[PATCH PR79663]Only reversely combine refs for ZERO length chains in predcom

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

Hi,
This patch resolves spec2k/mgrid regression as reported by PR79663.  Root cause has been
described thoroughly in comment #1/#2 of https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79663
This patch handles ZERO/non-ZERO length chains differently and only reversely combines refs
for ZERO length chains.  It also does small improvement with in place list swap.
Bootstrap and test on x86_64 and AArch64, is it OK?

Thanks,
bin

2017-01-21  Bin Cheng  <bin.cheng@arm.com>

	PR tree-optimization/79663
	* tree-predcom.c (combine_chains): Process refs in reverse order
	only for ZERO length chains, and add explaining comment.

Attachment: pr79663-20170221.txt
Description: pr79663-20170221.txt