The following deals with the situation where we have
<bb 2> [local count: 1073741824]:
_5 = bytes.D.25336._M_impl.D.24643._M_start;
_6 = bytes.D.25336._M_impl.D.24643._M_finish;
pretmp_66 = bytes.D.25336._M_impl.D.24643._M_end_of_storage;
if (_5 != _6)
goto <bb 3>; [70.00%]
else
goto <bb 4>; [30.00%]
...
<bb 6> [local count: 329045359]:
_89 = operator new (4);
_43 = bytes.D.25336._M_impl.D.24643._M_start;
_Num_44 = _137 - _43;
if (_Num_44 != 0)
but fail to see that _137 is equal to _5 and thus eventually _Num_44
is zero if not operator new would possibly clobber the global
bytes variable.
The following resolves this in value-numbering by using the
predicated values for _5 == _6 recorded for the dominating
condition.
Bootstrapped and tested on x86_64-unknown-linux-gnu, pushed.
PR tree-optimization/107852
* tree-ssa-sccvn.cc (visit_phi): Use equivalences recorded
as predicated values to elide more redundant PHIs.
* gcc.dg/tree-ssa/ssa-fre-101.c: New testcase.
---
gcc/testsuite/gcc.dg/tree-ssa/ssa-fre-101.c | 47 +++++++++++++++++++
gcc/tree-ssa-sccvn.cc | 51 ++++++++++++++++++++-
2 files changed, 97 insertions(+), 1 deletion(-)
create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/ssa-fre-101.c
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-fre-101.c
b/gcc/testsuite/gcc.dg/tree-ssa/ssa-fre-101.c
new file mode 100644
index 00000000000..c67f211dcf6
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-fre-101.c
@@ -0,0 +1,47 @@
+/* { dg-do compile } */
+/* { dg-options "-O -fdump-tree-fre1-details" } */
+
+int test1 (int i, int j)
+{
+ int k;
+ if (i != j)
+ k = i;
+ else
+ k = j;
+ return k;
+}
+
+int test2 (int i, int j)
+{
+ int k;
+ if (i != j)
+ k = j;
+ else
+ k = i;
+ return k;
+}
+
+int test3 (int i, int j)
+{
+ int k;
+ if (i == j)
+ k = j;
+ else
+ k = i;
+ return k;
+}
+
+int test4 (int i, int j)
+{
+ int k;
+ if (i == j)
+ k = i;
+ else
+ k = j;
+ return k;
+}
+
+/* We'd expect 4 hits but since we only keep one forwarder the
+ VN predication machinery cannot record something for the entry
+ block since it doesn't work on edges but on their source. */
+/* { dg-final { scan-tree-dump-times "equal on edge" 2 "fre1" } } */
diff --git a/gcc/tree-ssa-sccvn.cc b/gcc/tree-ssa-sccvn.cc
index 1f9c6c53b52..6895ae84d13 100644
--- a/gcc/tree-ssa-sccvn.cc
+++ b/gcc/tree-ssa-sccvn.cc
@@ -5814,7 +5814,8 @@ visit_phi (gimple *phi, bool *inserted, bool
backedges_varying_p)
/* See if all non-TOP arguments have the same value. TOP is
equivalent to everything, so we can ignore it. */
- FOR_EACH_EDGE (e, ei, gimple_bb (phi)->preds)
+ basic_block bb = gimple_bb (phi);
+ FOR_EACH_EDGE (e, ei, bb->preds)
if (e->flags & EDGE_EXECUTABLE)
{
tree def = PHI_ARG_DEF_FROM_EDGE (phi, e);
@@ -5859,6 +5860,54 @@ visit_phi (gimple *phi, bool *inserted, bool
backedges_varying_p)
&& known_eq (soff, doff))
continue;
}
+ /* There's also the possibility to use equivalences. */
+ if (!FLOAT_TYPE_P (TREE_TYPE (def)))
+ {
+ vn_nary_op_t vnresult;
+ tree ops[2];
+ ops[0] = def;
+ ops[1] = sameval;
+ tree val = vn_nary_op_lookup_pieces (2, EQ_EXPR,
+ boolean_type_node,
+ ops, &vnresult);
+ if (! val && vnresult && vnresult->predicated_values)
+ {
+ val = vn_nary_op_get_predicated_value (vnresult, e->src);
+ if (val && integer_truep (val))
+ {
+ if (dump_file && (dump_flags & TDF_DETAILS))
+ {
+ fprintf (dump_file, "Predication says ");
+ print_generic_expr (dump_file, def, TDF_NONE);
+ fprintf (dump_file, " and ");
+ print_generic_expr (dump_file, sameval, TDF_NONE);
+ fprintf (dump_file, " are equal on edge %d -> %d\n",
+ e->src->index, e->dest->index);
+ }
+ continue;
+ }
+ /* If on all previous edges the value was equal to def
+ we can change sameval to def. */
+ if (EDGE_COUNT (bb->preds) == 2
+ && (val = vn_nary_op_get_predicated_value
+ (vnresult, EDGE_PRED (bb, 0)->src))
+ && integer_truep (val))
+ {
+ if (dump_file && (dump_flags & TDF_DETAILS))
+ {
+ fprintf (dump_file, "Predication says ");
+ print_generic_expr (dump_file, def, TDF_NONE);
+ fprintf (dump_file, " and ");
+ print_generic_expr (dump_file, sameval, TDF_NONE);
+ fprintf (dump_file, " are equal on edge %d -> %d\n",
+ EDGE_PRED (bb, 0)->src->index,
+ EDGE_PRED (bb, 0)->dest->index);
+ }
+ sameval = def;
+ continue;
+ }
+ }
+ }
sameval = NULL_TREE;
break;
}
--
2.35.3
