[Bug tree-optimization/101120] New: vn_reference_maybe_forwprop_address can take much compile-time

[rguenth at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101120

            Bug ID: 101120
           Summary: vn_reference_maybe_forwprop_address can take much
                    compile-time
           Product: gcc
           Version: 11.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rguenth at gcc dot gnu.org
  Target Milestone: ---

For polyhedron doduc.f90 I see

 tree FRE                           :   5.05 ( 47%)   0.03 ( 14%)   5.16 ( 46%)
 5780k (  3%)

with -Ofast -march=haswell

and

 tree iv optimization               :   4.99 ( 40%)   0.13 ( 33%)   5.28 ( 40%)
  218M ( 51%)

with -Ofast

Resulting from the vectorizer generating chains of constant adds
(_1 = _2 + 8 + 8 + 8 + 8 spread across multiple stmts).  There's duplicate
bugs for this around but a possible cheap mitigation would be to fold those
to create cheaper to walk use-def chains for the passes between vectorization
and the next forwprop pass.