Re: [PATCH] PR tree-optimization/108697 - Create a lazy ssa_cache

[Richard Biener <richard.guenther@gmail.com>]

On Thu, Feb 16, 2023 at 3:34 PM Andrew MacLeod <amacleod@redhat.com> wrote:
>
>
> On 2/16/23 02:55, Richard Biener wrote:
> > On Wed, Feb 15, 2023 at 6:07 PM Andrew MacLeod via Gcc-patches
> > <gcc-patches@gcc.gnu.org> wrote:
> >> This patch implements the suggestion that we have an alternative
> >> ssa-cache which does not zero memory, and instead uses a bitmap to track
> >> whether a value is currently set or not.  It roughly mimics what
> >> path_range_query was doing internally.
> >>
> >> For sparsely used cases, expecially in large programs, this is more
> >> efficient.  I changed path_range_query to use this, and removed it old
> >> bitmap (and a hack or two around PHI calculations), and also utilized
> >> this is the assume_query class.
> >>
> >> Performance wise, the patch doesn't affect VRP (since that still uses
> >> the original version).  Switching to the lazy version caused a slowdown
> >> of 2.5% across VRP.
> >>
> >> There was a noticeable improvement elsewhere.,  across 230 GCC source
> >> files, threading ran over 12% faster!.  Overall compilation improved by
> >> 0.3%  Not sure it makes much difference in compiler.i, but it shouldn't
> >> hurt.
> >>
> >> bootstraps on x86_64-pc-linux-gnu with no regressions.   OK for trunk?
> >> or do you want to wait for the next release...
> > I see
> >
> > @@ -365,16 +335,8 @@ path_range_query::compute_ranges_in_phis (basic_block bb)
> >
> >         Value_Range r (TREE_TYPE (name));
> >         if (range_defined_in_block (r, name, bb))
> > -       {
> > -         unsigned v = SSA_NAME_VERSION (name);
> > -         set_cache (r, name);
> > -         bitmap_set_bit (phi_set, v);
> > -         // Pretend we don't have a cache entry for this name until
> > -         // we're done with all PHIs.
> > -         bitmap_clear_bit (m_has_cache_entry, v);
> > -       }
> > +       m_cache.set_global_range (name, r);
> >       }
> > -  bitmap_ior_into (m_has_cache_entry, phi_set);
> >   }
> >
> >   // Return TRUE if relations may be invalidated after crossing edge E.
> >
> > which I think is not correct - if we have
> >
> >   # _1 = PHI <..., _2>
> >   # _2 = PHI <..., _1>
> >
> > then their effects are supposed to be executed in parallel, that is,
> > both PHI argument _2 and _1 are supposed to see the "old" version.
> > The previous code tried to make sure the range of the new _1 doesn't
> > get seen when processing the argument _1 in the definition of _2.
> >
> > The new version drops this, possibly resulting in wrong-code.
>
> This is dropped because it is actually handled properly in
> range_defined_in_block now.  (which I think Aldy was describing).
>
> It didnt make sense to me why it was handled here like this, so I traced
> through the call chain to find out if it was still actually needed and
> discussed it with Aldy.  I think it was mostly a leftover wart.

Ah, thanks for checking.

> >
> > While I think it's appropriate to sort out compile-time issues like this
> > during stage4 at least the above makes me think it should be defered
> > to next stage1.
>
> I am happy to defer it since its a marginal increase anyway.

Sure - thus OK for stage1.

Thanks,
Richard.

>
> Andrew
>
>