[C++] Fix -Winvalid-offsetof warnings with g++ 4.4

Ref: https://sourceware.org/ml/gdb/2015-12/msg00014.html

Fixes the build in C++ mode with g++ 4.4:

 gdb/btrace.h: In function ‘size_t VEC_btrace_insn_s_embedded_size(int)’:
 gdb/btrace.h:84: error: invalid access to non-static data member ‘VEC_btrace_insn_s::vec’  of NULL object
 gdb/btrace.h:84: error: (perhaps the ‘offsetof’ macro was used incorrectly)
 gdb/btrace.h: In function ‘VEC_btrace_insn_s* VEC_btrace_insn_s_alloc(int)’:
 gdb/btrace.h:84: error: invalid access to non-static data member ‘VEC_btrace_insn_s::vec’  of NULL object
 gdb/btrace.h:84: error: (perhaps the ‘offsetof’ macro was used incorrectly)
 gdb/btrace.h: In function ‘VEC_btrace_insn_s* VEC_btrace_insn_s_copy(VEC_btrace_insn_s*)’:
 gdb/btrace.h:84: error: invalid access to non-static data member ‘VEC_btrace_insn_s::vec’  of NULL object
 gdb/btrace.h:84: error: (perhaps the ‘offsetof’ macro was used incorrectly)
 gdb/btrace.h: In function ‘VEC_btrace_insn_s* VEC_btrace_insn_s_merge(VEC_btrace_insn_s*, VEC_btrace_insn_s*)’:
 gdb/btrace.h:84: error: invalid access to non-static data member ‘VEC_btrace_insn_s::vec’  of NULL object
 gdb/btrace.h:84: error: (perhaps the ‘offsetof’ macro was used incorrectly)
 gdb/btrace.h: In function ‘int VEC_btrace_insn_s_reserve(VEC_btrace_insn_s**, int, const char*, unsigned int)’:
 gdb/btrace.h:84: error: invalid access to non-static data member ‘VEC_btrace_insn_s::vec’  of NULL object
 gdb/btrace.h:84: error: (perhaps the ‘offsetof’ macro was used incorrectly)

gdb/ChangeLog:
2015-12-16  Pedro Alves  <palves@redhat.com>

	* common/vec.h (vec_offset): New macro.
	(DEF_VEC_ALLOC_FUNC_I, DEF_VEC_ALLOC_FUNC_O): Use it instead of
	offsetof.

1 files changed, 34 insertions, 10 deletions

diff --git a/gdb/common/vec.h b/gdb/common/vec.h
index 6189283a03b..a188b020620 100644
--- a/gdb/common/vec.h
+++ b/gdb/common/vec.h
@@ -437,13 +437,23 @@ DEF_VEC_FUNC_O(T)							  \
 DEF_VEC_ALLOC_FUNC_O(T)							  \
 struct vec_swallow_trailing_semi
 
