[src/trunk]: src/usr.bin/xlint/lint1 lint: warn about unreachable case labels...

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

details:   https://anonhg.NetBSD.org/src/rev/df23abb33703
branches:  trunk
changeset: 983310:df23abb33703
user:      rillig <rillig%NetBSD.org@localhost>
date:      Sat May 15 19:12:14 2021 +0000

description:
lint: warn about unreachable case labels for '&&'

See octeon_gmxreg.h 1.2 from 2020-06-18 for an example, where
RXN_RX_INBND_SPEED was cleaned up without adjusting the corresponding
code in octeon_gmx.c.

diffstat:

 tests/usr.bin/xlint/lint1/Makefile     |   3 +-
 tests/usr.bin/xlint/lint1/expr_range.c |  38 +++++++++++++++++++--------------
 usr.bin/xlint/lint1/func.c             |  30 ++++++++++++++++++++++++--
 usr.bin/xlint/lint1/lint1.h            |   3 +-
 4 files changed, 53 insertions(+), 21 deletions(-)

diffs (167 lines):

diff -r 951ef9c24ea4 -r df23abb33703 tests/usr.bin/xlint/lint1/Makefile
--- a/tests/usr.bin/xlint/lint1/Makefile        Sat May 15 13:00:45 2021 +0000
+++ b/tests/usr.bin/xlint/lint1/Makefile        Sat May 15 19:12:14 2021 +0000
@@ -1,4 +1,4 @@
-# $NetBSD: Makefile,v 1.56 2021/05/14 21:14:55 rillig Exp $
+# $NetBSD: Makefile,v 1.57 2021/05/15 19:12:14 rillig Exp $
 
 NOMAN=         # defined
 MAX_MESSAGE=   343             # see lint1/err.c
@@ -104,6 +104,7 @@
 FILES+=                emit.exp
 FILES+=                emit.ln
 FILES+=                expr_range.c
+FILES+=                expr_range.exp
 FILES+=                feat_stacktrace.c
 FILES+=                feat_stacktrace.exp
 FILES+=                gcc_attribute.c
diff -r 951ef9c24ea4 -r df23abb33703 tests/usr.bin/xlint/lint1/expr_range.c
--- a/tests/usr.bin/xlint/lint1/expr_range.c    Sat May 15 13:00:45 2021 +0000
+++ b/tests/usr.bin/xlint/lint1/expr_range.c    Sat May 15 19:12:14 2021 +0000
@@ -1,37 +1,43 @@
-/*     $NetBSD: expr_range.c,v 1.1 2021/05/14 21:14:55 rillig Exp $    */
+/*     $NetBSD: expr_range.c,v 1.2 2021/05/15 19:12:14 rillig Exp $    */
 # 3 "expr_range.c"
 
 /*
- * Test whether lint can detect switch branches that are impossible to reach.
- * As of 2021-05-14, it cannot.  To do this, it would need to keep track of
- * the possible values of each variable or expression.  To do this reliably,
- * it would also need accurate control flow analysis, which as of 2021-05-14
- * works only for functions without 'goto'.
+ * In a switch statement that has (expr & constant) as the controlling
+ * expression, complain if one of the case branches is unreachable because
+ * the case label does can never match the controlling expression.
  *
- * GCC 10 does not complain the unreachable branch.  It knows that the branch
- * is unreachable though since it doesn't generate any code for it.  GCC once
- * had the option -Wunreachable-code, but that option was made a no-op in
- * 2011.
+ * GCC 10 does not complain about the unreachable branch.  It knows that the
+ * branch is unreachable though since it doesn't generate any code for it.
+ * GCC once had the option -Wunreachable-code, but that option was made a
+ * no-op in 2011.
  *
  * Clang 10 does not complain about this either, and just like GCC it doesn't
  * generate any code for this branch.  The code for tracking an expression's
  * possible values may be related to RangeConstraintManager, just guessing.
- *
- * There should be at least one static analysis tool that warns about this.
  */
 
