PR middle-end/31150
	* dse.c (struct store_info): Add const_rhs field.
	(clear_rhs_from_active_local_stores): Clear also const_rhs.
	(record_store): Try also cselib_expand_value_rtx to get a constant.
	(find_shift_sequence, get_stored_val): Use const_rhs instead of
	rhs if worthwhile.
	* cselib.c (cselib_record_sets): If !cselib_record_memory and
	there is just one set from read-only MEM, look at REG_EQUAL or
	REG_EQUIV note.

	* dse.c (struct store_info): Add redundant_reason field.
	(record_store): When storing the same constant as has been
	stored by an earlier store, set redundant_reason field
	to the earlier store's insn_info_t.  Don't delete cannot_delete
	insns.
	(find_shift_sequence): Remove read_info argument, add read_mode
	and require_cst arguments.  Return early if require_cst and
	constant wouldn't be returned.
	(get_stored_val): New function.
	(replace_read): Use it.
	(scan_insn): Put even cannot_delete insns with exactly 1 store
	into active_local_stores.
	(dse_step1): Don't delete cannot_delete insns.  Remove redundant
	constant stores if contains_cselib_groups and earlier store storing
	the same value hasn't been eliminated.
	(dse_step6): Renamed to dse_step7.  New function.
	(dse_step7): Renamed from dse_step6.
	(rest_of_handle_dse): Call dse_step6 and dse_step7 at the end.
	* cselib.c (cselib_expand_value_rtx): Don't wrap CONST_INTs
	into CONST unless really necessary.  Handle SUBREG, unary,
	ternary, bitfield and compares specially, to be able to simplify
	operations on constants.
	(expand_loc): Try to optimize LO_SUM.

	* dse.c (get_call_args): New function.
	(scan_insn): Don't handle BUILT_IN_BZERO.  For memset, attempt
	to get call arguments and if successful and both len and val are
	constants, handle the call as (mem:BLK) (const_int) store.

	* dse.c (struct store_info): Add is_large bool field, change
	positions_needed into a union of a bitmask and bitmap + count.
	(free_store_info): Free bitmap if is_large.
	(set_usage_bits): Don't look at stores where
	offset + width >= MAX_OFFSET.
	(set_position_unneeded, set_all_positions_unneeded,
	any_positions_needed_p, all_positions_needed_p): New static inline
	functions.
	(record_store): Handle BLKmode stores of CONST_INT, if
	MEM_SIZE is set on the MEM.  Use the new positions_needed
	accessor inlines.
	(replace_read): Handle reads from BLKmode CONST_INT stores.
	(check_mem_read_rtx): Use all_positions_needed_p function.
	(dse_step1): Free large positions_needed bitmaps and clear is_large.

	* dse.c (struct store_info): Change begin and end types to
	HOST_WIDE_INT.

	* dse.c (record_store): Fix check for unused store.

	* expr.c (block_clear_fn): No longer static.
	* expr.h (block_clear_fn): Declare.
	* dse.c (scan_insn): Memset and bzero can just read their
	arguments.

	* gcc.c-torture/execute/20081218-1.c: New test.

From-SVN: r142892