[Qemu-devel] [PATCH for 2.7] atomic: strip "const" from variables declared with typeof

[Paolo Bonzini]

With the latest clang, we have the following warning:

    /home/pranith/devops/code/qemu/include/qemu/seqlock.h:62:21: warning: 
passing 'typeof (*&sl->sequence) *' (aka 'const unsigned int *') to parameter 
of type 'unsigned int *' discards qualifiers 
[-Wincompatible-pointer-types-discards-qualifiers]
        return unlikely(atomic_read(&sl->sequence) != start);
                        ^~~~~~~~~~~~~~~~~~~~~~~~~~
    /home/pranith/devops/code/qemu/include/qemu/atomic.h:58:25: note: expanded 
from macro 'atomic_read'
        __atomic_load(ptr, &_val, __ATOMIC_RELAXED);     \
                           ^~~~~

Stripping const is a bit tricky due to promotions, but it is doable
with either C11 _Generic or GCC extensions.  Use the latter.

Reported-by: Pranith Kumar <address@hidden>
Signed-off-by: Paolo Bonzini <address@hidden>
---
 include/qemu/atomic.h | 48 ++++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 42 insertions(+), 6 deletions(-)

diff --git a/include/qemu/atomic.h b/include/qemu/atomic.h
index 7e13fca..fc9cfa2 100644
--- a/include/qemu/atomic.h
+++ b/include/qemu/atomic.h
@@ -18,6 +18,42 @@
 /* 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), 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)))))
+
 #ifdef __ATOMIC_RELAXED
 /* For C11 atomic ops */
 
@@ -54,7 +90,7 @@
 #define atomic_read(ptr)                              \
     ({                                                \
     QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *)); \
-    typeof(*ptr) _val;                                \
+    typeof_strip_qual(*ptr) _val;                     \
      __atomic_load(ptr, &_val, __ATOMIC_RELAXED);     \
     _val;                                             \
     })
@@ -80,7 +116,7 @@
 #define atomic_rcu_read(ptr)                          \
     ({                                                \
     QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *)); \
-    typeof(*ptr) _val;                                \
+    typeof_strip_qual(*ptr) _val;                     \
     atomic_rcu_read__nocheck(ptr, &_val);             \
     _val;                                             \
     })
@@ -103,7 +139,7 @@
 #define atomic_mb_read(ptr)                             \
     ({                                                  \
     QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *));   \
-    typeof(*ptr) _val;                                  \
+    typeof_strip_qual(*ptr) _val;                       \
      __atomic_load(ptr, &_val, __ATOMIC_RELAXED);       \
      smp_rmb();                                         \
     _val;                                               \
@@ -120,7 +156,7 @@
 #define atomic_mb_read(ptr)                             \
     ({                                                  \
     QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *));   \
-    typeof(*ptr) _val;                                  \
+    typeof_strip_qual(*ptr) _val;                       \
     __atomic_load(ptr, &_val, __ATOMIC_SEQ_CST);        \
     _val;                                               \
     })
@@ -137,7 +173,7 @@
 
 #define atomic_xchg(ptr, i)    ({                           \
     QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *));       \
-    typeof(*ptr) _new = (i), _old;                          \
+    typeof_strip_qual(*ptr) _new = (i), _old;               \
     __atomic_exchange(ptr, &_new, &_old, __ATOMIC_SEQ_CST); \
     _old;                                                   \
 })
@@ -146,7 +182,7 @@
 #define atomic_cmpxchg(ptr, old, new)                                   \
     ({                                                                  \
     QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *));                   \
-    typeof(*ptr) _old = (old), _new = (new);                            \
+    typeof_strip_qual(*ptr) _old = (old), _new = (new);                 \
     __atomic_compare_exchange(ptr, &_old, &_new, false,                 \
                               __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST);      \
     _old;                                                               \
-- 
2.7.4