Hello,
On Thu, 9 Sep 2021, Aldy Hernandez wrote:
> > Here there's no simple latch block to start with (the backedge comes
> > directly out of the loop exit block). So my suggested improvement
> > (testing if the latch was empty and only then reject the thread), would
> > solve this.
>
> Well, there's the thing. Loop discovery is marking BB5 as the latch, so
> it's not getting threaded:
Yes, it's a latch, but not an empty one. So the thread would make it just
even more non-empty, which might not be a problem anymore. So amending my
patch somewhere with a strategic
&& empty_block_p (latch)
and only then rejecting the thread should make this testcase work again.
(There's still a catch, though: if this non-empty latch, which is also the
exit test block, is threaded through and is followed by actual code, then
that code will be inserted onto the back edge, not into the latch block
before the exit test, and so also create a (new) non-empty latch. That
might or might not create problems downstreams, but not as many as
transformaing an empty into a non-empty latch would do; but it could
create for instance a second back edge (not in this testcase) and
suchlike)
> BTW, I'm not sure your check for the non-last position makes a difference:
>
> > diff --git a/gcc/tree-ssa-threadbackward.c b/gcc/tree-ssa-threadbackward.c
> > index 449232c7715..528a753b886 100644
> > --- a/gcc/tree-ssa-threadbackward.c
> > +++ b/gcc/tree-ssa-threadbackward.c
> > - threaded_through_latch = true;
> > + {
> > + threaded_through_latch = true;
> > + if (j != 0)
> > + latch_within_path = true;
> > + if (dump_file && (dump_flags & TDF_DETAILS))
> > + fprintf (dump_file, " (latch)");
> > + }
> > }
>
> If the last position being considered is a simple latch, it only has a simple
> outgoing jump. There's no need to thread that. You need a block with >= 2
> succ edges to thread anything.
So, you are saying that any candidate thread path wouldn't have the latch
in the last position if it were just an empty forwarder? I simply wasn't
sure about that, so was conservative and only wanted to reject things I
knew where positively bad (namely code in the path following the latch
that is in danger of being moved into the latch).
Ciao,
Michael.
