First, the motivation: LLVM currently does not realize that:
((2072 >> (L == 0)) >> 7) & 1 == 0
where L is some arbitrary value. Whether you right-shift 2072 by 7 or by 8, the lowest-order bit is always zero. There are obviously several ways to go about fixing this, but the generic solution pursued in this patch is to teach computeKnownBits something about shifts by a non-constant amount. Currently, we give up completely on these. Instead, in cases where we know something about the low-order bits of the shift-amount operand, we can combine (and together) the associated restrictions for all shift amounts consistent with that knowledge. As a further generalization, I refactored all of the logic for all three kinds of shifts to have this capability. This works well in the above case, for example, because the dynamic shift amount can only be 0 or 1, and thus we can say a lot about the known bits of the result.
This brings us to the second part of this patch: Even when we know all of the bits of a value via computeKnownBits, nothing currently constant-folds the result. The patch introduces the necessary code into InstCombine and InstSimplify. I've added it into both because:
- InstCombine won't automatically pick up the associated logic in InstSimplify (InstCombine uses InstSimplify, but not via the API that passes in the original instruction).
- Putting the logic in InstCombine allows the resulting simplifications to become part of the iterative worklist
- Putting the logic in InstSimplify allows the resulting simplifications to be used by everywhere else that calls SimplifyInstruction (inlining, unrolling, and many others).
And this requires a small change to our definition of an ephemeral value so that we don't break the rest case from r246696 (where the icmp feeding the @llvm.assume, is also feeding a br). Under the old definition, the icmp would not be considered ephemeral (because it is used by the br), but this causes the assume to remove itself (in addition to simplifying the branch structure), and it seems more-useful to prevent that from happening.
Please review, and thanks again!