https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68030
--- Comment #8 from amker at gcc dot gnu.org --- (In reply to rguent...@suse.de from comment #7) > On May 10, 2016 6:25:57 PM GMT+02:00, "amker at gcc dot gnu.org" > <gcc-bugzi...@gcc.gnu.org> wrote: > >https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68030 > > > >--- Comment #6 from amker at gcc dot gnu.org --- > >It's not only the vectorizer generating CSE sub-optimal code, pre and > >lim also > >do this kind of transform. > > In another PR I suggested swapping LIM and PRE to cleanup after LIM. IIRC > that had some testsuite regressions. > Not really, PRE and LIM do same transform here. What we need is to transform below: _970 = iy_186 + -2; _971 = _970 * 516; _979 = iy_186 + -1; _980 = _979 * 516; _985 = iy_186 * 516; _990 = iy_186 + 1; _991 = _990 * 516; _996 = iy_186 + 2; _997 = _996 * 516; into: _x = iy_186 * 516 _971 = _x - 516 * 2 _980 = _x - 516 _985 = _x _990 = _x + 516 _997 = _x + 516 * 2 I remember a way to handle reassociation is to assign different ranks to const/ssa_names, and re-associate expressions wrto to ranks. GCC creates new expressions time to time (for this case, it's cunroll), we may be able to do same re-association when we are creating new expressions, thus they can be easily handled by CSE (or something else).
