re PR tree-optimization/88709 (Improve store-merging)

re PR tree-optimization/88709 (Improve store-merging)

	PR tree-optimization/88709
	PR tree-optimization/90271
	* params.def (PARAM_STORE_MERGING_MAX_SIZE): New parameter.
	* gimple-ssa-store-merging.c (encode_tree_to_bitpos): Handle
	non-clobber CONSTRUCTORs with no elts.  Remove useless tmp_int
	variable.
	(imm_store_chain_info::coalesce_immediate_stores): Punt if the size
	of the store merging group is larger than
	PARAM_STORE_MERGING_MAX_SIZE parameter.
	(split_group): Add bzero_first argument.  If set, always emit first
	the first store which must be = {} of the whole area and then for the
	rest of the stores consider all zero bytes as paddings.
	(imm_store_chain_info::output_merged_store): Check if first store
	is = {} of the whole area and if yes, determine which setting of
	bzero_first for split_group gives smaller number of stores.  Adjust
	split_group callers.
	(lhs_valid_for_store_merging_p): Allow decls.
	(rhs_valid_for_store_merging_p): Allow non-clobber CONTRUCTORs with
	no elts.
	(pass_store_merging::process_store): Likewise.

	* gcc.dg/store_merging_26.c: New test.
	* gcc.dg/store_merging_27.c: New test.
	* gcc.dg/store_merging_28.c: New test.
	* gcc.dg/store_merging_29.c: New test.

From-SVN: r270924