[llvm] [RISCV][TTI] Cost non-power-of-two size changing casts (PR #101047)

[Philip Reames via llvm-commits]

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@@ -1030,17 +1030,28 @@ InstructionCost RISCVTTIImpl::getCastInstrCost(unsigned Opcode, Type *Dst,
   if (!IsVectorType)
     return BaseT::getCastInstrCost(Opcode, Dst, Src, CCH, CostKind, I);
 
-  bool IsTypeLegal = isTypeLegal(Src) && isTypeLegal(Dst) &&
-                     (Src->getScalarSizeInBits() <= ST->getELen()) &&
-                     (Dst->getScalarSizeInBits() <= ST->getELen());
-
-  // FIXME: Need to compute legalizing cost for illegal types.
-  if (!IsTypeLegal)
+  // FIXME: Need to compute legalizing cost for illegal types.  The current
+  // code handles only legal types and those which can be trivially
+  // promoted to legal.
+  if (!ST->hasVInstructions() || Src->getScalarSizeInBits() > ST->getELen() ||
+      Dst->getScalarSizeInBits() > ST->getELen())
     return BaseT::getCastInstrCost(Opcode, Dst, Src, CCH, CostKind, I);
 
   std::pair<InstructionCost, MVT> SrcLT = getTypeLegalizationCost(Src);
   std::pair<InstructionCost, MVT> DstLT = getTypeLegalizationCost(Dst);
 
+  // Our actual lowering for the case where a wider legal type is available
+  // uses promotion to the wider type.  This is reflected in the result of
+  // getTypeLegalizationCost, but BasicTTI assumes the widened cases are
+  // scalarized if the legalized Src and Dst are not equal sized.
+  const DataLayout &DL = this->getDataLayout();
+  if (!SrcLT.second.isVector() || !DstLT.second.isVector() ||
+      !TypeSize::isKnownLE(DL.getTypeSizeInBits(Src),
+                           SrcLT.second.getSizeInBits()) ||
+      !TypeSize::isKnownLE(DL.getTypeSizeInBits(Dst),
+                           DstLT.second.getSizeInBits()))
+    return BaseT::getCastInstrCost(Opcode, Dst, Src, CCH, CostKind, I);
+
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preames wrote:

The size checks should catch this case.  The only way it wouldn't would be if we split to larger types; that would be quite weird.

https://github.com/llvm/llvm-project/pull/101047