On Thu, Dec 3, 2015 at 9:29 PM, Jeff Law <law@redhat.com> wrote:
On 12/02/2015 08:35 AM, Richard Biener wrote:
The most interesting side effect, and one I haven't fully analyzed yet is
an
unexpected jump thread -- which I've traced back to differences in what
the
alias oracle is able to find when we walk unaliased vuses. Which makes
totally no sense that it's unable to find the unaliased vuse in the
simplified CFG, but finds it when we don't remove the unexecutable edge.
As
I said, it makes no sense to me yet and I'm still digging.
The walking of PHI nodes is quite simplistic to avoid doing too much work
so
an extra (not executable) edge may confuse it enough. So this might be
"expected". Adding a flag on whether EDGE_EXECUTABLE is to be
trusted would be an option (also helping SCCVN).
Found it. In the CFG with the unexectuable edges _not_ removed there is a
PHI associated with that edge which provides a dominating unaliased vuse.
Once that edge is removed, the PHI arg gets removed and thus we can't easily
see the unaliased vuse.
So all is working as expected. It wasn't ever a big issue, I just wanted to
make sure I thoroughly understood the somewhat counter-intuitive result.
Good. Btw, I remembered that with the unreachable tracking in SCCVN I
ran into cases where it didn't correctly track all unreachable blocks due
to the way the dominator walk works (which doesn't ensure we've visited
all predecessors). Like for
<unreachable>
| |
| <bb3>
| / \
<bb2> / \ /
\ <bb4> <reachable>
\ /
<bb5>
DOM order visits <unreachable>, <bb2>, <bb3>, <bb5> and then
the DOM children like <bb4>. So we fail to detect bb5 as unreachable
(didn't visit bb4 to mark outgoing edges unreachable yet).
The fix is easy (in testing right now). Simply track if the current block
was unreachable when visiting DOM children.
Didn't manage to produce a missed-optimization testcase (but only
tried for a few minutes), the cases I've seen it were involving
unreachable loops, but I don't have them anymore.
Sorry if that makes extracting the machinery harder ;)