[PATCH] D47681: [DAGCombiner] Bug 31275- Extract a shift from a constant mul or udiv if a rotate can be formed

[Sam Conrad via Phabricator via llvm-commits]

sameconrad marked an inline comment as done.
sameconrad added inline comments.


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Comment at: lib/CodeGen/SelectionDAG/DAGCombiner.cpp:5057
 // a rot[lr].
 SDNode *DAGCombiner::MatchRotate(SDValue LHS, SDValue RHS, const SDLoc &DL) {
   // Must be a legal type.  Expanded 'n promoted things won't work with rotates.
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lebedev.ri wrote:
> Is this always considered worth doing,
> or the check whether this should be done is on the caller's side?
It looks like MatchRotate always gets called from DAGCombiner::visitOr, I don't think there's any sort of profitability check, so it seems to assume its always worthwhile (however I'm still pretty new to LLVM and don't know if there are other places this could be done).  In any case, the new code would transform 4 instructions to 2 (2 mul/udiv ops, 1 shift, 1 or -> one mul/udiv op and 1 rotate).


https://reviews.llvm.org/D47681