+/* Avoid offsetof (or its usual C implementation) as it triggers
+   -Winvalid-offsetof warnings with enum_flags types with G++ <= 4.4,
+   even though those types are memcpyable.  This requires allocating a
+   dummy local VEC in all routines that use this, but that has the
+   advantage that it only works if T is default constructible, which
+   is exactly a check we want, to keep C compatibility.  */
+#define vec_offset(T, VPTR) ((size_t) ((char *) &(VPTR)->vec - (char *) VPTR))
+
 #define DEF_VEC_ALLOC_FUNC_I(T)						  \
 static inline VEC(T) *VEC_OP (T,alloc)					  \
      (int alloc_)							  \
 {									  \
+  VEC(T) dummy;							  	  \
+									  \
   /* We must request exact size allocation, hence the negation.  */	  \
   return (VEC(T) *) vec_o_reserve (NULL, -alloc_,			  \
-                                   offsetof (VEC(T),vec), sizeof (T));	  \
+                                   vec_offset (T, &dummy), sizeof (T));	  \
 }									  \
 									  \
 static inline VEC(T) *VEC_OP (T,copy) (VEC(T) *vec_)			  \
@@ -453,9 +463,11 @@ static inline VEC(T) *VEC_OP (T,copy) (VEC(T) *vec_)			  \
 									  \
   if (len_)								  \
     {									  \
+      VEC(T) dummy;							  \
+									  \
       /* We must request exact size allocation, hence the negation.  */	  \
       new_vec_ = (VEC (T) *)						  \
-	vec_o_reserve (NULL, -len_, offsetof (VEC(T),vec), sizeof (T));	  \
+	vec_o_reserve (NULL, -len_, vec_offset (T, &dummy), sizeof (T));	\
 									  \
       new_vec_->num = len_;						  \
       memcpy (new_vec_->vec, vec_->vec, sizeof (T) * len_);		  \
@@ -467,12 +479,13 @@ static inline VEC(T) *VEC_OP (T,merge) (VEC(T) *vec1_, VEC(T) *vec2_)	  \
 {									  \
   if (vec1_ && vec2_)							  \
     {									  \
+      VEC(T) dummy;							  \
       size_t len_ = vec1_->num + vec2_->num;				  \
       VEC (T) *new_vec_ = NULL;						  \
 									  \
       /* We must request exact size allocation, hence the negation.  */	  \
       new_vec_ = (VEC (T) *)						  \
-	vec_o_reserve (NULL, -len_, offsetof (VEC(T),vec), sizeof (T));	  \
+	vec_o_reserve (NULL, -len_, vec_offset (T, &dummy), sizeof (T));	  \
 									  \
       new_vec_->num = len_;						  \
       memcpy (new_vec_->vec, vec1_->vec, sizeof (T) * vec1_->num);	  \
@@ -505,12 +518,13 @@ static inline void VEC_OP (T,cleanup)					  \
 static inline int VEC_OP (T,reserve)					  \
      (VEC(T) **vec_, int alloc_ VEC_ASSERT_DECL)			  \
 {									  \
+  VEC(T) dummy;								  \
   int extend = !VEC_OP (T,space)					  \
 	(*vec_, alloc_ < 0 ? -alloc_ : alloc_ VEC_ASSERT_PASS);		  \
 									  \
   if (extend)								  \
     *vec_ = (VEC(T) *) vec_o_reserve (*vec_, alloc_,			  \
-				      offsetof (VEC(T),vec), sizeof (T)); \
+				      vec_offset (T, &dummy), sizeof (T)); \
 									  \
   return extend;							  \
 }									  \
@@ -581,7 +595,9 @@ static inline int VEC_OP (T,iterate)					  \
 static inline size_t VEC_OP (T,embedded_size)				  \
      (int alloc_)							  \
 {									  \
-  return offsetof (VEC(T),vec) + alloc_ * sizeof(T);			  \
+  VEC(T) dummy;								  \
+									  \
+  return vec_offset (T, &dummy) + alloc_ * sizeof(T);			  \
 }									  \
 									  \
 static inline void VEC_OP (T,embedded_init)				  \
@@ -863,7 +879,9 @@ static inline int VEC_OP (T,iterate)					  \
 static inline size_t VEC_OP (T,embedded_size)				  \
      (int alloc_)							  \
 {									  \
-  return offsetof (VEC(T),vec) + alloc_ * sizeof(T);			  \
+  VEC(T) dummy;								  \
+									  \
+  return vec_offset (T, &dummy) + alloc_ * sizeof(T);			  \
 }									  \
 									  \
 static inline void VEC_OP (T,embedded_init)				  \
@@ -998,9 +1016,11 @@ static inline unsigned VEC_OP (T,lower_bound)				  \
 static inline VEC(T) *VEC_OP (T,alloc)					  \
      (int alloc_)							  \
 {									  \
+  VEC(T) dummy;								  \
+									  \
   /* We must request exact size allocation, hence the negation.  */	  \
   return (VEC(T) *) vec_o_reserve (NULL, -alloc_,			  \
-                                   offsetof (VEC(T),vec), sizeof (T));	  \
+                                   vec_offset (T, &dummy), sizeof (T));	  \
 }									  \
 									  \
 static inline VEC(T) *VEC_OP (T,copy) (VEC(T) *vec_)			  \
@@ -1010,9 +1030,11 @@ static inline VEC(T) *VEC_OP (T,copy) (VEC(T) *vec_)			  \
 									  \
   if (len_)								  \
     {									  \
+      VEC(T) dummy;							  \
+									  \
       /* We must request exact size allocation, hence the negation.  */	  \
       new_vec_ = (VEC (T) *)						  \
-	vec_o_reserve  (NULL, -len_, offsetof (VEC(T),vec), sizeof (T));  \
+	vec_o_reserve  (NULL, -len_, vec_offset (T, &dummy), sizeof (T)); \
 									  \
       new_vec_->num = len_;						  \
       memcpy (new_vec_->vec, vec_->vec, sizeof (T) * len_);		  \
@@ -1024,12 +1046,13 @@ static inline VEC(T) *VEC_OP (T,merge) (VEC(T) *vec1_, VEC(T) *vec2_)	  \
 {									  \
   if (vec1_ && vec2_)							  \
     {									  \
+      VEC(T) dummy;							  \
       size_t len_ = vec1_->num + vec2_->num;				  \
       VEC (T) *new_vec_ = NULL;						  \
 									  \
       /* We must request exact size allocation, hence the negation.  */	  \
       new_vec_ = (VEC (T) *)						  \
-	vec_o_reserve (NULL, -len_, offsetof (VEC(T),vec), sizeof (T));	  \
+	vec_o_reserve (NULL, -len_, vec_offset (T, &dummy), sizeof (T));  \
 									  \
       new_vec_->num = len_;						  \
       memcpy (new_vec_->vec, vec1_->vec, sizeof (T) * vec1_->num);	  \
@@ -1062,12 +1085,13 @@ static inline void VEC_OP (T,cleanup)					  \
 static inline int VEC_OP (T,reserve)					  \
      (VEC(T) **vec_, int alloc_ VEC_ASSERT_DECL)			  \
 {									  \
+  VEC(T) dummy;								  \
   int extend = !VEC_OP (T,space) (*vec_, alloc_ < 0 ? -alloc_ : alloc_	  \
 				  VEC_ASSERT_PASS);			  \
 									  \
   if (extend)								  \
     *vec_ = (VEC(T) *)							  \
-	vec_o_reserve (*vec_, alloc_, offsetof (VEC(T),vec), sizeof (T)); \
+      vec_o_reserve (*vec_, alloc_, vec_offset (T, &dummy), sizeof (T));  \
 									  \
   return extend;							  \
 }									  \