[llvm] [RISCV][TTI] Add shuffle costing for masked slide lowering (PR #128537)

[Philip Reames via llvm-commits]

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@@ -413,6 +413,36 @@ bool llvm::getShuffleDemandedElts(int SrcWidth, ArrayRef<int> Mask,
   return true;
 }
 
+bool llvm::isMaskedSlidePair(ArrayRef<int> Mask, int NumElts,
+                             std::array<std::pair<int, int>, 2> &SrcInfo) {
+  const int SignalValue = NumElts * 2;
+  SrcInfo[0] = {-1, SignalValue};
+  SrcInfo[1] = {-1, SignalValue};
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preames wrote:

Not to my knowledge.  A valid shuffle should have a maximum index of 2 x the number of elements in the result - 1 at most.   This is asserted in variously places in code - the most important of which is ShuffleVectorInst::isValidOperands.

https://github.com/llvm/llvm-project/pull/128537