ipa-sra: Treat REFERENCE_TYPES as always dereferencable

ipa-sra: Treat REFERENCE_TYPES as always dereferencable

C++ and especially Fortran pass data by references which are not
pointers potentially pointing anywhere and so can be assumed to be
safely dereferencable.  This patch teaches IPA-SRA to treat them as
such and avoid the dance we do to prove that we can move loads from
them to the caller.

When we do not know that a dereference will happen all the time, we
need a heuristics so that we do not force memory accesses that normally
happen only rarely.  The patch simply uses the (possibly guessed)
profile and checks whether the (expected) number of loads is at least
half of function invocations invocations - the half is now
configurable with a param as requested by Honza.

gcc/ChangeLog:

2022-12-13  Martin Jambor  <mjambor@suse.cz>

	PR ipa/103585
	* params.opt (ipa-sra-deref-prob-threshold): New parameter.
	* doc/invoke.texi (ipa-sra-deref-prob-threshold): Document it.
	* ipa-sra.cc (struct gensum_param_access): New field load_count.
	(struct gensum_param_desc): New field safe_ref, adjusted comments.
	(by_ref_count): Renamed to unsafe_by_ref_count, adjusted all uses.
	(dump_gensum_access): Dump the new field.
	(dump_gensum_param_descriptor): Likewise.
	(create_parameter_descriptors): Set safe_ref field, move setting
	by_ref forward.  Only increment unsafe_by_ref_count for unsafe
	by_ref parameters.
	(allocate_access): Initialize new field.
	(mark_param_dereference): Adjust indentation.  Only add data to
	bb_dereferences for unsafe by_ref parameters.
	(scan_expr_access): For loads, accumulate BB counts.
	(dereference_probable_p): New function.
	(check_gensum_access): Fix leading comment, add parameter FUN.
	Check cumulative counts of loads for safe by_ref accesses instead
	of dereferences.
	(process_scan_results): Do not propagate dereference distances for
	safe by_ref parameters.  Pass fun to check_gensum_access.  Safe
	by_ref params do not need the postdominance check.

gcc/testsuite/ChangeLog:

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

	* g++.dg/ipa/ipa-sra-5.C: New test