On 11/21/2017 07:56 AM, Richard Biener wrote:
>>
>>
>> I considered trying to key behavior on EDGE_DFS_BACK (6->8). It'd be
>> something like don't record equivalences from a degenerate PHI where the
>> remaining edge(s) are EDGE_DFS_BACK. But I just couldn't convince
>> myself that was actually reasonable or sufficient in general.
>
> The key point is that you can never record "symbolic" equivalences on
> backedges. Because in SSA you rely on PHIs to fence SSA names from
> leaking cross iterations. That is, _1 in iteration 1 is obviously not equal
> to _1 in iteration 2.
>
> Other passes like VRP and value-numbering need to take care of this
> as well.
>
> You can of course record that _1 is 2 (constant) even across a backedge.
>
> So I think your consideration was exactly correct.
So finally getting back to this. Thankfully I was able to find an old
branch, revert the tree-ssa-dom.c bits and via some on-the-fly hackery
trigger the issue again.
This also enabled me to verify that triggering on EDGE_DFS_BACK would in
fact resolve this issue.
So attached are two patches. The first is the reversion. The second is
the change to avoid recording the equivalence for a backedge in the CFG.
Bootstrapped and regression tested on x86_64. Also verified by hand
(using that old branch) that we wouldn't get into the infinite loop if
we reverted and used EDGE_DFS_BACK to trigger ignoring the equivalence.
Installing on the trunk.
Jeff
commit 46a2536a74d7f6981a02da05041885d097d9e23c
Author: Jeff Law <l...@torsion.usersys.redhat.com>
Date: Mon Dec 18 19:18:47 2017 -0500
Revert
2017-11-19 Jeff Law <l...@redhat.com>
* tree-ssa-dom.c (record_equivalences_from_phis): Fix handling
of degenerates resulting from ignoring an edge.
diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index d165b5ccf41..b5854f7a67c 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,11 @@
+2017-12-18 Jeff Law <l...@redhat.com>
+
+ Revert
+ 2017-11-19 Jeff Law <l...@redhat.com>
+
+ * tree-ssa-dom.c (record_equivalences_from_phis): Fix handling
+ of degenerates resulting from ignoring an edge.
+
2017-12-18 Martin Sebor <mse...@redhat.com>
PR middle-end/83373
diff --git a/gcc/tree-ssa-dom.c b/gcc/tree-ssa-dom.c
index 93c992a9215..05bc807071f 100644
--- a/gcc/tree-ssa-dom.c
+++ b/gcc/tree-ssa-dom.c
@@ -1109,7 +1109,6 @@ record_equivalences_from_phis (basic_block bb)
tree rhs = NULL;
size_t i;
- bool ignored_phi_arg = false;
for (i = 0; i < gimple_phi_num_args (phi); i++)
{
tree t = gimple_phi_arg_def (phi, i);
@@ -1120,14 +1119,10 @@ record_equivalences_from_phis (basic_block bb)
if (lhs == t)
continue;
- /* We want to track if we ignored any PHI arguments because
- their associated edges were not executable. This impacts
- whether or not we can use any equivalence we might discover. */
+ /* If the associated edge is not marked as executable, then it
+ can be ignored. */
if ((gimple_phi_arg_edge (phi, i)->flags & EDGE_EXECUTABLE) == 0)
- {
- ignored_phi_arg = true;
- continue;
- }
+ continue;
t = dom_valueize (t);
@@ -1152,15 +1147,9 @@ record_equivalences_from_phis (basic_block bb)
a useful equivalence. We do not need to record unwind data for
this, since this is a true assignment and not an equivalence
inferred from a comparison. All uses of this ssa name are dominated
- by this assignment, so unwinding just costs time and space.
-
- Note that if we ignored a PHI argument and the resulting equivalence
- is SSA_NAME = SSA_NAME. Then we can not use the equivalence as the
- uses of the LHS SSA_NAME are not necessarily dominated by the
- assignment of the RHS SSA_NAME. */
+ by this assignment, so unwinding just costs time and space. */
if (i == gimple_phi_num_args (phi)
- && may_propagate_copy (lhs, rhs)
- && (!ignored_phi_arg || TREE_CODE (rhs) != SSA_NAME))
+ && may_propagate_copy (lhs, rhs))
set_ssa_name_value (lhs, rhs);
}
}
commit 61046656ff4e832bd1019e90c251b1c4c2c43883
Author: Jeff Law <l...@torsion.usersys.redhat.com>
Date: Mon Dec 18 23:28:37 2017 -0500
* tree-ssa-dom.c (record_equivalences_from_phis): Do not
record symbolic equivalences from backedges in the CFG.
diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index b5854f7a67c..96146e8934f 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,5 +1,8 @@
2017-12-18 Jeff Law <l...@redhat.com>
+ * tree-ssa-dom.c (record_equivalences_from_phis): Do not
+ record symbolic equivalences from backedges in the CFG.
+
Revert
2017-11-19 Jeff Law <l...@redhat.com>
diff --git a/gcc/tree-ssa-dom.c b/gcc/tree-ssa-dom.c
index 05bc807071f..663d07b6fe9 100644
--- a/gcc/tree-ssa-dom.c
+++ b/gcc/tree-ssa-dom.c
@@ -1126,6 +1126,12 @@ record_equivalences_from_phis (basic_block bb)
t = dom_valueize (t);
+ /* If T is an SSA_NAME and its associated edge is a backedge,
+ then quit as we can not utilize this equivalence. */
+ if (TREE_CODE (t) == SSA_NAME
+ && (gimple_phi_arg_edge (phi, i)->flags & EDGE_DFS_BACK))
+ break;
+
/* If we have not processed an alternative yet, then set
RHS to this alternative. */
if (rhs == NULL)
