Question about indirect functions and PGO

[Victor Rodriguez]

On Mon, Jul 13, 2020 at 6:41 AM Erick Ochoa
<erick.ochoa@theobroma-systems.com> wrote:
>
> Hi,
>
> I just wanted to answer myself.
> It seems that there are two thresholds that need to be met if a function
> is to be specialized within a particular context:
>
> 1. --param=hot-bb-count-ws-permille=. This option controls the hotness
> threshold of basic blocks and is needed for function specialization
> during LTO. If a callsite is not in a hot basic block, it seems that the
> callsite will not be specialized for a particular parameter.
> 2. --param=ipa-cp-eval-threshold=. This option controls a heuristic that
> lets constant propagation happen if a function is a good candidate for
> cloning. This parameter is used for both: function specialization within
> a particular context and for all contexts.
>
> On 10/07/2020 13:19, Erick Ochoa wrote:
> > Forgot to mention that these functions take a function pointer as a
> > parameter and as a result, the specialized functions are able to replace
> > the indirect function call with a direct function call.
> >
> > On 10/07/2020 13:17, Erick Ochoa wrote:
> >> Hello,
> >>
> >> I'm working on an optimization and I encountered this interesting
> >> behaviour. There are a couple of functions that are specialized when
> >> the program is not compiled with PGO (-fprofile-generate and
> >> -fprofile-use)
> >>
> >> However, when the program is compiled with PGO the compiler does not
> >> specialize the function calls.
> >>
> >> I printing the program just after materializing all clones.
> >>
> >> I am running this version of GCC:
> >> Author: GCC Administrator <gccadmin@gcc.gnu.org>
> >> Date:   Fri Jul 10 00:16:28 2020 +0000
> >>
> >>      Daily bump.
> >>
> >> I can imagine that the profiling information was used to determine
> >> that specializing these functions is a bad tradeoff between binary
> >> size and speed. But I do not know this for sure. How can I find out
> >> why these functions were not specialized? (I.e. is there a threshold
> >> that wasn't met, and if so, where is it located and what's its value?)
> >>

Have you tried the new option-fprofile-partial-training ?

-fprofile-partial-training can now be used to inform the compiler that
code paths not covered by the training run should not be optimized for
size.

I tested by myself this week and it literally do not optimize at all
the functions not touched by the training

Regards

Victor Rodriguez

> >> Thanks!