This patch fixes a wrong -Wimplicit-fallthrough warning for
case 0:
if (1) // wrong may fallthrough
return 0;
case 1:
which in .gimple looks like
: // case 0
if (1 != 0) goto ; else goto ;
:
D.1987 = 0;
// predicted unlikely by early return (on trees) predictor.
return D.1987;
: // dead
: // case 1
and the warning thinks that : falls through to :. It
does not know that is effectively a dead label, only reachable
through fallthrough from previous instructions, never jumped to. To
that effect, Jakub introduced UNUSED_LABEL_P, which is set on such dead
labels.
collect_fallthrough_labels has code to deal with cases like
case 2:
if (e != 10)
i++; // this may fallthru, warn
else
return 44;
case 3:
which collects labels that may fall through. Here it sees the "goto ;"
at the end of the then branch and so when the warning reaches
...
: // from if-then
: // case 3
it knows it should warn about the possible fallthrough. But an UNUSED_LABEL_P
is not a label that can fallthrough like that, so it should ignore those.
However, we still want to warn about this:
case 0:
if (1)
n++; // falls through
case 1:
so collect_fallthrough_labels needs to return the "n = n + 1;" statement, rather
than the dead label.
Co-authored-by: Jakub Jelinek
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
PR middle-end/103597
gcc/ChangeLog:
* gimplify.cc (collect_fallthrough_labels): Don't push UNUSED_LABEL_Ps
into labels. Maybe set prev to the statement preceding UNUSED_LABEL_P.
(gimplify_cond_expr): Set UNUSED_LABEL_P.
* tree.h (UNUSED_LABEL_P): New.
gcc/testsuite/ChangeLog:
* c-c++-common/Wimplicit-fallthrough-39.c: New test.
---
gcc/gimplify.cc | 54 ++-
.../c-c++-common/Wimplicit-fallthrough-39.c | 140 ++
gcc/tree.h| 6 +
3 files changed, 194 insertions(+), 6 deletions(-)
create mode 100644 gcc/testsuite/c-c++-common/Wimplicit-fallthrough-39.c
diff --git a/gcc/gimplify.cc b/gcc/gimplify.cc
index f62f150fc08..2588824dce2 100644
--- a/gcc/gimplify.cc
+++ b/gcc/gimplify.cc
@@ -2250,9 +2250,9 @@ last_stmt_in_scope (gimple *stmt)
}
}
-/* Collect interesting labels in LABELS and return the statement preceding
- another case label, or a user-defined label. Store a location useful
- to give warnings at *PREVLOC (usually the location of the returned
+/* Collect labels that may fall through into LABELS and return the statement
+ preceding another case label, or a user-defined label. Store a location
+ useful to give warnings at *PREVLOC (usually the location of the returned
statement or of its surrounding scope). */
static gimple *
@@ -2331,8 +2331,12 @@ collect_fallthrough_labels (gimple_stmt_iterator *gsi_p,
if (gsi_end_p (*gsi_p))
break;
- struct label_entry l = { false_lab, if_loc };
- labels->safe_push (l);
+ /* A dead label can't fall through. */
+ if (!UNUSED_LABEL_P (false_lab))
+ {
+ struct label_entry l = { false_lab, if_loc };
+ labels->safe_push (l);
+ }
/* Go to the last statement of the then branch. */
gsi_prev (gsi_p);
@@ -2359,6 +2363,17 @@ collect_fallthrough_labels (gimple_stmt_iterator *gsi_p,
labels->safe_push (l);
}
}
+ /* This case is about
+ if (1 != 0) goto ; else goto ;
+ :
+ n = n + 1; // #1
+ : // #2
+ : // #3
+where #2 is UNUSED_LABEL_P and we want to warn about #1 falling
+through to #3. So set PREV to #1. */
+ else if (UNUSED_LABEL_P (false_lab))
+ prev = gsi_stmt (*gsi_p);
+
/* And move back. */
gsi_next (gsi_p);
}
@@ -4461,9 +4476,19 @@ gimplify_cond_expr (tree *expr_p, gimple_seq *pre_p,
fallback_t fallback)
if (TREE_OPERAND (expr, 1) == NULL_TREE
&& !have_else_clause_p
&& TREE_OPERAND (expr, 2) != NULL_TREE)
- label_cont = label_true;
+ {
+ /* For if (0) {} else { code; } tell -Wimplicit-fallthrough
+handling that label_cont == label_true can be only reached
+through fallthrough from { code; }. */
+ if (integer_zerop (COND_EXPR_COND (expr)))
+ UNUSED_LABEL_P (label_true) = 1;
+ label_cont = label_true;
+ }
else
{
+ bool then_side_effects
+ = (TREE_OPERAND (expr, 1)
+ && TREE_SIDE_EFFECTS (TREE_OPERAND (expr, 1)));
gimplify_seq_add_stmt (, gimple_build_label (label_true));
have_then_clause_p = gimplify_stmt (_OPERAND (expr, 1), );
/* For if (...) { code; } else {} or
@@ -4477,6 +4502,16 @@
