[DAGCombiner] Recognize another rotation idiom

[Richard Sandiford]

Sorry for the slow reply!

Adam Nemet <anemet at apple.com> writes:
> This is the new idiom:
>
>   x<<(y&31) | x>>((0-y)&31)
>
> which is recognized as:
>
>   x ROTL (y&31)
>
> The change refines matchRotateSub.  In
> Neg & (OpSize - 1) == (OpSize - Pos) & (OpSize - 1), if Pos is
> Pos' & (OpSize - 1) we can just use Pos' instead of Pos.
>
> There are two patches attached.  I cover the actual functional change in
> the second.  In the first I tried to fix two minor issues that confused
> me somewhat while trying to understand matchRotateSub:
>
> 1. Slightly change the wording in the function comment. Originally, it can be
> understood as if we turned the input into two subsequent rotates.
>
> 2. Better connect the comment which talks about Mask and the code which used
> LoBits.  Renamed variable to MaskLoBits.
>
> Thanks,
> Adam
>
>
> diff --git a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
> index 5057850..2305687 100644
> --- a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
> +++ b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
> @@ -3352,7 +3352,7 @@ static bool MatchRotateHalf(SDValue Op, SDValue &Shift, SDValue &Mask) {
>  //
>  //     (or (shift1 X, Neg), (shift2 X, Pos))
>  //
> -// reduces to a rotate in direction shift2 by Pos and a rotate in direction
> +// reduces to a rotate in direction shift2 by Pos or a rotate in direction
>  // shift1 by Neg.  The range [0, OpSize) means that we only need to consider
>  // shift amounts with defined behavior.
>  static bool matchRotateSub(SDValue Pos, SDValue Neg, unsigned OpSize) {

I take your point, but "or" kind-of suggests that these are two separate
conditions, whereas they're really just two ways of expressing the same
condition.  Maybe "or (equivalently)"?

Both patches LGTM otherwise, thanks.

Richard