Enable prefetching at -O3 for AMD cpus.

Enable prefetching at -O3 for AMD cpus.

2010-06-25  Changpeng Fang  <changpeng.fang@amd.com>

	* common.opt (fprefetch-loop-arrays): Re-define
	-fprefetch-loop-arrays as a tri-state option with the initial
	value of -1.
	* tree-ssa-loop.c (gate_tree_ssa_loop_prefetch): Invoke prefetch
	pass only when flag_prefetch_loop_arrays > 0.
	* toplev.c (process_options): Note that, with tri-states,
	flag_prefetch_loop_arrays>0 means prefetching is enabled.
	* config/i386/i386.c (override_options): Enable prefetching at -O3
	for a set of CPUs that sw prefetching is helpful.
	(software_prefetching_beneficial_p): New.  Return TRUE if software
	prefetching is beneficial for the given CPU.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@161391 138bc75d-0d04-0410-961f-82ee72b054a4