[llvm] [Analysis] Extend llvm.experimental.cttz.elts to type-based-cost (PR #184578)

[Luke Lau via llvm-commits]

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@@ -3413,6 +3384,53 @@ class BasicTTIImplBase : public TargetTransformInfoImplCRTPBase<T> {
 
   InstructionCost getVectorSplitCost() const { return 1; }
 
+  // TODO: The costs below reflect the expansion code in
+  // SelectionDAGBuilder, but we may want to sacrifice some accuracy in
+  // favour of compile time.
+  // This path should only be taken if Targets cannot custom lower this
+  // intrinsic.
+  InstructionCost getCttzEltsCost(const IntrinsicCostAttributes &ICA,
+                                  bool ZeroIsPoison,
+                                  TTI::TargetCostKind CostKind) const {
+    const IntrinsicInst *I = ICA.getInst();
+    Type *ArgTy = ICA.getArgTypes()[0];
+    EVT ArgType = getTLI()->getValueType(DL, ArgTy, true);
+    Type *RetTy = ICA.getReturnType();
+    FastMathFlags FMF = ICA.getFlags();
+
+    // Find the smallest "sensible" element type to use for the expansion.
+    ConstantRange VScaleRange(APInt(64, 1), APInt::getZero(64));
+    if (isa<ScalableVectorType>(ArgTy) && I && I->getCaller())
+      VScaleRange = getVScaleRange(I->getCaller(), 64);
+
+    unsigned EltWidth = getTLI()->getBitWidthForCttzElements(
+        RetTy, ArgType.getVectorElementCount(), ZeroIsPoison, &VScaleRange);
+    Type *NewEltTy = IntegerType::getIntNTy(RetTy->getContext(), EltWidth);
+
+    // Create the new vector type & get the vector length
+    Type *NewVecTy =
+        VectorType::get(NewEltTy, cast<VectorType>(ArgTy)->getElementCount());
+
+    IntrinsicCostAttributes StepVecAttrs(Intrinsic::stepvector, NewVecTy, {},
+                                         FMF);
+    InstructionCost Cost =
+        thisT()->getIntrinsicInstrCost(StepVecAttrs, CostKind);
+
+    Cost +=
+        thisT()->getArithmeticInstrCost(Instruction::Sub, NewVecTy, CostKind);
+    Cost += thisT()->getCastInstrCost(Instruction::SExt, NewVecTy, ArgTy,
+                                      TTI::CastContextHint::None, CostKind);
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lukel97 wrote:

The cost for this sext is super high, like 10 for ArgTy = <2 x i32> because the dest type is smaller than the source type, i.e. ArgTy why <2 x i8>

Not a fault with this patch, but something we should definitely fix

https://github.com/llvm/llvm-project/pull/184578