linux-kernel - Re: [llvmlinux] percpu | bitmap issue? (Cannot boot on bare metal due to a kernel NULL pointer dereference)

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Date:	Sat, 12 Sep 2015 23:22:45 +0200
From:	Sedat Dilek <sedat.dilek@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Baoquan He <bhe@...hat.com>, Denys Vlasenko <dvlasenk@...hat.com>,
	Tejun Heo <tj@...nel.org>, Christoph Lameter <cl@...ux.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Thomas Graf <tgraf@...g.ch>, Ingo Molnar <mingo@...nel.org>,
	"the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: [llvmlinux] percpu | bitmap issue? (Cannot boot on bare metal due
 to a kernel NULL pointer dereference)

On Wed, Sep 9, 2015 at 2:54 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Wed, Sep 09, 2015 at 12:05:50PM +0200, Sedat Dilek wrote:
>> I can boot into a CLANG v3.7 compiled Linux-kernel when lib/bitmap is
>> compiled with GCC (here: v4.9).
>>
>> CONFIG_OPTIMIZE_INLINING has no effect on this.
>>
>> Attached are the single llvmlinux patch for AMD64 (x86_64), my
>> kernel-config and 2 objdumps with bitmap.o file alltogether.
>
> I suggest you go tell the LLVM people their compiler is broken.

It looks like an inline-optimization bug in CLANG when the compiler's
optimization-level is higher than -O2.

Level -O0 and -O1 are OK - I can boot in such Linux-kernel v4.2.

With Level -Oz when CONFIG_CC_OPTIMIZE_FOR_SIZE=y (equivalent to GCC's
-Os) the objdump looked sane but I cannot boot on bare metal.
Re-compiling lib/bitmap with -O0 or -O1 is fine again.

- Sedat -

[1] http://lists.linuxfoundation.org/pipermail/llvmlinux/2015-September/001355.html
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