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

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Date:	Mon, 14 Sep 2015 10:13:22 +0800
From:	Fengguang Wu <fengguang.wu@...el.com>
To:	Sedat Dilek <sedat.dilek@...il.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Use (two) different compilers at build-time?

On Sun, Sep 13, 2015 at 05:21:41PM +0200, Sedat Dilek wrote:
> On Thu, Sep 10, 2015 at 2:25 AM, Fengguang Wu <fengguang.wu@...el.com> wrote:
> > On Mon, Sep 07, 2015 at 09:12:58PM +0200, Sedat Dilek wrote:
> >> Hi,
> >>
> >> is it possible to use a different compiler at build-time?
> >
> > btw, it'd be great if clang can just work on mainline kernel.
> >
> 
> I am not a member of that LLVMLinux team, but they upstreamed a lot of patches.
> 
> > I tried to run clang in 0day kbuild tests, however make aborts
> > quickly in seconds. There are dozens of clang patches provided in
> >
> >     http://llvm.linuxfoundation.org/index.php/Main_Page
> >
> > however such our-of-tree patches are not bisect friendly.
> >
> 
> I agree, there is a high potential for improvements in LLVMLinux :-).
> 
> Unfortunately, I struggled here some days with $COMPILER's
> inline-optimization (disable | force| noinline) and it turned out to
> be a "known" x86-hweight issue (a patch was archived and thrown away
> from the series of x86 patches).
> 
> Finally, I have a "buildable" and "running-on-bare-metal"
> llvmlinux-patched Linux v4.2 here on Ubuntu/precise AMD64.

That's great!

> It is everytime interesting to see where the root cause sits.
> It's like in everyday life - never assume - just ask - stay curious
> and attentive :-).

If upstream kernels can just work (at least work for one kconfig),
I'll be able to follow up add run regular 0day build tests for LLVM.
Which should guarantee new break ups are detected ASAP and hopefully
the developer that breaks it may help fix it up "automatically". :-)

Thanks,
Fengguang
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