[Xen-devel] [PATCH] clang: disable the gcc-compat warnings for read_atomic

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

clang gcc-compat warnings can wrongly fire when certain constructions are used,
at least the following flow:

switch ( ... )
{
case ...:
    while ( ({ int x; switch ( foo ) { case 1: x = 1; break; } x }) )
    {
        ...

Will cause clang to emit the following warning "'break' is bound to loop, GCC
binds it to switch", which is a false positive, and both gcc and clang bound
the break to the inner switch. In order to workaround this issue, disable the
gcc-compat checks for the usage of the read_atomic macro.

This has been reported upstream as http://bugs.llvm.org/show_bug.cgi?id=32595.

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
 xen/include/asm-x86/atomic.h |  2 ++
 xen/include/xen/compiler.h   | 27 +++++++++++++++++++++++++++
 2 files changed, 29 insertions(+)

diff --git a/xen/include/asm-x86/atomic.h b/xen/include/asm-x86/atomic.h
index 2fbe705518..b997a1726b 100644
--- a/xen/include/asm-x86/atomic.h
+++ b/xen/include/asm-x86/atomic.h
@@ -45,6 +45,7 @@ void __bad_atomic_size(void);
 
 #define read_atomic(p) ({                                 \
     unsigned long x_;                                     \
+    CLANG_DISABLE_WARN_GCC_COMPAT_START                   \
     switch ( sizeof(*(p)) ) {                             \
     case 1: x_ = read_u8_atomic((uint8_t *)(p)); break;   \
     case 2: x_ = read_u16_atomic((uint16_t *)(p)); break; \
@@ -52,6 +53,7 @@ void __bad_atomic_size(void);
     case 8: x_ = read_u64_atomic((uint64_t *)(p)); break; \
     default: x_ = 0; __bad_atomic_size(); break;          \
     }                                                     \
+    CLANG_DISABLE_WARN_GCC_COMPAT_END                     \
     (typeof(*(p)))x_;                                     \
 })
 
diff --git a/xen/include/xen/compiler.h b/xen/include/xen/compiler.h
index 16aeeea7f1..17d1f33a2d 100644
--- a/xen/include/xen/compiler.h
+++ b/xen/include/xen/compiler.h
@@ -100,4 +100,31 @@
 # define ASM_FLAG_OUT(yes, no) no
 #endif
 
+/*
+ * NB: we need to disable the gcc-compat warnings for clang in some places or
+ * else it will complain with: "'break' is bound to loop, GCC binds it to
+ * switch" when a switch is used inside of a while expression inside of a
+ * switch statement, ie:
+ *
+ * switch ( ... )
+ * {
+ * case ...:
+ *      while ( ({ int x; switch ( foo ) { case 1: x = 1; break; } x }) )
+ *      {
+ *              ...
+ *
+ * This has already been reported upstream:
+ * http://bugs.llvm.org/show_bug.cgi?id=32595
+ */
+#ifdef __clang__
+# define CLANG_DISABLE_WARN_GCC_COMPAT_START                    \
+    _Pragma("clang diagnostic push")                            \
+    _Pragma("clang diagnostic ignored \"-Wgcc-compat\"")
+# define CLANG_DISABLE_WARN_GCC_COMPAT_END                      \
+    _Pragma("clang diagnostic pop")
+#else
+# define CLANG_DISABLE_WARN_GCC_COMPAT_START
+# define CLANG_DISABLE_WARN_GCC_COMPAT_END
+#endif
+
 #endif /* __LINUX_COMPILER_H */
-- 
2.11.0 (Apple Git-81)


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