On Mon, Jul 26, 2021 at 4:18 PM Richard Sandiford
<richard.sandiford@arm.com> wrote:
>
> Aldy Hernandez <aldyh@redhat.com> writes:
> > This patch replaces the evrp_range_analyzer in the loop versioning code
> > with an on-demand ranger.
> >
> > Everything was pretty straightforward, except that range_of_expr requires
> > a gimple statement as context to provide context aware ranges.  I didn't see
> > a convient place where the statement was saved, so I made a vector indexed
> > by SSA names.  As an alternative, I tried to use the loop's first statement,
> > but that proved to be insufficient.
>
> The mapping is one-to-many though: there can be multiple statements
> for each SSA name.  Maybe that doesn't matter in this context and
> any of the statements can act as a representative.
>
> I'm surprised that the loop's first statement didn't work though,
> since the SSA name is supposedly known to be loop-invariant.  What went
> wrong when you tried that?

I was looking at the first statement of loop_info->block_list and one
of the dg.exp=loop-versioning* tests failed.  Perhaps I should have
used the loop itself, as in the attached patch.  With this patch all
of the loop-versioning tests pass.

>
> > I am not familiar with loop versioning, but if the DOM walk was only
> > necessary for the calls to record_ranges_from_stmt, this too could be
> > removed as the ranger will work without it.
>
> Yeah, that was the only reason.  If the information is available at
> version_for_unity (I guess it is) then we should just avoid recording
> the versioning there if so.
>
> How expensive is the check?  If the result is worth caching, perhaps
> we should have two bitmaps: the existing one, and one that records
> whether we've checked a particular SSA name.
>
> If the check is relatively cheap then that won't be worth it though.

If you're asking about the range_of_expr check, that's all cached, so
it should be pretty cheap.  Besides, we're no longer calculating
ranges for each statement in the IL, as we were doing in lv_dom_walker
with evrp's record_ranges_from_stmt.  Only statements of interest are
queried.

How about this patch, pending tests?

Aldy