[llvm-commits] [llvm] r131887 - in /llvm/trunk: lib/Transforms/InstCombine/InstCombineMulDivRem.cpp test/Transforms/InstCombine/shift.ll

[Duncan Sands]

Hi Chris,

> Transform any logical shift of a power of two into an exact/NUW shift when
> in a known-non-zero context.

in "x div (1 << y)" this will mark the shift as "nuw" always, due to the
following logic: (A) isPowerOfTwo will say that "1 << y" is a power of
two, because either the 1 is shifted off the end so yielding an undef which
can be assumed to be a power of two, or the 1 isn't shifted off the end in
which case it sure is a power of two; (B) then your logic will deduce that
1 was not shifted off the end, and will add a nsw flag to the shift.

Thus this transform potentially upgrades an undef value to a trap value: if
later on some pass discovers for example that y is big enough that it shifts 1
off the end then now you get a trap value while before this transform you got
an undef.  Yet trap values are defined to only occur when violating a nsw/nuw/
exact flag on an operation, but there was no such flag in the original code!
I don't know if this matters but it seems a bit dubious to me.

Ciao, Duncan.