RE: [PATCH 2/2]middle-end: Support recognition of three-way max/min.

[Tamar Christina <Tamar.Christina@arm.com>]

> > > > When this function replaces the edge it doesn't seem to update the
> > > dominators.
> > > > Since It's replacing the middle BB we then end up with an error
> > > >
> > > > gcc/testsuite/gcc.dg/tree-ssa/minmax-14.c:17:1: error: dominator
> > > > of 5 should be 4, not 2
> > > >
> > > > during early verify. So instead, I replace the BB but defer its
> > > > deletion until cleanup which removes it and updates the dominators.
> > >
> > > Hmm, for a diamond shouldn't you replace
> > >
> > >   if (EDGE_SUCC (cond_block, 0)->dest == bb)
> > >     edge_to_remove = EDGE_SUCC (cond_block, 1);
> > >   else
> > >     edge_to_remove = EDGE_SUCC (cond_block, 0);
> > >
> > > with
> > >
> > >   if (EDGE_SUCC (cond_block, 0)->dest == bb)
> > >     edge_to_remove = EDGE_SUCC (cond_block, 1);
> > >   else if (EDGE_SUCC (cond_block, 1)->dest == bb)
> > >     edge_to_remove = EDGE_SUCC (cond_block, 0);
> > >
> > > thus, the code expects to be left with a fallthru to the PHI block
> > > which is expected to have the immediate dominator being cond_block
> > > but with a diamond there's a (possibly empty) block inbetween and
> > > dominators are wrong.
> >
> > Agreed, but the (EDGE_SUCC (cond_block, 1)->dest == bb) doesn't seem
> > like the Right one since for a diamond there will be a block in
> > between the two.  Did you perhaps mean  EDGE_SUCC (EDGE_SUCC
> > (cond_block, 1)->dest, 0)->dest == bb? i.e. that that destination across the
> diamond be bb, and then you remove the middle block?
> 
> Hmm, I think my condition was correct - the code tries to remove the edge to
> the middle-block and checks the remaining edge falls through to the merge
> block.  With a true diamond there is no fallthru to the merge block to keep so
> we better don't remove any edge?
> 
> > For the minmax diamond we want both edges removed, since all the code
> > in the middle BBs are now dead.  But this is probably not true in the general
> sense.

Ah! Sorry I was firing a few cylinders short, I get what you mean now:

@@ -425,8 +439,19 @@ replace_phi_edge_with_variable (basic_block cond_block,
   edge edge_to_remove;
   if (EDGE_SUCC (cond_block, 0)->dest == bb)
     edge_to_remove = EDGE_SUCC (cond_block, 1);
-  else
+  else if (EDGE_SUCC (cond_block, 1)->dest == bb)
     edge_to_remove = EDGE_SUCC (cond_block, 0);
+  else
+    {
+      /* If neither edge from the conditional is the final bb
+        then we must have a diamond block, in which case
+        the true edge was changed by SET_USE above and we must
+        mark the other edge as the false edge.  */
+      gcond *cond = as_a <gcond *> (last_stmt (cond_block));
+      gimple_cond_make_false (cond);
+      return;
+    }
+

Bootstrapped Regtested on aarch64-none-linux-gnu and no issues.

Ok with this Change?

Thanks,
Tamar