[llvm] [CostModel] Move default expand cost for partial reductions to BasicTTIImpl (PR #189905)

[Sander de Smalen via llvm-commits]

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@@ -3435,6 +3435,47 @@ class BasicTTIImplBase : public TargetTransformInfoImplCRTPBase<T> {
     return RedCost + MulCost + 2 * ExtCost;
   }
 
+  InstructionCost getPartialReductionCost(
+      unsigned Opcode, Type *InputTypeA, Type *InputTypeB, Type *AccumType,
+      ElementCount VF, TTI::PartialReductionExtendKind OpAExtend,
+      TTI::PartialReductionExtendKind OpBExtend, std::optional<unsigned> BinOp,
+      TTI::TargetCostKind CostKind,
+      std::optional<FastMathFlags> FMF) const override {
+    unsigned Ratio =
+        AccumType->getScalarSizeInBits() / InputTypeA->getScalarSizeInBits();
+    if (VF.getKnownMinValue() <= Ratio)
+      return InstructionCost::getInvalid();
+
+    Type *InputVectorType = VectorType::get(InputTypeA, VF);
+    Type *ExtInputVectorType = VectorType::get(AccumType, VF);
+    Type *AccumVectorType =
+        VectorType::get(AccumType, VF.divideCoefficientBy(Ratio));
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sdesmalen-arm wrote:

On your comment: "(1) if `AccumType` is narrower than `InputTypeA`, integer division yields `Ratio == 0`, which will later be used in `VF.divideCoeffcientBy(Ratio) (division by zero)"
This should be caught by `if (VF.getKnownMinValue() <= Ratio)`, because if Ratio is `0` there is no value for `VF.getKnownMinValue()` (which is an unsigned value) for which this function does not return `InstructionCost::getInvalid()`.

https://github.com/llvm/llvm-project/pull/189905