[src/trunk]: src/tests/usr.bin/xlint/lint1 tests/lint: test cast to union wit...

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

details:   https://anonhg.NetBSD.org/src/rev/9332b193eb19
branches:  trunk
changeset: 374435:9332b193eb19
user:      rillig <rillig%NetBSD.org@localhost>
date:      Sat Apr 22 19:45:04 2023 +0000

description:
tests/lint: test cast to union with stricter alignment

diffstat:

 tests/usr.bin/xlint/lint1/msg_135.c |  31 +++++++++++++++++++++++++++++--
 1 files changed, 29 insertions(+), 2 deletions(-)

diffs (49 lines):

diff -r 95f7ff81484c -r 9332b193eb19 tests/usr.bin/xlint/lint1/msg_135.c
--- a/tests/usr.bin/xlint/lint1/msg_135.c       Sat Apr 22 19:41:15 2023 +0000
+++ b/tests/usr.bin/xlint/lint1/msg_135.c       Sat Apr 22 19:45:04 2023 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: msg_135.c,v 1.12 2023/03/28 14:44:34 rillig Exp $      */
+/*     $NetBSD: msg_135.c,v 1.13 2023/04/22 19:45:04 rillig Exp $      */
 # 3 "msg_135.c"
 
 // Test for message: converting '%s' to '%s' increases alignment from %u to %u [135]
@@ -22,7 +22,7 @@ read_uint(const unsigned short **pp)
 struct incomplete;
 
 struct complete {
-    int member;
+       int member;
 };
 
 /*
@@ -74,3 +74,30 @@ plain_char_to_unsigned_type(char *cp)
        usp = (unsigned short *)cp;
        sink(usp);
 }
+
+/*
+ * Converting a pointer with a low alignment requirement to a union that
+ * includes other types with higher alignment requirements is safe.  While
+ * accessing any other member of the union might trigger an alignment
+ * violation, such an access would invoke undefined behavior anyway.
+ *
+ * A practical case for this pattern are tagged unions, in which the first
+ * member of the struct determines how the remaining members are interpreted.
+ * See sbin/newfs_udf, function udf_validate_tag_and_crc_sums for an example.
+ */
+double
+cast_to_union(void)
+{
+       int align_4 = 0;
+       double align_8 = 0.0;
+       union both {
+               int p_align_4;
+               double p_align_8;
+       } *both;
+
+       /* TODO: don't warn here. */
+       /* expect+1: warning: converting 'pointer to int' to 'pointer to union both' increases alignment from 4 to 8 [135] */
+       both = (union both *)&align_4;
+       both = (union both *)&align_8;
+       return both->p_align_8;
+}