On Wed, 3 Aug 2022, Tamar Christina wrote:
>
> > -----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
I meant to merge the keep_edge and the existing else case by setting
keep_edge the obvious way in the other two if cases. Sorry for not
being clear ...
OK with that change.
> @@ -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
>
--
Richard Biener <rguent...@suse.de>
SUSE Software Solutions Germany GmbH, Frankenstrasse 146, 90461 Nuernberg,
Germany; GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman;
HRB 36809 (AG Nuernberg)
