* 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.


git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@56618 138bc75d-0d04-0410-961f-82ee72b054a4

1 files changed, 54 insertions, 2 deletions

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 1ce61bd451d..e956baa902a 100644
--- a/gcc/doc/invoke.texi
+++ b/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
@@ -211,7 +211,7 @@ in the following sections.
 @xref{Warning Options,,Options to Request or Suppress Warnings}.
 @gccoptlist{
 -fsyntax-only  -pedantic  -pedantic-errors @gol
--w  -W  -Wall  -Waggregate-return @gol
+-w  -W  -Wall -Waggregate-return @gol
 -Wcast-align  -Wcast-qual  -Wchar-subscripts  -Wcomment @gol
 -Wconversion  -Wno-deprecated-declarations @gol
 -Wdisabled-optimization  -Wno-div-by-zero  -Werror @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