[Bug tree-optimization/51005] New: -ftree-tail-merge slows down compilation of 20001226-1.c

[vries at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51005

             Bug #: 51005
           Summary: -ftree-tail-merge slows down compilation of
                    20001226-1.c
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: vr...@gcc.gnu.org


Reported by David Miller in
http://gcc.gnu.org/ml/gcc-patches/2011-10/msg02780.html:
...
My check-gcc runs are completely dominated by the time it takes to
test compile.exp=20001226-1.c

[...]

Unfortunately, at -O2 this testcase causes exponential cost problems
which are beyond my skills to fix.  Specifically,
tail_merge_optimize() applies it's clusters and
iterate_fix_dominators() hits the worst case O(n) complexity of
et_splay for a CFG structured like this. :-/  %38 of the compilation
time is spent in et_splay().
...