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lint: add more rationale for removing effect-less code

This commit is contained in:
rillig 2021-01-04 23:17:03 +00:00
parent 8f3340157a
commit fb2d84bf80
1 changed files with 16 additions and 12 deletions
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28
usr.bin/xlint/lint1/tree.c
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@ -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;