> -----Original Message-----
> From: Richard Biener <richard.guent...@gmail.com>
> Sent: Tuesday, August 2, 2022 10:11 AM
> To: Tamar Christina <tamar.christ...@arm.com>
> Cc: Richard Biener <rguent...@suse.de>; ja...@redhat.com; nd
> <n...@arm.com>; gcc-patches@gcc.gnu.org
> Subject: Re: [PATCH 2/2]middle-end: Support recognition of three-way
> max/min.
>
> On Tue, Aug 2, 2022 at 10:33 AM Tamar Christina via Gcc-patches <gcc-
> patc...@gcc.gnu.org> wrote:
> >
> > > > > > 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;
> > + }
> > +
>
> Note there is already
>
> if (EDGE_COUNT (edge_to_remove->dest->preds) == 1)
> {
> ...
> }
> else
> {
> /* If there are other edges into the middle block make
> CFG cleanup deal with the edge removal to avoid
> updating dominators here in a non-trivial way. */
> gcond *cond = as_a <gcond *> (last_stmt (cond_block));
> if (edge_to_remove->flags & EDGE_TRUE_VALUE)
> gimple_cond_make_false (cond);
> else
> gimple_cond_make_true (cond);
> }
>
> I'm not sure how you can say 'e' is always the true edge? May I suggest to
> amend the first condition with edge_to_remove && (and initialize that to
> NULL) and use e->flags instead of edge_to_remove in the else, of course
> also inverting the logic since we're keeping 'e'?
As discussed on IRC, here's the version using keep_edge:
@@ -422,12 +436,17 @@ replace_phi_edge_with_variable (basic_block cond_block,
SET_USE (PHI_ARG_DEF_PTR (phi, e->dest_idx), new_tree);
/* Remove the empty basic block. */
- edge edge_to_remove;
+ edge edge_to_remove = NULL, keep_edge = NULL;
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);
- if (EDGE_COUNT (edge_to_remove->dest->preds) == 1)
+ else if ((keep_edge = find_edge (cond_block, e->src)))
+ ;
+ else
+ gcc_unreachable ();
+
+ if (edge_to_remove && EDGE_COUNT (edge_to_remove->dest->preds) == 1)
{
e->flags |= EDGE_FALLTHRU;
e->flags &= ~(EDGE_TRUE_VALUE | EDGE_FALSE_VALUE);
@@ -438,6 +457,18 @@ replace_phi_edge_with_variable (basic_block cond_block,
gsi = gsi_last_bb (cond_block);
gsi_remove (&gsi, true);
}
+ else if (keep_edge)
+ {
+ /* If we're in a diamond then we have identified the edge
+ that we want to keep. Since the dominators will require
+ updating in a non-trivial way we leave it to CFG cleanup
+ but mark the condition as appropriately true/false. */
+ gcond *cond = as_a <gcond *> (last_stmt (cond_block));
+ if (keep_edge->flags & EDGE_FALSE_VALUE)
+ gimple_cond_make_false (cond);
+ else if (keep_edge->flags & EDGE_TRUE_VALUE)
+ gimple_cond_make_true (cond);
+ }
else
{
/* If there are other edges into the middle block make
@@ -1733,15 +1764,52 @@ value_replacement (basic_block cond_bb, basic_block
middle_bb,
return 0;
}
Bootstrapped Regtested on aarch64-none-linux-gnu and no issues.
Ok with change?
Thanks,
Tamar
>
> > Bootstrapped Regtested on aarch64-none-linux-gnu and no issues.
> >
> > Ok with this Change?
> >
> > Thanks,
> > Tamar
