[src/trunk]: src/usr.bin/xlint/lint1 lint: add more rationale for removing ef...

[rillig <rillig%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/3629e17905df
branches:  trunk
changeset: 979665:3629e17905df
user:      rillig <rillig%NetBSD.org@localhost>
date:      Mon Jan 04 23:17:03 2021 +0000

description:
lint: add more rationale for removing effect-less code

diffstat:

 usr.bin/xlint/lint1/tree.c |  28 ++++++++++++++++------------
 1 files changed, 16 insertions(+), 12 deletions(-)

diffs (51 lines):

diff -r 18275c417209 -r 3629e17905df usr.bin/xlint/lint1/tree.c
--- a/usr.bin/xlint/lint1/tree.c        Mon Jan 04 22:41:56 2021 +0000
+++ b/usr.bin/xlint/lint1/tree.c        Mon Jan 04 23:17:03 2021 +0000
@@ -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 @@
                 * 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.
+                *
+                * The argument of the innermost cast or conversion must not
+                * be parenthesized.
                 *
-                * 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 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 implicit conversion is never parenthesized.  Therefore
-                * this must be a cast that is later converted, to build a
-                * chain.
+                * 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;