c++: Handle __builtin_clear_padding on non-trivially-copyable types [PR102586]

c++: Handle __builtin_clear_padding on non-trivially-copyable types [PR102586]

On Fri, Feb 11, 2022 at 07:55:50PM +0100, Jakub Jelinek via Gcc-patches wrote:
> Something like the https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102586#c16
> will still be needed with adjusted testcase from
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102586#c15 such that
> __builtin_clear_padding is called directly on var addresses rather than
> in separate functions.

Here is an updated version of the
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102586#c15
patch which uses FIELD_DECL in the langhook instead of its TREE_TYPE,
and the testcases have been adjusted for the builtin accepting
pointers to non-trivially-copyable types only if it is address of a
declaration.

2022-04-07  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/102586
gcc/
	* langhooks.h (struct lang_hooks_for_types): Add classtype_as_base
	langhook.
	* langhooks-def.h (LANG_HOOKS_CLASSTYPE_AS_BASE): Define.
	(LANG_HOOKS_FOR_TYPES_INITIALIZER): Add it.
	* gimple-fold.cc (clear_padding_type): Use ftype instead of
	TREE_TYPE (field) some more.  For artificial FIELD_DECLs without
	name try the lang_hooks.types.classtype_as_base langhook and
	if it returns non-NULL, use that instead of ftype for recursive call.
gcc/cp/
	* cp-objcp-common.h (cp_classtype_as_base): Declare.
	(LANG_HOOKS_CLASSTYPE_AS_BASE): Redefine.
	* cp-objcp-common.cc (cp_classtype_as_base): New function.
gcc/testsuite/
	* g++.dg/torture/builtin-clear-padding-5.C: New test.
	* g++.dg/cpp2a/builtin-clear-padding1.C (bar): Uncomment one
	call that is now accepted.