re PR tree-optimization/83326 (SPEC CPU2017 648.exchange2_s ~6% performance regression with r255267 (reproducer attached))

2017-12-14  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/83326
	* tree-ssa-loop-ivcanon.c (try_unroll_loop_completely): Add
	may_be_zero parameter and handle it by not marking the first
	peeled copy as not exiting the loop.
	(try_peel_loop): Likewise.
	(canonicalize_loop_induction_variables): Use number_of_iterations_exit
	to handle the case of constant or zero iterations and perform
	loop header copying on-the-fly.

	* gcc.dg/tree-ssa/pr81388-2.c: Adjust.

From-SVN: r255635