[llvm] [AArch64] Consider histcnt smaller than i32 in the cost model (PR #108521)

[Graham Hunter via llvm-commits]

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@@ -517,25 +517,31 @@ static bool isUnpackedVectorVT(EVT VecVT) {
 static InstructionCost getHistogramCost(const IntrinsicCostAttributes &ICA) {
   Type *BucketPtrsTy = ICA.getArgTypes()[0]; // Type of vector of pointers
   Type *EltTy = ICA.getArgTypes()[1];        // Type of bucket elements
+  unsigned TotalHistCnts = 1;
 
-  // Only allow (32b and 64b) integers or pointers for now...
+  // Only allow (up to 64b) integers or pointers
   if ((!EltTy->isIntegerTy() && !EltTy->isPointerTy()) ||
-      (EltTy->getScalarSizeInBits() != 32 &&
-       EltTy->getScalarSizeInBits() != 64))
+      EltTy->getScalarSizeInBits() > 64)
     return InstructionCost::getInvalid();
 
-  // FIXME: Hacky check for legal vector types. We can promote smaller types
-  //        but we cannot legalize vectors via splitting for histcnt.
   // FIXME: We should be able to generate histcnt for fixed-length vectors
   //        using ptrue with a specific VL.
-  if (VectorType *VTy = dyn_cast<VectorType>(BucketPtrsTy))
-    if ((VTy->getElementCount().getKnownMinValue() != 2 &&
-         VTy->getElementCount().getKnownMinValue() != 4) ||
-        VTy->getPrimitiveSizeInBits().getKnownMinValue() > 128 ||
-        !VTy->isScalableTy())
+  if (VectorType *VTy = dyn_cast<VectorType>(BucketPtrsTy)) {
+    unsigned EC = VTy->getElementCount().getKnownMinValue();
+    if (!isPowerOf2_64(EC) || !VTy->isScalableTy())
       return InstructionCost::getInvalid();
 
-  return InstructionCost(BaseHistCntCost);
+    bool Element64b = EltTy->isIntegerTy(64);
----------------
huntergr-arm wrote:

Using the `EltSize` suggestion from above:

```suggestion
    // HistCnt only supports 32b and 64b element types.
    unsigned LegalEltSize = EltSize <= 32 ? 32 : 64;
    if (EC == 2 || (LegalEltSize == 32 && EC == 4))
      return InstructionCost(BaseHistCntCost);

    unsigned NaturalVectorWidth = AArch64::SVEBitsPerBlock / LegalEltSize;
```

https://github.com/llvm/llvm-project/pull/108521