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[PATCH 2/2] compiler.h: fix C++ uninitialized const issue

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C++ does not like the union { typeof(x) __val; char __c[1]; } __u
construct for const types:
 error: uninitialized const member in
 'union atomic_read(const atomic_t*)::<anonymous>'
Address this by creating a C++ version of READ_ONCE where this union is
initialized: union { void _u(){}; typeof(x) __val; char __c[1]; } __u={0}

To please gcc 6.3.0 also add in a _u(){} as default ctor.

This makes C++ happy enough to build.

Signed-off-by: Joakim Tjernlund <joakim.tjernlund@infinera.com>
---
 include/linux/compiler.h | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index cf0fa5d..0a047fd 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -300,6 +300,7 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
  * required ordering.
  */
 
+#ifndef __cplusplus
 #define __READ_ONCE(x, check)						\
 ({									\
 	union { typeof(x) __val; char __c[1]; } __u;			\
@@ -309,6 +310,17 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
 		__read_once_size_nocheck(&(x), __u.__c, sizeof(x));	\
 	__u.__val;							\
 })
+#else
+#define __READ_ONCE(x, check)						\
+({									\
+	union { void _u(){}; typeof(x) __val; char __c[1]; } __u={0};	\
+	if (check)							\
+		__read_once_size(&(x), __u.__c, sizeof(x));		\
+	else								\
+		__read_once_size_nocheck(&(x), __u.__c, sizeof(x));	\
+	__u.__val;							\
+})
+#endif
 #define READ_ONCE(x) __READ_ONCE(x, 1)
 
 /*
-- 
2.10.2

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[PATCH 1/2] Correct function definition for C++ Joakim Tjernlund <joakim.tjernlund@infinera.com> - 2017-02-21 16:40 +0100
  [PATCH 2/2] compiler.h: fix C++ uninitialized const issue Joakim Tjernlund <joakim.tjernlund@infinera.com> - 2017-02-21 16:40 +0100
  Re: [PATCH 1/2] Correct function definition for C++ Greg KH <greg@kroah.com> - 2017-02-22 08:20 +0100
    Re: [PATCH 1/2] Correct function definition for C++ "greg@kroah.com" <greg@kroah.com> - 2017-02-22 14:10 +0100
      Re: [PATCH 1/2] Correct function definition for C++ "greg@kroah.com" <greg@kroah.com> - 2017-02-22 15:30 +0100
    Re: [PATCH 1/2] Correct function definition for C++ Thomas Backlund <tmb@mageia.org> - 2017-03-01 02:00 +0100

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