[PATCH][RFC][match.pd] optimize (X & C) == N when C is power of 2

[Segher Boessenkool]

On Fri, Jul 24, 2015 at 09:09:39AM +0100, Kyrill Tkachov wrote:
> This transformation folds (X % C) == N into
> X & ((1 << (size - 1)) | (C - 1))) == N
> for constants C and N where N is positive and C is a power of 2.

For N = 0 you can transform it to

	((unsigned)X % C) == N

and for 0 < N < C you can transform it to

	X > 0 && ((unsigned)X % C) == N          (or X >= 0)

and for -C < N < 0 it is

	X < 0 && ((unsigned)X % C) == N + C      (or X <= 0)

and for other N it is

	0.

For N not a constant, well, do you really care?  :-)

(That second case might eventually fold to your original expression).


Segher