------- Comment #2 from raeburn at raeburn dot org 2008-08-16 18:18 ------- Just noting for future reference: I looked at the VRP results and that does seem to be where the optimization opportunity is missed; x%y with constant y is VARYING if x is, though it seems to me the result should be [-abs(y)+1,abs(y)-1] for signed math and [0,abs(y)-1] for unsigned math. If x can be constrained to either nonnegative or nonpositive, that reduces the range in the signed case, and if it has a shorter range [x1,x2] where floor(x1/y)==floor(x2/y) it might be constrained further still. If y has a range, the result is still bound by the larger of the absolute values of the ends of the range.
(Similarly, though not related to this case, x/const can't be larger than type_max(x)/const. Seems to me that in general there are a few cases where varying should be treated as [type_min,type_max] and processed like any other range, but maybe I'm missing something.) I experimented with a patch to tree-vrp.c to support TRUNC_MOD_EXPR(varying,const), and found I also had to get VRP to understand BIT_NOT_EXPR, but then it generated the same code for both test functions. The patch needs work still, and I need to understand the infinity and overflow stuff in tree-vrp.c a little better to make sure I'm not breaking something. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33259
