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

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Message-ID: <CA+icZUWGmnYq8JjpuPF8M96V1u0gKBwo-soWqE_eKBgFdwXFnQ@mail.gmail.com>
Date:	Tue, 8 Sep 2015 00:30:07 +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 11:20 PM, Sedat Dilek <sedat.dilek@...il.com> wrote:
> 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.
>

Sadly, this trick does not work here with Linux v4.2.

- Sedat -
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