@....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 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