linux-kernel - Re: Use (two) different compilers at build-time?

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Date:	Mon, 7 Sep 2015 22:53:50 +0200
From:	Sedat Dilek <sedat.dilek@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Use (two) different compilers at build-time?

On Mon, Sep 7, 2015 at 10:42 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Mon, Sep 7, 2015 at 1:31 PM, Sedat Dilek <sedat.dilek@...il.com> wrote:
>>
>> So, how do I do that conveniently?
>
> For a single file? Just compile everything with the primary compiler,
> and then delete the single object file, and do "make" again.
>

That does not work.

I copied a gcc-compiled percpu.o OR deleted/renamed percpu.o and
re-invoked make - this starts a complete new build from scratch.

$ cd $KERNEL_BUILD_DIR
$ mv mm/percpu.o mm/percpu.o_CLANG
$ make CC=gcc HOSTCC=gcc

> Use a wrapper around the compiler (and point to that wrapper with the
> "to switch compilers from under the make, without the build paths
> changing (because otherwise our makefile auto-machinery notices that
> flags and command changed).
>
> Use CC (or CROSS_COMPILE) to point at your wrapper.
>

No idea how to realize that, sorry.

> What we've occasionally wanted to do for compiler bugs is to have
> something that can do the above not for single files, but some kind of
> file bisection thing (so bisect not in history, but in the list of
> files compiled). Sadly I don't think anybody has ever made that work.
>

Sounds like a nice feature.

- Sedat -
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