ipa: Careful processing ANCESTOR jump functions and NULL pointers (PR 103083)

ipa: Careful processing ANCESTOR jump functions and NULL pointers (PR 103083)

IPA_JF_ANCESTOR jump functions are constructed also when the formal
parameter of the caller is first checked whether it is NULL and left
as it is if it is NULL, to accommodate C++ casts to an ancestor class.

The jump function type was invented for devirtualization and IPA-CP
propagation of tree constants is also careful to apply it only to
existing DECLs(*) but as PR 103083 shows, the part propagating "known
bits" was not careful about this, which can lead to miscompilations.

This patch introduces a flag to the ancestor jump functions which
tells whether a NULL-check was elided when creating it and makes the
bits propagation behave accordingly, masking any bits otherwise would
be known to be one.  This should safely preserve alignment info, which
is the primary ifnormation that we keep in bits for pointers.

(*) There still may remain problems when a DECL resides on address
zero (with -fno-delete-null-pointer-checks ...I hope it cannot happen
otherwise).  I am looking into that now but I think it will be easier
for everyone if I do so in a follow-up patch.

gcc/ChangeLog:

2022-02-11  Martin Jambor  <mjambor@suse.cz>

	PR ipa/103083
	* ipa-prop.h (ipa_ancestor_jf_data): New flag keep_null;
	(ipa_get_jf_ancestor_keep_null): New function.
	* ipa-prop.c (ipa_set_ancestor_jf): Initialize keep_null field of the
	ancestor function.
	(compute_complex_assign_jump_func): Pass false to keep_null
	parameter of ipa_set_ancestor_jf.
	(compute_complex_ancestor_jump_func): Pass true to keep_null
	parameter of ipa_set_ancestor_jf.
	(update_jump_functions_after_inlining): Carry over keep_null from the
	original ancestor jump-function or merge them.
	(ipa_write_jump_function): Stream keep_null flag.
	(ipa_read_jump_function): Likewise.
	(ipa_print_node_jump_functions_for_edge): Print the new flag.
	* ipa-cp.c (class ipcp_bits_lattice): Make various getters const.  New
	member function known_nonzero_p.
	(ipcp_bits_lattice::known_nonzero_p): New.
	(ipcp_bits_lattice::meet_with_1): New parameter drop_all_ones,
	observe it.
	(ipcp_bits_lattice::meet_with): Likewise.
	(propagate_bits_across_jump_function): Simplify.  Pass true in
	drop_all_ones when it is necessary.
	(propagate_aggs_across_jump_function): Take care of keep_null
	flag.
	(ipa_get_jf_ancestor_result): Propagate NULL accross keep_null
	jump functions.

gcc/testsuite/ChangeLog:

2021-11-25  Martin Jambor  <mjambor@suse.cz>

	* gcc.dg/ipa/pr103083-1.c: New test.
	* gcc.dg/ipa/pr103083-2.c: Likewise.

(cherry picked from commit 7ea3a73c195a79e6740ae594ee1a14c8bf7a938d)