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

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Message-ID: <20190509185701.hocym3zlc4x4pmx5@treble>
Date:   Thu, 9 May 2019 13:57:01 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Randy Dunlap <rdunlap@...radead.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        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 11:48:43AM -0700, Randy Dunlap wrote:
> On 5/9/19 11:47 AM, Josh Poimboeuf wrote:
> > On Thu, May 09, 2019 at 01:45:31PM -0500, Josh Poimboeuf wrote:
> >> 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.
> > 
> > /me curses URL encoding
> > 
> > https://lkml.kernel.org/r/201905040509.iqQ2CrOU%25lkp@intel.com
> > 
> 
> Still fails for me.

Sorry, another URL fail.  It wasn't on lkml.  I should actually try
clicking my own links.

https://lists.01.org/pipermail/kbuild-all/2019-May/060554.html

-- 
Josh