On December 23, 2017 9:33:03 AM GMT+01:00, Jakub Jelinek <ja...@redhat.com>
wrote:
>Hi!
>
>The problem here is that for loops that have constant 0/false condition
>the C++ FE wants to correctly emit if (0) { body; incr-expr; }
>but doesn't just build3 (COND_EXPR, ...), but fold_build3, and
>COND_EXPR
>folding with constant condition optimizes away the unused branch
>completely
>if it doesn't contain any labels. In this case it doesn't contain any
>labels, but contains CASE_LABEL_EXPR that we should also take into
>account
>(but can ignore it when during the walk we are inside a SWITCH_BODY).
>
>Bootstrapped/regtested on x86_64-linux and i686-linux, ok for
>trunk/7.3?
OK.
Richard.
>2017-12-23 Jakub Jelinek <ja...@redhat.com>
>
> PR c++/83553
> * fold-const.c (struct contains_label_data): New type.
> (contains_label_1): Return non-NULL even for CASE_LABEL_EXPR, unless
> inside of a SWITCH_BODY seen during the walk.
> (contains_label_p): Use walk_tree instead of
> walk_tree_without_duplicates, prepare data for contains_label_1 and
> provide own pset.
>
> * c-c++-common/torture/pr83553.c: New test.
>
>--- gcc/fold-const.c.jj 2017-12-21 09:43:17.000000000 +0100
>+++ gcc/fold-const.c 2017-12-23 00:22:54.447669504 +0100
>@@ -11218,22 +11218,48 @@ fold_binary_loc (location_t loc, enum tr
> } /* switch (code) */
> }
>
>+/* Used by contains_label_[p1]. */
>+
>+struct contains_label_data
>+{
>+ hash_set<tree> *pset;
>+ bool inside_switch_p;
>+};
>+
>/* Callback for walk_tree, looking for LABEL_EXPR. Return *TP if it is
>- a LABEL_EXPR; otherwise return NULL_TREE. Do not check the
>subtrees
>- of GOTO_EXPR. */
>+ a LABEL_EXPR or CASE_LABEL_EXPR not inside of another SWITCH_EXPR;
>otherwise
>+ return NULL_TREE. Do not check the subtrees of GOTO_EXPR. */
>
> static tree
>-contains_label_1 (tree *tp, int *walk_subtrees, void *data
>ATTRIBUTE_UNUSED)
>+contains_label_1 (tree *tp, int *walk_subtrees, void *data)
> {
>+ contains_label_data *d = (contains_label_data *) data;
> switch (TREE_CODE (*tp))
> {
> case LABEL_EXPR:
> return *tp;
>
>+ case CASE_LABEL_EXPR:
>+ if (!d->inside_switch_p)
>+ return *tp;
>+ return NULL_TREE;
>+
>+ case SWITCH_EXPR:
>+ if (!d->inside_switch_p)
>+ {
>+ if (walk_tree (&SWITCH_COND (*tp), contains_label_1, data,
>d->pset))
>+ return *tp;
>+ d->inside_switch_p = true;
>+ if (walk_tree (&SWITCH_BODY (*tp), contains_label_1, data,
>d->pset))
>+ return *tp;
>+ d->inside_switch_p = false;
>+ *walk_subtrees = 0;
>+ }
>+ return NULL_TREE;
>+
> case GOTO_EXPR:
> *walk_subtrees = 0;
>-
>- /* fall through */
>+ return NULL_TREE;
>
> default:
> return NULL_TREE;
>@@ -11246,8 +11272,9 @@ contains_label_1 (tree *tp, int *walk_su
> static bool
> contains_label_p (tree st)
> {
>- return
>- (walk_tree_without_duplicates (&st, contains_label_1 , NULL) !=
>NULL_TREE);
>+ hash_set<tree> pset;
>+ contains_label_data data = { &pset, false };
>+ return walk_tree (&st, contains_label_1, &data, &pset) != NULL_TREE;
> }
>
> /* Fold a ternary expression of code CODE and type TYPE with operands
>--- gcc/testsuite/c-c++-common/torture/pr83553.c.jj 2017-12-23
>00:30:20.548180122 +0100
>+++ gcc/testsuite/c-c++-common/torture/pr83553.c 2017-12-23
>00:29:11.000000000 +0100
>@@ -0,0 +1,29 @@
>+/* PR c++/83553 */
>+/* { dg-do run } */
>+
>+int a[3];
>+
>+int
>+foo (int n)
>+{
>+ switch (n)
>+ {
>+ case 0:
>+ for (n = 7, a[0]++; 0; a[2] = a[1] + 1)
>+ {
>+ case 2:
>+ a[1] = a[0] + 1;
>+ }
>+ }
>+ return n;
>+}
>+
>+int
>+main ()
>+{
>+ if (foo (0) != 7 || a[0] != 1 || a[1] || a[2])
>+ __builtin_abort ();
>+ if (foo (2) != 2 || a[0] != 1 || a[1] != 2 || a[2] != 3)
>+ __builtin_abort ();
>+ return 0;
>+}
>
> Jakub
