On Mon, 30 Nov 2015, Marek Polacek wrote:
> On Sat, Nov 28, 2015 at 08:50:12AM +0100, Richard Biener wrote:
> > Different approach: after the FE folds (unexpectedly?), scan the result for
> > SAVE_EXPRs and if found, drop the folding.
>
> Neither this fixes this problem completely, because we simply don't know where
> those SAVE_EXPRs might be introduced: it might be convert(), but e.g. when I
> changed the original testcase a tiny bit (added -), then those SAVE_EXPRs were
> introduced in a different spot (via c_process_stmt_expr -> c_fully_fold).
So the following "disables" save_expr generation from generic-match.c
by failing to simplify if save_expr would end up not returning a
non-save_expr.
I expect this will make fixing PR68590 difficult (w/o re-introducing
some fold-const.c code or changing genmatch to "special-case"
things).
The other option for this PR is to re-introduce the TREE_SIDE_EFFECTS
check I removed earlier (to avoid un-CSEing large expressions at
-O0 for example) and thus only FAIL if the save_expr were needed
for correctness.
Richard.
Index: gcc/tree.c
===================================================================
--- gcc/tree.c (revision 231065)
+++ gcc/tree.c (working copy)
@@ -3231,8 +3231,6 @@ decl_address_ip_invariant_p (const_tree
not handle arithmetic; that's handled in skip_simple_arithmetic and
tree_invariant_p). */
-static bool tree_invariant_p (tree t);
-
static bool
tree_invariant_p_1 (tree t)
{
@@ -3282,7 +3280,7 @@ tree_invariant_p_1 (tree t)
/* Return true if T is function-invariant. */
-static bool
+bool
tree_invariant_p (tree t)
{
tree inner = skip_simple_arithmetic (t);
Index: gcc/tree.h
===================================================================
--- gcc/tree.h (revision 231065)
+++ gcc/tree.h (working copy)
@@ -4320,6 +4320,10 @@ extern tree staticp (tree);
extern tree save_expr (tree);
+/* Return true if T is function-invariant. */
+
+extern bool tree_invariant_p (tree);
+
/* Look inside EXPR into any simple arithmetic operations. Return the
outermost non-arithmetic or non-invariant node. */
Index: gcc/genmatch.c
===================================================================
--- gcc/genmatch.c (revision 231065)
+++ gcc/genmatch.c (working copy)
@@ -3106,7 +3106,9 @@ dt_simplify::gen_1 (FILE *f, int indent,
else if (is_a <predicate_id *> (opr))
is_predicate = true;
/* Search for captures used multiple times in the result expression
- and dependent on TREE_SIDE_EFFECTS emit a SAVE_EXPR. */
+ and check if we can safely evaluate it multiple times. Otherwise
+ fail, avoiding a SAVE_EXPR because that confuses the C FE
+ const expression folding. */
if (!is_predicate)
for (int i = 0; i < s->capture_max + 1; ++i)
{
@@ -3114,8 +3116,8 @@ dt_simplify::gen_1 (FILE *f, int indent,
continue;
if (cinfo.info[i].result_use_count > 1)
fprintf_indent (f, indent,
- "captures[%d] = save_expr (captures[%d]);\n",
- i, i);
+ "if (! tree_invariant_p (captures[%d])) "
+ "return NULL_TREE;\n", i);
}
for (unsigned j = 0; j < e->ops.length (); ++j)
{
