On Fri, 14 Mar 2014, Prathamesh Kulkarni wrote:
On Fri, Mar 14, 2014 at 9:25 PM, Marc Glisse <marc.gli...@inria.fr> wrote:
On Fri, 14 Mar 2014, Prathamesh Kulkarni wrote:
The patterns mentioned in the links were:
a) (X >> CST1) >= CST2 -> X >= CST2 << CST1
however, an expression Y >= CST gets folded to Y > CST - 1
so the transform I wrote:
(X >> CST1) > CST2 -> X > CST2 << CST1
That's not the same, try X=1, CST1=1, CST2=0.
Ah yes. Shall following be correct ?
(X >> CST1) > CST2 -> X > ( (CST2 + 1) << CST1 ) - 1
Works correctly for X=1, CST1 = 1, CST2 = 0
Looks better. Though there is still the case where the new constant
overflows, in which case we can fold the comparison to false.
b) (X & ~CST) == 0 -> X <= CST
Uh, that can't be true for all constants, only some with a very specific
shape (7 is 2^3-1).
Agreed. Shall the pattern be folded if CST is 2^(n-1) ?
Wrong parentheses. And I didn't really think about it, so that may not be
the right test.
I think it would be a good idea to write, in comments, next to each
non-trivial transformation, a short "proof" (at least some form of
explanation). It would help people re-reading it later see quickly why the
conditions are what they are.
--
Marc Glisse
