include: move typeof_strip_qual to compiler.h, use it in QAPI_LIST_LENGTH()

The typeof_strip_qual() is most useful for the atomic fetch-and-modify
operations in atomic.h, but it can be used elsewhere as well.  For example,
QAPI_LIST_LENGTH() assumes that the argument is not const, which is not a
requirement.

Move the macro to compiler.h and, while at it, move it under #ifndef
__cplusplus to emphasize that it uses C-only constructs.  A C++ version
of typeof_strip_qual() using type traits is possible[1], but beyond the
scope of this patch because the little C++ code that is in QEMU does not
use QAPI.

The patch was tested by changing the declaration of strv_from_str_list()
in qapi/qapi-type-helpers.c to:

    char **strv_from_str_list(const strList *const list)

This is valid C code, and it fails to compile without this change.

[1] https://lore.kernel.org/qemu-devel/20240624205647.112034-1-flwu@google.com/

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Tested-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

3 files changed, 47 insertions, 43 deletions

diff --git a/include/qapi/util.h b/include/qapi/util.h
index 20dfea8..b825424 100644
--- a/include/qapi/util.h
+++ b/include/qapi/util.h
@@ -62,7 +62,7 @@ int parse_qapi_name(const char *name, bool complete);
 #define QAPI_LIST_LENGTH(list)                                      \
     ({                                                              \
         size_t _len = 0;                                            \
-        typeof(list) _tail;                                         \
+        typeof_strip_qual(list) _tail;                              \
         for (_tail = list; _tail != NULL; _tail = _tail->next) {    \
             _len++;                                                 \
         }                                                           \
diff --git a/include/qemu/atomic.h b/include/qemu/atomic.h
index 99110ab..dc4118d 100644
--- a/include/qemu/atomic.h
+++ b/include/qemu/atomic.h
@@ -20,48 +20,6 @@
 /* Compiler barrier */
 #define barrier()   ({ asm volatile("" ::: "memory"); (void)0; })
 
-/* The variable that receives the old value of an atomically-accessed
- * variable must be non-qualified, because atomic builtins return values
- * through a pointer-type argument as in __atomic_load(&var, &old, MODEL).
- *
- * This macro has to handle types smaller than int manually, because of
- * implicit promotion.  int and larger types, as well as pointers, can be
- * converted to a non-qualified type just by applying a binary operator.
- */
-#define typeof_strip_qual(expr)                                                    \
-  typeof(                                                                          \
-    __builtin_choose_expr(                                                         \
-      __builtin_types_compatible_p(typeof(expr), bool) ||                          \
-        __builtin_types_compatible_p(typeof(expr), const bool) ||                  \
-        __builtin_types_compatible_p(typeof(expr), volatile bool) ||               \
-        __builtin_types_compatible_p(typeof(expr), const volatile bool),           \
-        (bool)1,                                                                   \
-    __builtin_choose_expr(                                                         \
-      __builtin_types_compatible_p(typeof(expr), signed char) ||                   \
-        __builtin_types_compatible_p(typeof(expr), const signed char) ||           \
-        __builtin_types_compatible_p(typeof(expr), volatile signed char) ||        \
-        __builtin_types_compatible_p(typeof(expr), const volatile signed char),    \
-        (signed char)1,                                                            \
-    __builtin_choose_expr(                                                         \
-      __builtin_types_compatible_p(typeof(expr), unsigned char) ||                 \
-        __builtin_types_compatible_p(typeof(expr), const unsigned char) ||         \
-        __builtin_types_compatible_p(typeof(expr), volatile unsigned char) ||      \
-        __builtin_types_compatible_p(typeof(expr), const volatile unsigned char),  \
-        (unsigned char)1,                                                          \
-    __builtin_choose_expr(                                                         \
-      __builtin_types_compatible_p(typeof(expr), signed short) ||                  \
-        __builtin_types_compatible_p(typeof(expr), const signed short) ||          \
-        __builtin_types_compatible_p(typeof(expr), volatile signed short) ||       \
-        __builtin_types_compatible_p(typeof(expr), const volatile signed short),   \
-        (signed short)1,                                                           \
-    __builtin_choose_expr(                                                         \
-      __builtin_types_compatible_p(typeof(expr), unsigned short) ||                \
-        __builtin_types_compatible_p(typeof(expr), const unsigned short) ||        \
-        __builtin_types_compatible_p(typeof(expr), volatile unsigned short) ||     \
-        __builtin_types_compatible_p(typeof(expr), const volatile unsigned short), \
-        (unsigned short)1,                                                         \
-      (expr)+0))))))
-
 #ifndef __ATOMIC_RELAXED
 #error "Expecting C11 atomic ops"
 #endif
diff --git a/include/qemu/compiler.h b/include/qemu/compiler.h
index c797f0d..554c5ce 100644
--- a/include/qemu/compiler.h
+++ b/include/qemu/compiler.h
@@ -227,4 +227,50 @@
 #define SECOND_ARG(first, second, ...) second
 #define IS_EMPTY_(junk_maybecomma)     SECOND_ARG(junk_maybecomma 1, 0)
 
