[Bug tree-optimization/51005] -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

--- Comment #2 from vries at gcc dot gnu.org 2011-11-07 10:05:03 UTC ---
Created attachment 25733
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=25733
possible patch

This patch brings the computation time back down:
...
$ time gcc src/gcc/testsuite/gcc.c-torture/compile/20001226-1.c -S -O2
-ftree-tail-merge

real    0m35.600s
user    0m35.210s
sys    0m0.270s
...

The patch delegates the updating of the vops to TODO_update_ssa_only_virtuals.
That way, the dominators don't need to be updated after every replacement, just
after every iteration.