Re: [PATCH] Properly honor param_max_fsm_thread_path_insns in backwards threader

[Aldy Hernandez <aldyh@redhat.com>]

Feel free to blame me for everything except the profitability code and the
generic block copier. That stuff was all there before and I mostly avoided
it.

:-)

Thanks for the work in this space.
Aldy

On Tue, Aug 2, 2022, 15:29 Richard Biener <rguenther@suse.de> wrote:

> On Tue, 2 Aug 2022, Aldy Hernandez wrote:
>
> > On Tue, Aug 2, 2022 at 1:59 PM Richard Biener <rguenther@suse.de> wrote:
> > >
> > > On Tue, 2 Aug 2022, Aldy Hernandez wrote:
> > >
> > > > On Tue, Aug 2, 2022 at 1:45 PM Richard Biener <rguenther@suse.de>
> wrote:
> > > > >
> > > > > On Tue, 2 Aug 2022, Aldy Hernandez wrote:
> > > > >
> > > > > > Unfortunately, this was before my time, so I don't know.
> > > > > >
> > > > > > That being said, thanks for tackling these issues that my work
> > > > > > triggered last release.  Much appreciated.
> > > > >
> > > > > Ah.  But it was your r12-324-g69e5544210e3c0 that did
> > > > >
> > > > > -  else if (n_insns > 1)
> > > > > +  else if (!m_speed_p && n_insns > 1)
> > > > >
> > > > > causing the breakage on the 12 branch.  That leads to a simpler
> > > > > fix I guess.  Will re-test and also backport to GCC 12 if
> successful.
> > > >
> > > > Huh.  It's been a while, but that looks like a typo.  That patch was
> > > > supposed to be non-behavior changing.
> > >
> > > Exactly my thinking so reverting it shouldn't be a reason for
> > > detailed questions.  Now, the contains_hot_bb computation is,
> >
> > Sorry for the pain.
>
> So - actually the change was probably done on purpose (even if
> reverting - which I've now already one - caused no testsuite regressions).
> That's because the whole function is invoked N + 1 times for a path
> of length N and we definitely want to avoid using the size optimization
> heuristics when the path is not complete yet.  I think the proper
> way is to do
>
> diff --git a/gcc/tree-ssa-threadbackward.cc
> b/gcc/tree-ssa-threadbackward.cc
> index ba114e98a41..6979398ef76 100644
> --- a/gcc/tree-ssa-threadbackward.cc
> +++ b/gcc/tree-ssa-threadbackward.cc
> @@ -767,7 +767,11 @@ back_threader_profitability::profitable_path_p (const
> vec<basic_block> &m_path,
>       as in PR 78407 this leads to noticeable improvements.  */
>    if (m_speed_p
>        && ((taken_edge && optimize_edge_for_speed_p (taken_edge))
> -         || contains_hot_bb))
> +         || contains_hot_bb
> +         /* Avoid using the size heuristics when not doing the final
> +            thread evaluation, we get called for each added BB
> +            to the path.  */
> +         || !taken_edge))
>      {
>        if (n_insns >= param_max_fsm_thread_path_insns)
>         {
>
> thus assume there'll be a hot BB in the future.
>
> That said, the very best fix would be to not call this function
> N + 1 times (I have a patch to call it only N times - yay), but
> instead factor out parts to be called per BB plus keeping enough
> state so we can incrementally collect info.
>
> There's more "odd" things in the backward threader, of course :/
>
> I'm looking for things applicable to the GCC 12 branch right now
> so will try the above.
>
> Richard.
>
>