c-common.c (warn_abi): New variable.

	* c-common.c (warn_abi): New variable.
	* c-common.h (warn_abi): Likewise.
	* c-opts.c (COMMAND_LINE_OPTIONS): Add -Wabi.
	(c_common_decode_option): Handle it.
	* doc/invoke.texi:P Document -Wabi.

	* class.c (layout_virtual_bases): Warn about bugs in G++ that
	result in incorrect object layouts.
	(layout_class_type): Likewise.

	* testsuite/g++.dg/abi/bitfield5.C: New test.
	* testsuite/g++.dg/abi/vbase10.C: Likewise.

From-SVN: r56618

gcc/ChangeLog

+2002-08-27  Mark Mitchell  <mark@codesourcery.com>
+
+	* c-common.c (warn_abi): New variable.
+	* c-common.h (warn_abi): Likewise.
+	* c-opts.c (COMMAND_LINE_OPTIONS): Add -Wabi.
+	(c_common_decode_option): Handle it.
+	* doc/invoke.texi:P Document -Wabi.
+	
 Tue Aug 27 23:03:52 2002  Nicola Pero  <n.pero@mi.flashnet.it>
 
 	* c-common.c (warn_undeclared_selector): New variable.

gcc/c-common.c

@@ -566,6 +566,11 @@ int flag_permissive;
 
 int flag_enforce_eh_specs = 1;
 
+/* Nonzero means warn about things that will change when compiling
+   with an ABI-compliant compiler.  */
+
+int warn_abi = 0;
+
 /* Nonzero means warn about implicit declarations.  */
 
 int warn_implicit = 1;

gcc/c-common.h

@@ -737,6 +737,11 @@ extern int flag_permissive;
 
 extern int flag_enforce_eh_specs;
 
+/* Nonzero means warn about things that will change when compiling
+   with an ABI-compliant compiler.  */
+
+extern int warn_abi;
+
 /* Nonzero means warn about implicit declarations.  */
 
 extern int warn_implicit;

gcc/c-opts.c

@@ -124,6 +124,7 @@ static void sanitize_cpp_opts PARAMS ((void));
   OPT("MQ",                     CL_ALL | CL_ARG, OPT_MQ)		     \
   OPT("MT",                     CL_ALL | CL_ARG, OPT_MT)		     \
   OPT("P",                      CL_ALL,   OPT_P)			     \
+  OPT("Wabi",                   CL_CXX,   OPT_Wabi)                          \
   OPT("Wall",			CL_ALL,   OPT_Wall)			     \
   OPT("Wbad-function-cast",	CL_C,     OPT_Wbad_function_cast)	     \
   OPT("Wcast-qual",		CL_ALL,   OPT_Wcast_qual)		     \
@@ -684,6 +685,10 @@ c_common_decode_option (argc, argv)
       cpp_opts->no_line_commands = 1;
       break;
 
+    case OPT_Wabi:
+      warn_abi = on;
+      break;
+
     case OPT_Wall:
       set_Wunused (on);
       set_Wformat (on);

gcc/cp/ChangeLog

+2002-08-27  Mark Mitchell  <mark@codesourcery.com>
+
+	* class.c (layout_virtual_bases): Warn about bugs in G++ that
+	result in incorrect object layouts.
+	(layout_class_type): Likewise.
+	
 2002-08-24  Matt Austern  <austern@apple.com>
 
 	* tree.c (lvalue_p_1): Add argument for whether casts of lvalues

gcc/cp/class.c

@@ -4562,6 +4562,7 @@ layout_virtual_bases (t, offsets)
 {
   tree vbases, dsize;
   unsigned HOST_WIDE_INT eoc;
+  bool first_vbase = true;
 
   if (CLASSTYPE_N_BASECLASSES (t) == 0)
     return;
@@ -4589,6 +4590,7 @@ layout_virtual_bases (t, offsets)
 
       if (!TREE_VIA_VIRTUAL (vbases))
 	continue;
+
       vbase = binfo_for_vbase (BINFO_TYPE (vbases), t);
 
       if (!BINFO_PRIMARY_P (vbase))
@@ -4606,7 +4608,6 @@ layout_virtual_bases (t, offsets)
 	  /* Add padding so that we can put the virtual base class at an
 	     appropriately aligned offset.  */
 	  dsize = round_up (dsize, desired_align);
-
 	  usize = size_binop (CEIL_DIV_EXPR, dsize, bitsize_unit_node);
 
 	  /* We try to squish empty virtual bases in just like
@@ -4634,11 +4635,30 @@ layout_virtual_bases (t, offsets)
 					      CLASSTYPE_SIZE (basetype)));
 	    }
 
+	  /* If the first virtual base might have been placed at a
+	     lower address, had we started from CLASSTYPE_SIZE, rather
+	     than TYPE_SIZE, issue a warning.  There can be both false
+	     positives and false negatives from this warning in rare
+	     cases; to deal with all the possibilities would probably
+	     require performing both layout algorithms and comparing
+	     the results which is not particularly tractable.  */
+	  if (warn_abi
+	      && first_vbase
+	      && tree_int_cst_lt (size_binop (CEIL_DIV_EXPR,
+					      round_up (CLASSTYPE_SIZE (t),
+							desired_align),
+					      bitsize_unit_node),
+				  BINFO_OFFSET (vbase)))
+	    warning ("offset of virtual base `%T' is not ABI-compliant and may change in a future version of GCC",
+		     basetype);
+
 	  /* Keep track of the offsets assigned to this virtual base.  */
 	  record_subobject_offsets (BINFO_TYPE (vbase), 
 				    BINFO_OFFSET (vbase),
 				    offsets,
 				    /*vbases_p=*/0);
+
+	  first_vbase = false;
 	}
     }
 
