linux-kernel - Re: [PATCH v2 18/18] arm64: select ARCH_SUPPORTS_LTO

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Message-ID: <20171116161731.GA94341@samitolvanen.mtv.corp.google.com>
Date:   Thu, 16 Nov 2017 08:17:31 -0800
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Will Deacon <will.deacon@....com>
Cc:     Alex Matveev <alxmtvv@...il.com>, Andi Kleen <ak@...ux.intel.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Greg Hackmann <ghackmann@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kbuild@...r.kernel.org,
        linux-kernel@...r.kernel.org, Mark Rutland <mark.rutland@....com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Maxim Kuvyrkov <maxim.kuvyrkov@...aro.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Yury Norov <ynorov@...iumnetworks.com>,
        Matthias Kaehlcke <mka@...omium.org>, peterz@...radead.org,
        paulmck@...ux.vnet.ibm.com
Subject: Re: [PATCH v2 18/18] arm64: select ARCH_SUPPORTS_LTO_CLANG

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.

Sami