[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:

Ok, the code is actually correct, but only somewhat accidentally.  The getShuffleCost API is a bit of a mess.  The type parameter is sometimes the source type, and sometimes the destination type.  For the PermuteSingleSrc and PermuteTwoSrc forms, at least the getInstructionCost code normalizes to the wider type, and passes in that one.  (i.e. from the costing perspective, there is no length changing shuffle.)

I have not audited other callers of this API, and make no claims about whether they handle this correctly.  However, given the above code, if they don't, it would appear to be a bug in the caller.  

I hate this API for the record.  It's full of these special cases and it makes working on it really hard to do without introducing bugs.  

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