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

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Message-ID: <CA+icZUUr6Tso7gJoXRUnkuBvFdV5dn1D65oyK4WSmj2Dm9CKqg@mail.gmail.com>
Date:	Mon, 7 Sep 2015 23:20:08 +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 10:59 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Mon, Sep 7, 2015 at 1:53 PM, Sedat Dilek <sedat.dilek@...il.com> wrote:
>>
>> That does not work.
>
> .. because you didn't do what I told you to do.
>
>> I copied a gcc-compiled percpu.o OR deleted/renamed percpu.o and
>> re-invoked make - this starts a complete new build from scratch.
>
> Right. Because you changed the compiler name, so now the build system
> realizes that the old build instructions are stale.
>
> Which is why you have to:
>
>>> 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.
>
> Literally just do something like this:
>
>  - have a shell script call "mycompiler" and make it do gcc/llvm "$@".
>
>  - or even just use a symlink (the script has the advantage that you
> can play with the options etc too)
>
>  - change the shell script (or symlink) itself, and make sure to use
> the same CC for "make" at all times, so that the build script never
> sees that the underlying command is now different.
>
> It should work fine, I've done it a couple of times (although
> admittedly not recently)
>

OK, I have created a mycompiler shell-script and use that for CC and
HOSTCC in my own build-script.

Using CLANG...

[ /usr/bin/mycompiler ]

#!/bin/bash

clang "$@"
- EOF -

$ mycompiler --version
clang version 3.7.0 (tags/RELEASE_370/final)
Target: x86_64-unknown-linux-gnu
Thread model: posix

Switching to GCC...

[ /usr/bin/mycompiler ]

#!/bin/bash

gcc-4.9 "$@"
- EOF -

$ mycompiler --version
gcc-4.9 (Ubuntu 4.9.2-0ubuntu1~12.04) 4.9.2
Copyright (C) 2014 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Thanks, that helped me a lot.

- Sedat -
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