 /* lint1-extra-flags: -chap */
 
+void println(const char *);
+
 void
 example(unsigned x)
 {
        switch (x & 6) {
-       case 1:
-               /* This branch is unreachable. */
-               printf("one\n");
+       case 0:
+               println("0 is reachable");
+               break;
+       case 1:                 /* expect: statement not reached */
+               println("1 is not reachable");
                break;
        case 2:
-               printf("two\n");
+               println("2 is reachable");
+               break;
+       case 6:
+               println("6 is reachable");
+               break;
+       case 7:                 /* expect: statement not reached */
+               println("7 is not reachable");
                break;
        }
 }
diff -r 951ef9c24ea4 -r df23abb33703 usr.bin/xlint/lint1/func.c
--- a/usr.bin/xlint/lint1/func.c        Sat May 15 13:00:45 2021 +0000
+++ b/usr.bin/xlint/lint1/func.c        Sat May 15 19:12:14 2021 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: func.c,v 1.107 2021/05/03 07:08:54 rillig Exp $        */
+/*     $NetBSD: func.c,v 1.108 2021/05/15 19:12:14 rillig Exp $        */
 
 /*
  * Copyright (c) 1994, 1995 Jochen Pohl
@@ -37,7 +37,7 @@
 
 #include <sys/cdefs.h>
 #if defined(__RCSID) && !defined(lint)
-__RCSID("$NetBSD: func.c,v 1.107 2021/05/03 07:08:54 rillig Exp $");
+__RCSID("$NetBSD: func.c,v 1.108 2021/05/15 19:12:14 rillig Exp $");
 #endif
 
 #include <stdlib.h>
@@ -440,6 +440,25 @@
 }
 
 static void
+check_case_label_bitand(const tnode_t *case_expr, const tnode_t *switch_expr)
+{
+       uint64_t case_value, mask;
+
+       if (switch_expr->tn_op != BITAND ||
+           switch_expr->tn_right->tn_op != CON)
+               return;
+
+       lint_assert(case_expr->tn_op == CON);
+       case_value = case_expr->tn_val->v_quad;
+       mask = switch_expr->tn_right->tn_val->v_quad;
+
+       if ((case_value & ~mask) != 0) {
+               /* statement not reached */
+               warning(193);
+       }
+}
+
+static void
 check_case_label_enum(const tnode_t *tn, const cstk_t *ci)
 {
        /* similar to typeok_enum in tree.c */
@@ -483,6 +502,7 @@
                return;
        }
 
+       check_case_label_bitand(tn, ci->c_switch_expr);
        check_case_label_enum(tn, ci);
 
        lint_assert(ci->c_switch_type != NULL);
@@ -694,12 +714,16 @@
                tp->t_tspec = INT;
        }
 
+       /* leak the memory, for check_case_label_bitand */
+       expr_save_memory();
+
        check_getopt_begin_switch();
-       expr(tn, true, false, true, false);
+       expr(tn, true, false, false, false);
 
        begin_control_statement(CS_SWITCH);
        cstmt->c_switch = true;
        cstmt->c_switch_type = tp;
+       cstmt->c_switch_expr = tn;
 
        set_reached(false);
        seen_fallthrough = true;
diff -r 951ef9c24ea4 -r df23abb33703 usr.bin/xlint/lint1/lint1.h
--- a/usr.bin/xlint/lint1/lint1.h       Sat May 15 13:00:45 2021 +0000
+++ b/usr.bin/xlint/lint1/lint1.h       Sat May 15 19:12:14 2021 +0000
@@ -1,4 +1,4 @@
-/* $NetBSD: lint1.h,v 1.100 2021/04/18 17:47:32 rillig Exp $ */
+/* $NetBSD: lint1.h,v 1.101 2021/05/15 19:12:14 rillig Exp $ */
 
 /*
  * Copyright (c) 1996 Christopher G. Demetriou.  All Rights Reserved.
@@ -416,6 +416,7 @@
        bool    c_had_return_value : 1; /* had "return expr;" */
 
        type_t  *c_switch_type;         /* type of switch expression */
+       tnode_t *c_switch_expr;
        case_label_t *c_case_labels;    /* list of case values */
 
        struct  memory_block *c_for_expr3_mem; /* saved memory for end of loop