Module Name: src
Committed By: rillig
Date: Mon Jan 4 23:17:04 UTC 2021
Modified Files:
src/usr.bin/xlint/lint1: tree.c
Log Message:
lint: add more rationale for removing effect-less code
To generate a diff of this commit:
cvs rdiff -u -r1.129 -r1.130 src/usr.bin/xlint/lint1/tree.c
Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.
Modified files:
Index: src/usr.bin/xlint/lint1/tree.c
diff -u src/usr.bin/xlint/lint1/tree.c:1.129 src/usr.bin/xlint/lint1/tree.c:1.130
--- src/usr.bin/xlint/lint1/tree.c:1.129 Mon Jan 4 22:41:56 2021
+++ src/usr.bin/xlint/lint1/tree.c Mon Jan 4 23:17:03 2021
@@ -1,4 +1,4 @@
-/* $NetBSD: tree.c,v 1.129 2021/01/04 22:41:56 rillig Exp $ */
+/* $NetBSD: tree.c,v 1.130 2021/01/04 23:17:03 rillig Exp $ */
/*
* Copyright (c) 1994, 1995 Jochen Pohl
@@ -37,7 +37,7 @@
#include <sys/cdefs.h>
#if defined(__RCSID) && !defined(lint)
-__RCSID("$NetBSD: tree.c,v 1.129 2021/01/04 22:41:56 rillig Exp $");
+__RCSID("$NetBSD: tree.c,v 1.130 2021/01/04 23:17:03 rillig Exp $");
#endif
#include <float.h>
@@ -4017,18 +4017,22 @@ check_precedence_confusion(tnode_t *tn)
* that demonstrates an actual change in behavior when this
* bug gets fixed.
*
- * Right now, the condition is always false. To make it true
- * after fixing the typo, the right-hand operand must be an
- * explicit cast or an implicit conversion that is
- * parenthesized. For the right-hand operand itself, this
- * would already be done using the line below the loop.
+ * rn must be a chain of casts and conversions, and at least
+ * one of these must be a parenthesized cast.
*
- * To make a difference, the right-hand operand must not be
- * parenthesized, but its indirect cast or conversion must be.
+ * The argument of the innermost cast or conversion must not
+ * be parenthesized.
*
- * An implicit conversion is never parenthesized. Therefore
- * this must be a cast that is later converted, to build a
- * chain.
+ * The argument of the innermost cast or conversion must be
+ * an expression with confusing precedence. Since all these
+ * expressions have lower precedence than a cast, these can
+ * only appear as a parenthesized expression. This in turn
+ * makes the whole loop superfluous.
+ *
+ * An edge case might be due to constant folding, if the
+ * nodes created from constant folding did not preserve
+ * tn_parenthesized properly. But that would be another bug,
+ * so it doesn't count as an argument.
*/
for (rn = tn->tn_right; tn->tn_op == CVT; rn = rn->tn_left)
rparn |= rn->tn_parenthesized;