+#ifndef __cplusplus
+/*
+ * Useful in macros that need to declare temporary variables.  For example,
+ * the variable that receives the old value of an atomically-accessed
+ * variable must be non-qualified, because atomic builtins return values
+ * through a pointer-type argument as in __atomic_load(&var, &old, MODEL).
+ *
+ * This macro has to handle types smaller than int manually, because of
+ * implicit promotion.  int and larger types, as well as pointers, can be
+ * converted to a non-qualified type just by applying a binary operator.
+ */
+#define typeof_strip_qual(expr)                                                    \
+  typeof(                                                                          \
+    __builtin_choose_expr(                                                         \
+      __builtin_types_compatible_p(typeof(expr), bool) ||                          \
+        __builtin_types_compatible_p(typeof(expr), const bool) ||                  \
+        __builtin_types_compatible_p(typeof(expr), volatile bool) ||               \
+        __builtin_types_compatible_p(typeof(expr), const volatile bool),           \
+        (bool)1,                                                                   \
+    __builtin_choose_expr(                                                         \
+      __builtin_types_compatible_p(typeof(expr), signed char) ||                   \
+        __builtin_types_compatible_p(typeof(expr), const signed char) ||           \
+        __builtin_types_compatible_p(typeof(expr), volatile signed char) ||        \
+        __builtin_types_compatible_p(typeof(expr), const volatile signed char),    \
+        (signed char)1,                                                            \
+    __builtin_choose_expr(                                                         \
+      __builtin_types_compatible_p(typeof(expr), unsigned char) ||                 \
+        __builtin_types_compatible_p(typeof(expr), const unsigned char) ||         \
+        __builtin_types_compatible_p(typeof(expr), volatile unsigned char) ||      \
+        __builtin_types_compatible_p(typeof(expr), const volatile unsigned char),  \
+        (unsigned char)1,                                                          \
+    __builtin_choose_expr(                                                         \
+      __builtin_types_compatible_p(typeof(expr), signed short) ||                  \
+        __builtin_types_compatible_p(typeof(expr), const signed short) ||          \
+        __builtin_types_compatible_p(typeof(expr), volatile signed short) ||       \
+        __builtin_types_compatible_p(typeof(expr), const volatile signed short),   \
+        (signed short)1,                                                           \
+    __builtin_choose_expr(                                                         \
+      __builtin_types_compatible_p(typeof(expr), unsigned short) ||                \
+        __builtin_types_compatible_p(typeof(expr), const unsigned short) ||        \
+        __builtin_types_compatible_p(typeof(expr), volatile unsigned short) ||     \
+        __builtin_types_compatible_p(typeof(expr), const volatile unsigned short), \
+        (unsigned short)1,                                                         \
+      (expr)+0))))))
+#endif
+
 #endif /* COMPILER_H */