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

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Message-ID: <20190319101123.GX6058@hirez.programming.kicks-ass.net>
Date:   Tue, 19 Mar 2019 11:11:23 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     torvalds@...ux-foundation.org, tglx@...utronix.de, hpa@...or.com,
        julien.thierry@....com, will.deacon@....com, luto@...capital.net,
        mingo@...nel.org, catalin.marinas@....com, james.morse@....com,
        valentin.schneider@....com, brgerst@...il.com, luto@...nel.org,
        bp@...en8.de, dvlasenk@...hat.com, linux-kernel@...r.kernel.org,
        dvyukov@...gle.com, rostedt@...dmis.org
Subject: Re: [PATCH 02/25] tracing: Improve "if" macro code generation

On Mon, Mar 18, 2019 at 06:37:20PM -0500, Josh Poimboeuf wrote:
> On Mon, Mar 18, 2019 at 04:38:42PM +0100, Peter Zijlstra wrote:
> > 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.
> > 
> > Make it easier for GCC to optimize by keeping the conditional as a
> > conditional rather than converting it to an integer.  This shrinks the
> > generated code quite a bit, and also makes the code sane enough for
> > objtool to understand.
> > 
> > Cc: rostedt@...dmis.org
> > Cc: valentin.schneider@....com
> > Cc: luto@...capital.net
> > Cc: brgerst@...il.com
> > Cc: catalin.marinas@....com
> > Cc: mingo@...nel.org
> > Cc: dvlasenk@...hat.com
> > Cc: will.deacon@....com
> > Cc: julien.thierry@....com
> > Cc: torvalds@...ux-foundation.org
> > Cc: dvyukov@...gle.com
> > Cc: bp@...en8.de
> > Cc: tglx@...utronix.de
> > Cc: james.morse@....com
> > Cc: luto@...nel.org
> > Cc: hpa@...or.com
> > Reported-by: Peter Zijlstra <peterz@...radead.org>
> > Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > Link: https://lkml.kernel.org/r/20190307174802.46fmpysxyo35hh43@treble
> 
> Missing "From" me.

Right, sorry, quilt eats that and I sometimes forget to fix up :/