[C++] Add -fnull-this-pointer

[Jan Hubicka <hubicka at ucw dot cz>]

Hi,
according to Trevor, the assumption about THIS pointer being non-NULL breaks
several bigger C++ packages (definitly including Firefox, but I believe
kdevelop was mentioned, too).  This patch makes the feature to be controlable
by a dedicated flag.  I am not sure about the default. We now have ubsan check
for the bug so I would hope the codebases to be updated soon, but it did not
happen for Firefox for quite a while despite the fact that Martin Liska reported
it.

This patch defaults to -fno-null-this-pointer, but I would be OK with changing
the default and setting it on only in GCC 6. Main point of the patch is to
avoid need of those packages to be built with -fno-delete-null-pointer-checks
(which still subsumes the flag).

The patch is bit inconsistent, becuase C++ FE wil still assume that this pointer
is non-NULL when expanding multiple inheritance accesses.  We did this from
very beginning. I do not know FE enough to see if it is easy to change the
behaviour here or if it is desired.

Bootstrapped/regtsted x86_64-linux.

Honza

	* c-family/c.opt (fnull-this-pointer): New flag.
	(nonnull_arg_p): Honnor flag_null_this_pointer.
Index: tree.c
===================================================================
--- tree.c	(revision 232553)
+++ tree.c	(working copy)
@@ -14016,6 +14022,7 @@ nonnull_arg_p (const_tree arg)
   /* THIS argument of method is always non-NULL.  */
   if (TREE_CODE (TREE_TYPE (cfun->decl)) == METHOD_TYPE
       && arg == DECL_ARGUMENTS (cfun->decl)
+      && flag_null_this_pointer
       && flag_delete_null_pointer_checks)
     return true;
 
Index: c-family/c.opt
===================================================================
--- c-family/c.opt	(revision 232553)
+++ c-family/c.opt	(working copy)
@@ -1321,6 +1321,10 @@ Enum(ivar_visibility) String(public) Val
 EnumValue
 Enum(ivar_visibility) String(package) Value(IVAR_VISIBILITY_PACKAGE)
 
+fnull-this-pointer
+C++ ObjC++ Optimization Report Var(flag_null_this_pointer)
+Allow calling methods of NULL pointer
+
 fnonansi-builtins
 C++ ObjC++ Var(flag_no_nonansi_builtin, 0)
 
Index: doc/invoke.texi
===================================================================
--- doc/invoke.texi	(revision 232553)
+++ doc/invoke.texi	(working copy)
@@ -232,6 +232,7 @@ Objective-C and Objective-C++ Dialects}.
 -fobjc-std=objc1 @gol
 -fno-local-ivars @gol
 -fivar-visibility=@r{[}public@r{|}protected@r{|}private@r{|}package@r{]} @gol
+-fnull-this-pointer @gol
 -freplace-objc-classes @gol
 -fzero-link @gol
 -gen-decls @gol
@@ -2361,6 +2362,11 @@ errors if these functions are not inline
 Disable Wpedantic warnings about constructs used in MFC, such as implicit
 int and getting a pointer to member function via non-standard syntax.
 
+@item -fnull-this-pointer
+@opindex fnull-this-pointer
+Disable optimization which take advantage of the fact that calling method
+of @code{NULL} pointer is undefined.
+
 @item -fno-nonansi-builtins
 @opindex fno-nonansi-builtins
 Disable built-in declarations of functions that are not mandated by