https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90037
Jeffrey A. Law <law at redhat dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Priority|P3 |P2
Component|middle-end |tree-optimization
--- Comment #8 from Jeffrey A. Law <law at redhat dot com> ---
So, if we change phionlycprop to look for other const/copy initializations that
can be eliminated and run a pass between DOM and the erroneous-path isolation
pass, then the false positive is eliminated (as expected).
There's two things I don't like about that. First, it turns phionlycprop into
a full-fledged constant propagation pass. phionlycprop is supposed to be so
fast that we never really notice it. It accomplishes this by only looking at
PHI nodes that are degenerates and any constants exposed by propagating way the
degenerate PHI. Essentially it's just cleaning up painfully obvious cruft left
by jump threading.
To pick up this case we'd have to scan statements in blocks. We could restrict
that to blocks where we eliminated degenerate PHI. But still. Ugh.
Second, once phionlycprop is doing more work, I'm less inclined to want to add
another instance of the pass.
Finally, once phionlycprop is doing more work one could legitimately ask if we
should just drop the code and use the lattice copy propagator.
Just for fun I replaced all the phi-only cprop calls with calls into the
lattice propagator (including the one I added between DOM and erroneous-path
optimization). As expected that fixes the testcase too. It also happens to
clean up things slightly better at an earlier point in the optimizer pipeline.
I don't know if it's a good trade-off though.
