On Jun 28, 2007, at 3:09 AM, Duncan Sands wrote: > Hi, > >>> It seems like folding undef/X to undef isn't safe either though, >>> with >>> the way it sounds like undef is intended to work. This code: >>> >>> %x = udiv i32 undef, %intmax >>> %y = udiv i32 %x, 2 >>> >>> will always set %y to 0. Maybe instcombine can fold the second >>> udiv by looking through its operands, but it can't safely fold the >>> first. The best it could do is try to fold away all of %x's uses so >>> that %x isn't needed anymore. > > presumably undef/X should be folded to 0. [This means that undef/0 > gets > folded to 0, not sure if that's OK].
Ah, duncan's right. I confused myself. We can fold undef/x -> 0 because there is some value of the undef where it is always safe to produce zero for any x. In particular, if the undef was 0, x can be anything (if it is also zero, the result is undefined, so zero is fine). -Chris _______________________________________________ llvm-commits mailing list llvm-commits@cs.uiuc.edu http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
