pr44576 Avoid un-necessary prefetch analysis by distributing the cost models

pr44576 Avoid un-necessary prefetch analysis by distributing the cost models

2010-07-09  Changpeng Fang  <changpeng.fang@amd.com>

	PR tree-optimization/44576
	* tree-ssa-loop-prefetch.c (trip_count_to_ahead_ratio_too_small_p):
	New.  Pull out from is_loop_prefetching_profitable to implement
	the trip count to ahead ratio heuristic.
	(mem_ref_count_reasonable_p): New.  Pull out from
	is_loop_prefetching_profitable to implement the instruction to
	memory reference ratio heuristic.  Also consider not reasonable if
	the memory reference count is above a threshold (to avoid
	explosive compilation time.
	(insn_to_prefetch_ratio_too_small_p): New.  Pull out from
	is_loop_prefetching_profitable to implement the instruction to
	prefetch ratio heuristic.
	(is_loop_prefetching_profitable): Removed.
	(loop_prefetch_arrays): Distribute the cost analysis across the
	function to allow early exit of the prefetch analysis.
	is_loop_prefetching_profitable is splitted into three functions,
	with each one called as early as possible.
	(PREFETCH_MAX_MEM_REFS_PER_LOOP): New.  Threshold above which the
	number of memory references in a loop is considered too many.

From-SVN: r162023