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Date:   Fri, 3 Nov 2017 18:29:01 +0000
From:   Mark Rutland <>
To:     Nick Desaulniers <>
Cc:     Sami Tolvanen <>,
        LKML <>,,
        Matthias Kaehlcke <>,
        Kees Cook <>,
        Greg Hackmann <>,
        George Burgess <>,
        Stephen Hines <>,
        Pirama Arumuga Nainar <>,
        Manoj Gupta <>
Subject: Re: [PATCH 00/15] Add support for clang LTO

On Fri, Nov 03, 2017 at 11:07:04AM -0700, Nick Desaulniers wrote:
> On Fri, Nov 3, 2017 at 10:51 AM, Mark Rutland <> 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.


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.

This is a little less than ideal. :/

> Clang _can_ be built with all other back ends disabled but one, but I
> have a problem with this approach:  distributions of Clang are rarely
> configured this way (all backends enabled by default), and it seems we
> just found this past week that Clang if configured with just one
> backend, it will silently ignore -target flags for other backends and
> generate code for the configured backend (this led to:
>  I consider this a bug in Clang,
> so I just filed:

Ouch. Thanks for attacking that.

> >   CC      arch/arm64/crypto/aes-ce-cipher.o
> > fatal error: error in backend: Do not know how to split the result of this operator!
> Yep, we've seen this.  It was the FINAL bug in Clang for compiling the
> kernel for arm64.  It was recently fixed upstream in llvm by
>, but so recent that you'll need to either compiler
> Clang from source from ToT or work around it like we have in Android
> with:

Sure. I can live with patch 7 for now, assuming there's nothing else
lurking in the shadows.


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