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

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Date:	Mon, 14 Sep 2015 09:35:04 +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>,
	llvmlinux@...ts.linuxfoundation.org
Subject: Re: [llvmlinux] percpu | bitmap issue? (Cannot boot on bare metal due
 to a kernel NULL pointer dereference)

On Mon, Sep 14, 2015 at 9:12 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Sun, Sep 13, 2015 at 04:33:39AM +0200, Sedat Dilek wrote:
>> > It looks like an inline-optimization bug in CLANG when the compiler's
>> > optimization-level is higher than -O2.
>
>> > [1] http://lists.linuxfoundation.org/pipermail/llvmlinux/2015-September/001355.html
>>
>> After some discussion on #llvm it turned out to be a known issue in LLVMLinux!
>>
>> Unfortunately, an existing patch [1] got archived which is still
>> required to build x86_64 correctly.
>
>> [1] http://git.linuxfoundation.org/?p=llvmlinux.git;a=blob_plain;f=arch/x86_64/patches/ARCHIVE/0029-Fix-ARCH_HWEIGHT-for-compilation-with-clang.patch;hb=HEAD
>
> As long as LLVM cannot do things like that and requires full function
> calls I cannot see it being a sensible compiler to use from a
> performance POV.
>
> There's a fairly large difference between an inline POPCNT instruction
> and a full out-of-line function call.
>
> /me goes back to ignoring LLVM for the time being.

[ CC llvmlinux ML ]

Hi Peter,

Clear words and a clear statement!
( Men want clear statements see [1]. )

The issue was reported as LLVM PR #9457 [1] and the bug-history [2] says...

   Reported: 2011-03-11 07:27 CST by PaX Team

...so it is a known and old issue.

Cannot say if "ignoring" is really helpful or not :-).
Sanjoy Das (a LLVM/Clang developer) joined the vital discussion in the
thread (see [3])...

"[llvmlinux] [PATCH] x86/hweight: LLVMLinux: Fix __arch_hweight{32,
64}() for compilation with clang"

Beyond "fixing" or implementing missing (GCC) features on the LLVM-Clang-side...
People there have some discussion and ideas on a "workaround" or fix
it on the Linux-kernel-side.

Ignoring does not help - ignorance is a hard (hardest?) punishment for men.

I invite you to join the discussion at LLVMLinux...
As I know... "YES, you can." Linux x86/x86_64 (assembler) Kung-Fu.
( I admit I have not these skillz. )

Regards,
- Sedat -

[1] https://llvm.org/bugs/show_bug.cgi?id=9457
[2] https://llvm.org/bugs/show_activity.cgi?id=9457
[3] [1] http://lists.linuxfoundation.org/pipermail/llvmlinux/2015-September/001369.html
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