On Thu, Oct 8, 2015 at 8:15 PM, Bernd Schmidt <bschm...@redhat.com> wrote: > On 10/08/2015 08:03 PM, Joseph Myers wrote: >> >> On Thu, 8 Oct 2015, Bernd Schmidt wrote: >> >>> On 10/07/2015 11:54 AM, Hurugalawadi, Naveen wrote: >>>> >>>> Move Fold X & (X ^ Y) as X & ~Y to match.pd. >>>> Move Fold X & (Y ^ X) as ~Y & X to match.pd. >>> >>> >>> I wonder if we shouldn't try to autogenerate patterns such as these. I >>> did >>> something like that for a different project a long time ago. Generate >>> expressions up to a certain level of complexity, identify which ones are >>> equivalent, and pick the one with the lowest cost for simplifications... >> >> >> Any bitwise expression whose ultimate operands are X, Y, 0 and -1 >> (possibly with conversions among types of the same width) could be >> canonicalized to one of: 0, -1, X, Y, ~X, ~Y, X^Y, X^~Y, A&B or A|B (where >> A is X or ~X and B is Y or ~Y). I don't guarantee those are the best >> canonical forms, but if you're folding this sort of expression you ought >> to be able to make GCC fold all such expressions down to some such form >> (and fold away all equality comparisons among such expressions with >> constant value). > > > I was actually thinking of also doing this for more complex expressions with > more than two different operands.
Note that the optimization problem in general is more complicated and involves multiple such expressions which should be associated/optimized with CSE in mind to result in the minimal number of overall instructions needed to compute all required values. But yes, we could auto-generate patterns up to a specific depth or up to a point where more complex patterns are handled by composition. Patches welcome ;) Richard. > > Bernd
