[src/trunk]: src/tests/usr.bin/xlint/lint1 tests/lint: investigate how compil...

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

details:   https://anonhg.NetBSD.org/src/rev/d473dfc255e5
branches:  trunk
changeset: 373265:d473dfc255e5
user:      rillig <rillig%NetBSD.org@localhost>
date:      Sat Jan 28 08:30:12 2023 +0000

description:
tests/lint: investigate how compilers interpret bit-fields

diffstat:

 tests/usr.bin/xlint/lint1/gcc_bit_field_types.c |  63 ++++++++++++++++++++++++-
 1 files changed, 62 insertions(+), 1 deletions(-)

diffs (76 lines):

diff -r a1b4d88542d6 -r d473dfc255e5 tests/usr.bin/xlint/lint1/gcc_bit_field_types.c
--- a/tests/usr.bin/xlint/lint1/gcc_bit_field_types.c   Sat Jan 28 07:45:21 2023 +0000
+++ b/tests/usr.bin/xlint/lint1/gcc_bit_field_types.c   Sat Jan 28 08:30:12 2023 +0000
@@ -1,6 +1,11 @@
-/*     $NetBSD: gcc_bit_field_types.c,v 1.6 2022/01/15 14:22:03 rillig Exp $   */
+/*     $NetBSD: gcc_bit_field_types.c,v 1.7 2023/01/28 08:30:12 rillig Exp $   */
 # 3 "gcc_bit_field_types.c"
 
+struct incompatible {
+       int dummy;
+};
+void reveal_type(struct incompatible);
+
 /*
  * https://gcc.gnu.org/onlinedocs/gcc/Structures-unions-enumerations-and-bit-fields-implementation.html
  *
@@ -35,3 +40,59 @@
         */
        return lbf.member & 0xf;
 }
+
+/*
+ * C99 6.7.2.1p4 says: "A bit-field shall have a type that is a qualified or
+ * unqualified version of _Bool, signed int, unsigned int, or some other
+ * implementation-defined type."
+ *
+ * The wording of that constraint does not disambiguate whether it is talking
+ * about the declared underlying type of the storage unit or the expression
+ * type when evaluating a bit-field as an rvalue.
+ */
+void
+type_of_bit_field(void)
+{
+       struct {
+               unsigned bits:3;
+       } s;
+
+       /*
+        * Lint interprets the type of the bit-field is 'unsigned:3', which
+        * matches the non-bit-field type 'unsigned' in the _Generic
+        * expression.  (XXX: May or may not be intended.)
+        *
+        * The _Generic expression prevents the integer promotions from
+        * getting applied as part of the function argument conversions.
+        *
+        * GCC 11 says: error: '_Generic' selector of type 'unsigned char:3'
+        * is not compatible with any association
+        *
+        * Clang 15 says: error: controlling expression type 'unsigned int'
+        * not compatible with any generic association type
+        *
+        * TCC says: error: type 'unsigned int' does not match any association
+        *
+        * MSVC 19 says: error C7702: no compatible type for 'unsigned int'
+        * in _Generic association list
+        *
+        * ICC 2021.7.1 says: error: no association matches the selector type
+        * "unsigned int"
+        */
+       /* expect+4: warning: passing 'pointer to unsigned int' to incompatible 'struct incompatible', arg #1 [155] */
+       reveal_type(_Generic(s.bits,
+           int: (int *)0,
+           unsigned int: (unsigned int *)0
+       ));
+
+       /*
+        * When lint promotes the bit-field as part of the function argument
+        * conversions, the type 'unsigned:3' gets promoted to 'int', as that
+        * is the smallest candidate type that can represent all possible
+        * values from 'unsigned:3', see promote_c90.  Maybe that's wrong,
+        * maybe not, the compilers disagree so lint can offer yet another
+        * alternative interpretation.
+        */
+       /* expect+1: warning: passing 'unsigned int:3' to incompatible 'struct incompatible', arg #1 [155] */
+       reveal_type(s.bits);
+}