Re: C++ conversion status update

[Pedro Alves <palves@redhat.com>]

On 12/15/2015 08:02 PM, Simon Marchi wrote:
> On 15 December 2015 at 06:46, Jose E. Marchesi <jose.marchesi@oracle.com> wrote:
>> g++ -g -O2   -I. -I../../gdb -I../../gdb/common -I../../gdb/config -DLOCALEDIR="\"/usr/local/share/locale\"" -DHAVE_CONFIG_H -I../../gdb/../include/opcode -I../../gdb/../opcodes/.. -I../../gdb/../readline/.. -I../../gdb/../zlib -I../bfd -I../../gdb/../bfd -I../../gdb/../include -I../libdecnumber -I../../gdb/../libdecnumber  -I../../gdb/gnulib/import -Ibuild-gnulib/import   -DTUI=1  -I/usr/include/python2.6 -I/usr/include/python2.6 -Wall -Wpointer-arith -Wno-unused -Wunused-value -Wunused-function -Wno-switch -Wno-char-subscripts -Wempty-body -Wno-sign-compare -Wno-write-strings -Wno-narrowing -Wformat-nonliteral -Werror -c -o sparc64-tdep.o -MT sparc64-tdep.o -MMD -MP -MF .deps/sparc64-tdep.Tpo ../../gdb/sparc64-tdep.c
>> cc1plus: warnings being treated as errors
>> In file included from ../../gdb/target.h:74,
>>                  from ../../gdb/exec.h:23,
>>                  from ../../gdb/gdbcore.h:29,
>>                  from ../../gdb/sparc64-tdep.c:27:
>> ../../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)
>> At global scope:
>> cc1plus: error: unrecognized command line option "-Wno-narrowing"
>> make[2]: *** [sparc64-tdep.o] Error 1
>> make[2]: Leaving directory `/home/jemarch/couts2/binutils-gdb/build/gdb'
>> make[1]: *** [all-gdb] Error 2
>> make[1]: Leaving directory `/home/jemarch/couts2/binutils-gdb/build'
>> make: *** [all] Error 2
> 
> This can be reproduced with g++ 4.4 on any architecture, i think.
> 
> This seems to be caused by the interaction of enum_flags and the
> vector implementation.  After preprocessing we have this:
> 
>   __builtin_offsetof (VEC_btrace_insn_s, vec)
> 
> However, VEC_btrace_insn_s is a non-POD data type, because it
> ultimately contains an enum_flags object, which is non-POD.
> 
>   VEC_btrace_insn_s
>     btrace_insn_s vec[1];
>       btrace_insn_flags flags; (enum_flags <enum btrace_insn_flag>)
> 
> From what I read, using offsetof on a non-POD structure is an
> undefined behavior, at least in C++98.  I don't know if it has changed
> in later standard versions, but later g++ do not complain.

Yeah, C++11 relaxed the rules a lot.  enum_flags is designed to be
POD-like, though it has user-defined constructors.  It's quite safe
to memcpy it.

> 
> For g++ 4.4, it is possible to silence the warning/error with
> -Wno-invalid-offsetof, if we know that it still produces good results.
> 

Yeah, it should work.  The real fix of course would be to
reimplement vec.h using real C++ templates (until we can get rid of
C mode, that is).

An alternative is to avoid offsetof in the code itself, like in the
patch below.  It's really offsetof-but-avoid-NULL.

An advantage over disabling the warning is that it's contained in
the VEC code, which lets G++ catch issues elsewhere.  OTOH, disabling
warning is simpler.  WDYT?

From c0803a1aca62cde14c47a103aa996f1e1f4a2875 Mon Sep 17 00:00:00 2001
From: Pedro Alves <palves@redhat.com>
Date: Tue, 15 Dec 2015 22:43:06 +0000
Subject: [PATCH] no offsetof

---
 gdb/common/vec.h | 44 ++++++++++++++++++++++++++++++++++----------
 1 file changed, 34 insertions(+), 10 deletions(-)

diff --git a/gdb/common/vec.h b/gdb/common/vec.h
index 6189283..a188b02 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;							  \
 }									  \
-- 
1.9.3