MIPS GCC test failure: gcc.dg/tree-ssa/ssa-dom-thread-4.c

["Steve Ellcey " <sellcey@mips.com>]

Jeff and Richard,

I was looking at the test failure of gcc.dg/tree-ssa/ssa-dom-thread-4.c
on MIPS.  The failure is in the scan of how many jumps are threaded
which has changed from 6 to 4 on MIPS.  I tracked down the change to
this patch:

2013-11-19  Jeff Law  <law@redhat.com>

        * tree-ssa-threadedge.c (thread_across_edge): After threading
        through a joiner, allow threading a normal block requiring
        duplication.

        * tree-ssa-threadupdate.c (thread_block_1): Improve code to detect
        jump threading requests that would muck up the loop structures.

        * tree-ssa-threadupdate.c: Fix trailing whitespace.
        * tree-ssa-threadupdate.h: Likewise.

Now my initial thought is to just change the mips scan to look for
4 'Threaded' lines instead of 6 and be done with it, but the test has
a long explanation of why 6 is the right answer on MIPS and I don't know
what to do with this.  Even after looking at the new and old dom1 dump
files I can't really explain why 4 is the right number now instead of 6.

I am not sure if this is broken on arc or avr now either, I didn't see
any test results from these platforms in gcc-testsuite mailing list
so I am not sure if they were affected or not.

I also noticed that while the dom1 dump is different the final code
generation for MIPS has not changed for this test.

So my question is, can either of you help me with the MIPS explanation
of why 4 is the right number now and/or can I just remove the explanation
and change the MIPS scan?  Should I leave avr and arc alone (split them
off from mips) since I have no evidence that they are failing?

Steve Ellcey
sellcey@mips.com