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

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Date:	Tue, 8 Sep 2015 07:55:34 +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 Tue, Sep 8, 2015 at 7:45 AM, Sedat Dilek <sedat.dilek@...il.com> wrote:
> On Tue, Sep 8, 2015 at 12:30 AM, Sedat Dilek <sedat.dilek@...il.com> wrote:
>> 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.
>>
>
> So, the problem is any change to your make-lines.
> Means $COMPILER and/or compiler flags!
>
> I simplified in Makefile...
>
> COMPILER := clang
> export COMPILER
>
> and then did a symlink gcc-4.9 -> clang.
>
> This did NOT work because clang uses a compiler-flag
> '-no-integrated-as' which does not exist for gcc!
>
> So, switching from gcc -> clang or vice-versa is not possible with
> your compiler-wrapper-script trick :-(.
>
> Anyway, I need a different solution.
>
> One of my ideas was to hack the mm/Makefile.
>
> --- a/mm/Makefile
> +++ b/mm/Makefile
> @@ -2,6 +2,9 @@
>  # Makefile for the linux memory manager.
>  #
>
> +COMPILER := gcc
> +export COMPILER
> +
>  KASAN_SANITIZE_slab_common.o := n
>  KASAN_SANITIZE_slub.o := n
>
> @@ -78,3 +81,6 @@ obj-$(CONFIG_CMA)     += cma.o
>  obj-$(CONFIG_MEMORY_BALLOON) += balloon_compaction.o
>  obj-$(CONFIG_PAGE_EXTENSION) += page_ext.o
>  obj-$(CONFIG_CMA_DEBUGFS) += cma_debug.o
> +
> +COMPILER := clang
> +export COMPILER
>
> Not sure if this works.
>

This does not work.

And if it had worked - it cannot due to passing invalid compiler-flags to gcc.

Empty head.

- Sedat -
--
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