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tree-optimization/107852 - missed optimization with PHIs

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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.

	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.
This commit is contained in:
Richard Biener 2022-11-29 12:56:22 +01:00
parent 063ba138ea
commit fd8dd6c038
2 changed files with 97 additions and 1 deletions
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47
gcc/testsuite/gcc.dg/tree-ssa/ssa-fre-101.c Normal file
View File

@ -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" } } */

51
gcc/tree-ssa-sccvn.cc
View File

@ -5793,7 +5793,8 @@ 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);
@ -5838,6 +5839,54 @@ 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;
}