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

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Message-ID: <20150914071231.GM18489@twins.programming.kicks-ass.net>
Date:	Mon, 14 Sep 2015 09:12:31 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Sedat Dilek <sedat.dilek@...il.com>
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 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.
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