re PR target/92038 (Extremely inefficient x86_64 code for trivally copyable types passed in registers.)
PR target/92038
* gimple-ssa-store-merging.c (find_constituent_stores): For return
value only, return non-NULL if there is a single non-clobber
constituent store even if there are constituent clobbers and return
one of clobber constituent stores if all constituent stores are
clobbers.
(split_group): Handle clobbers.
(imm_store_chain_info::output_merged_store): When computing
bzero_first, look after all clobbers at the start. Don't count
clobber stmts in orig_num_stmts, except if the first orig store is
a clobber covering the whole area and split_stores cover the whole
area, consider equal number of stmts ok. Punt if split_stores
contains only ->orig stores and their number plus number of original
clobbers is equal to original number of stmts. For ->orig, look past
clobbers in the constituent stores.
(imm_store_chain_info::output_merged_stores): Don't remove clobber
stmts.
(rhs_valid_for_store_merging_p): Don't return false for clobber stmt
rhs.
(store_valid_for_store_merging_p): Allow clobber stmts.
(verify_clear_bit_region_be): Fix up a thinko in function comment.
* g++.dg/opt/store-merging-1.C: New test.
* g++.dg/opt/store-merging-2.C: New test.
* g++.dg/opt/store-merging-3.C: New test.
From-SVN: r277963
