Module Name: src
Committed By: rillig
Date: Sat Jan 28 08:30:12 UTC 2023
Modified Files:
src/tests/usr.bin/xlint/lint1: gcc_bit_field_types.c
Log Message:
tests/lint: investigate how compilers interpret bit-fields
To generate a diff of this commit:
cvs rdiff -u -r1.6 -r1.7 src/tests/usr.bin/xlint/lint1/gcc_bit_field_types.c
Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.
Modified files:
Index: src/tests/usr.bin/xlint/lint1/gcc_bit_field_types.c
diff -u src/tests/usr.bin/xlint/lint1/gcc_bit_field_types.c:1.6 src/tests/usr.bin/xlint/lint1/gcc_bit_field_types.c:1.7
--- src/tests/usr.bin/xlint/lint1/gcc_bit_field_types.c:1.6 Sat Jan 15 14:22:03 2022
+++ src/tests/usr.bin/xlint/lint1/gcc_bit_field_types.c Sat Jan 28 08:30:12 2023
@@ -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 @@ promote_large_bit_field(struct large_bit
*/
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);
+}
