[PATCH GCC]More conservative interchanging small loops with const initialized simple reduction

[Bin Cheng <Bin dot Cheng at arm dot com>]

Hi,
This simple patch makes interchange even more conservative for small loops with constant initialized simple reduction.
The reason is undoing such reduction introduces new data reference and cond_expr, which could cost too much in a small
loop.
Test gcc.target/aarch64/pr62178.c is fixed with this patch.  Is it OK if test passes?

Thanks,
bin
2017-12-08  Bin Cheng  <bin.cheng@arm.com>

	* gimple-loop-interchange.cc (struct loop_cand): New field.
	(loop_cand::loop_cand): Init new field in constructor.
	(loop_cand::classify_simple_reduction): Record simple reduction
	initialized with constant value.
	(should_interchange_loops): New parameter.  Skip interchange if loop
	has few data references and constant intitialized simple reduction.
	(tree_loop_interchange::interchange): Update call to above function.
	(should_interchange_loop_nest): Ditto.

Attachment: const-simple-reduc.txt
Description: const-simple-reduc.txt