linux-kernel - Re: [PATCH 02/25] tracing: Improve "if" macro code generation

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Date:   Thu, 9 May 2019 12:44:03 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        David Laight <David.Laight@...lab.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "hpa@...or.com" <hpa@...or.com>,
        "julien.thierry@....com" <julien.thierry@....com>,
        "will.deacon@....com" <will.deacon@....com>,
        "luto@...capital.net" <luto@...capital.net>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "james.morse@....com" <james.morse@....com>,
        "valentin.schneider@....com" <valentin.schneider@....com>,
        "brgerst@...il.com" <brgerst@...il.com>,
        "luto@...nel.org" <luto@...nel.org>, "bp@...en8.de" <bp@...en8.de>,
        "dvlasenk@...hat.com" <dvlasenk@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dvyukov@...gle.com" <dvyukov@...gle.com>
Subject: Re: [PATCH 02/25] tracing: Improve "if" macro code generation

On Thu, May 9, 2019 at 12:28 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> But it probably would probably still be good to know why this fixes the
> issues you see.

Looks like a compiler bug, plain and simple.

The simplified case really simplifies a lot for the internal IR, and
has a single conditional that then increments a constant location and
returns a constant value. That means that any branch following code in
the compiler has a much simpler setup, and doesn't need to try to
follow any chain of condirional branches with the same conditional to
generate sane code.

But the code generation problem is clearly a compiler bug, although
I'd not be surprised if it's also triggered by whatever horrid things
ASAN does internally to gcc state.

                 Linus