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Date: Thu, 16 Nov 2017 17:30:54 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Sami Tolvanen <samitolvanen@google.com>
Cc: Will Deacon <will.deacon@arm.com>, Alex Matveev <alxmtvv@gmail.com>,
        Andi Kleen <ak@linux.intel.com>,
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Subject: Re: [PATCH v2 18/18] arm64: select ARCH_SUPPORTS_LTO_CLANG
Message-ID: <20171116163054.kcsdsomr7u2mqql2@hirez.programming.kicks-ass.net>
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On Thu, Nov 16, 2017 at 08:17:31AM -0800, Sami Tolvanen wrote:
> On Thu, Nov 16, 2017 at 11:58:11AM +0000, Will Deacon wrote:
> > I'll be honest with you: I'm absolutely terrified about enabling this.
> 
> That's understandable, I wouldn't want to enable this by default
> quite yet either. This patch doesn't enable LTO for arm64, just makes
> it possible to enable the feature. I'm perfectly fine with marking
> CONFIG_LTO_CLANG experimental if it makes people more comfortable.
> 
> > How much testing has this seen?
> 
> I've been running clang LTO kernels for a few months on a Pixel 2 device
> without any issues. This is on a 4.4 kernel though.
> 
> > Right now, the C standard isn't on our side here and we're relying on
> > the compiler not doing this kind of thing. Can we continue to rely on
> > that in the face of LTO?
> 
> I'll have to check with our LLVM experts, but I have not run into these
> issues with current compiler versions. Looking at Andi's old patches,
> looks like gcc might be more aggressive in reordering things with LTO
> than clang.

Ideally we'd get the toolchain people to commit to supporting the kernel
memory model along side the C11 one. That would help a ton.