@@ -4776,6 +4796,8 @@ layout_class_type (t, empty_p, vfuns_p, virtuals_p)
   /* Maps offsets (represented as INTEGER_CSTs) to a TREE_LIST of
      types that appear at that offset.  */
   splay_tree empty_base_offsets;
+  /* True if the last field layed out was a bit-field.  */
+  bool last_field_was_bitfield = false;
 
   /* Keep track of the first non-static data member.  */
   non_static_data_members = TYPE_FIELDS (t);
@@ -4865,6 +4887,18 @@ layout_class_type (t, empty_p, vfuns_p, virtuals_p)
       layout_nonempty_base_or_field (rli, field, NULL_TREE,
 				     empty_base_offsets, t);
 
+      /* If a bit-field does not immediately follow another bit-field,
+	 and yet it starts in the middle of a byte, we have failed to
+	 comply with the ABI.  */
+      if (warn_abi
+	  && DECL_C_BIT_FIELD (field) 
+	  && !last_field_was_bitfield
+	  && !integer_zerop (size_binop (TRUNC_MOD_EXPR,
+					 DECL_FIELD_BIT_OFFSET (field),
+					 bitsize_unit_node)))
+	cp_warning_at ("offset of `%D' is not ABI-compliant and may change in a future version of GCC", 
+		       field);
+
       /* If we needed additional padding after this field, add it
 	 now.  */
       if (padding)
@@ -4882,6 +4916,8 @@ layout_class_type (t, empty_p, vfuns_p, virtuals_p)
 					 NULL_TREE, 
 					 empty_base_offsets, t);
 	}
+
+      last_field_was_bitfield = DECL_C_BIT_FIELD (field);
     }
 
   /* It might be the case that we grew the class to allocate a

gcc/doc/invoke.texi

@@ -187,7 +187,7 @@ in the following sections.
 -fno-optional-diags  -fpermissive @gol
 -frepo  -fno-rtti  -fstats  -ftemplate-depth-@var{n} @gol
 -fuse-cxa-atexit  -fvtable-gc  -fno-weak  -nostdinc++ @gol
--fno-default-inline  -Wctor-dtor-privacy @gol
+-fno-default-inline -Wabi -Wctor-dtor-privacy @gol
 -Wnon-virtual-dtor  -Wreorder @gol
 -Weffc++  -Wno-deprecated @gol
 -Wno-non-template-friend  -Wold-style-cast @gol
@@ -1475,6 +1475,58 @@ Do not assume @samp{inline} for functions defined inside a class scope.
 functions will have linkage like inline functions; they just won't be
 inlined by default.
 
+@item -Wabi @r{(C++ only)}
+@opindex Wabi
+Warn when G++ generates code that is probably not compatible with the
+vendor-neutral C++ ABI.  Although an effort has been made to warn about
+all such cases, there are probably some cases that are not warned about, 
+even though G++ is generating incompatible code.  There may also be
+cases where warnings are emitted even though the code that is generated
+will be compatible.
+
+You should rewrite your code to avoid these warnings if you are
+concerned about the fact that code generated by G++ may not be binary
+compatible with code generated by other compilers.
+
+The known incompatibilites at this point include:
+
+@itemize @bullet
+
+@item
+Incorrect handling of tail-padding for bit-fields.  G++ may attempt to
+pack data into the same byte as a base class.  For example:
+
+@smallexample
+struct A @{ virtual void f(); int f1 : 1; @};
+struct B : public A @{ int f2 : 1; @};
+@end smallexample
+
+@noindent
+In this case, G++ will place @code{B::f2} into the same byte
+as@code{A::f1}; other compilers will not.  You can avoid this problem 
+by explicitly padding @code{A} so that its size is a multiple of the
+byte size on your platform; that will cause G++ and other compilers to
+layout @code{B} identically.
+
+@item
+Incorrect handling of tail-padding for virtual bases.  G++ does not use
+tail padding when laying out virtual bases.  For example:
+
+@smallexample
+struct A @{ virtual void f(); char c1; @};
+struct B @{ B(); char c2; @};
+struct C : public A, public virtual B @{@};
+@end smallexample
+
+@noindent
+In this case, G++ will not place @code{B} into the tail-padding for
+@code{A}; other compilers will.  You can avoid this problem by
+explicitly padding @code{A} so that its size is a multiple of its
+alignment (ignoring virtual base classes); that will cause G++ and other
+compilers to layout @code{C} identically.
+
+@end itemize
+
 @item -Wctor-dtor-privacy @r{(C++ only)}
 @opindex Wctor-dtor-privacy
 Warn when a class seems unusable, because all the constructors or

gcc/testsuite/ChangeLog

+2002-08-27  Mark Mitchell  <mark@codesourcery.com>
+
+	* testsuite/g++.dg/abi/bitfield5.C: New test.
+	* testsuite/g++.dg/abi/vbase10.C: Likewise.
+
 Tue Aug 27 22:23:22 2002  Nicola Pero  <n.pero@mi.flashnet.it>
 
 	* objc.dg/undeclared-selector.m: New test.

gcc/testsuite/g++.dg/abi/bitfield5.C

+// { dg-do compile } 
+// { dg-options "-Wabi" }
+
+struct A { 
+  virtual void f(); 
+  int f1 : 1; 
+};
+
+struct B : public A {
+  int f2 : 1;  // { dg-warning "ABI" }
+  int : 0;
+  int f3 : 4; 
+  int f4 : 3;
+};

gcc/testsuite/g++.dg/abi/vbase10.C

+// { dg-do compile }
+// { dg-options "-Wabi" }
+
+struct A { virtual void f(); char c1; };
+struct B { B(); char c2; };
+struct C : public A, public virtual B {}; // { dg-warning "ABI" }
+