On Tue, Jan 19, 2016 at 01:11:44PM +0100, Jan Hubicka wrote:
> Hi,
> according to Trevor, the assumption about THIS pointer being non-NULL breaks
That was Markus, not me.
> 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
fwiw I find the naming a bit confusing maybe I'm just tired but it takes
some puzlling for me to know which way is being strict and which way is
allowing this.
> 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).
Personally I'd rather try and be strict. I suspect it often will be
easy to find and fix the bugs when the optimization is enabled. Of
course if some projects don't care they can pass flags themselves.
Trev
>
> 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
