On 25/07/15 03:19, Segher Boessenkool wrote:
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).
Yeah, these avoid the potentially expensive mask, but introduce more operations,
which I believe may not be desirable at this stage.
Unless these transformations are ok for match.pd I'll try to implement this
transformation
at RTL expansion time.
Thanks,
Kyrill
Segher
