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

[Josh Poimboeuf]

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@xxxxxxxxxxxxxxxxxxxx> wrote:
> 
> > On Thu, May 9, 2019 at 6:01 AM Steven Rostedt <rostedt@xxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> Reported-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>

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@xxxxxxxxx

which Linus' patch fixes for some reason.

-- 
Josh