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

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Message-ID: <20190509184531.jhinxi2x2pdfaefb@treble>
Date:   Thu, 9 May 2019 13:45:31 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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 09, 2019 at 02:29:02PM -0400, Steven Rostedt wrote:
> On Thu, 9 May 2019 09:51:59 -0700
> Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
> > On Thu, May 9, 2019 at 6:01 AM Steven Rostedt <rostedt@...dmis.org> wrote:
> > >
> > > This patch works. Can I get your Signed-off-by for it?  
> > 
> > Yes. Please write some kind of comprehensible commit log for it, but
> 
> How's this:
> 
> "Peter Zijlstra noticed that with CONFIG_PROFILE_ALL_BRANCHES, the "if"
> macro converts the conditional to an array index.  This can cause GCC
> to create horrible code.  When there are nested ifs, the generated code
> uses register values to encode branching decisions.
> 
> Josh Poimboeuf found that replacing the define "if" macro from using
> the condition as an array index and incrementing the branch statics
> with an if statement itself, reduced the asm complexity and shrinks the
> generated code quite a bit.
> 
> But this can be simplified even further by replacing the internal if
> statement with a ternary operator.
> 
> Reported-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Reported-by: Josh Poimboeuf <jpoimboe@...hat.com>

Actually, my original fix already went in:

  37686b1353cf ("tracing: Improve "if" macro code generation")

But it introduced a regression:

  https://lkml.kernel.org/r/201905040509.iqQ2CrOU%lkp@intel.com

which Linus' patch fixes for some reason.

-- 
Josh