Re: [PATCH 02/25] tracing: Improve "if" macro code generation
From: Josh Poimboeuf
Date: Thu May 09 2019 - 15:07:03 EST
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@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:
> which Linus' patch fixes for some reason.
/me curses URL encoding