[Bug tree-optimization/42108] [4.4/4.5 Regression] 50% performance regression

["rguenth at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #30 from rguenth at gcc dot gnu dot org  2009-12-14 11:23 -------
I fail to see why FRE does not remove the redundant load of *n_9(D).  Oh, it
is because we first value-number D.1537_58 = *n_9(D); and only after it
we value-number D.1529_45 = *n_9(D);

This is because while we visit the SCC members in RPO order we do not impose
any order on visiting SCCs and those two stmts are not dependent on each
other (we neither account for virtual operands nor control dependences there).  
Old problem.

The fix for this is to either wait for the VN rewrite or to collect all SCCs,
sort them in RPO order and only then process them.  Note that it still can
be difficult to impose a total ordering on SCCs (but at least this case
should be easy).  Another possibility is to artificially grow SCCs and
their dependencies by honoring dominating virtual operand uses, not only
defs (ugh).

For non-memory the missing ordering is not a problem as we do not rely on
walking stmts during expression lookup (and that walking only visits
dominating expressions).  Something to keep in mind for the VN
re-implementation as well.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42108