[llvm] [LV] Vectorize histogram operations (PR #99851)

[Florian Hahn via llvm-commits]

================
@@ -958,6 +964,65 @@ void VPWidenCallRecipe::print(raw_ostream &O, const Twine &Indent,
     O << ")";
   }
 }
+#endif
+
+void VPHistogramRecipe::execute(VPTransformState &State) {
+  State.setDebugLocFrom(getDebugLoc());
+  IRBuilderBase &Builder = State.Builder;
+
+  for (unsigned Part = 0; Part < State.UF; ++Part) {
+    Value *Address = State.get(getOperand(0), Part);
+    Value *IncVec = State.get(getOperand(1), Part);
+    VectorType *VTy = cast<VectorType>(Address->getType());
+
+    // The histogram intrinsic requires a mask even if the recipe doesn't;
+    // if the mask operand was omitted then all lanes should be executed and
+    // we just need to synthesize an all-true mask.
+    Value *Mask = nullptr;
+    if (VPValue *VPMask = getMask())
+      Mask = State.get(VPMask, Part);
+    else
+      Mask = Builder.CreateVectorSplat(
+          VTy->getElementCount(), ConstantInt::getTrue(Builder.getInt1Ty()));
+
+    // Not sure how to make IncAmt stay scalar yet. For now just extract the
+    // first element and tidy up later.
+    // FIXME: Do we actually want this to be scalar? We just splat it in the
+    //        backend anyway...
+    Value *IncAmt = Builder.CreateExtractElement(IncVec, Builder.getInt64(0));
+
+    // If this is a subtract, we want to invert the increment amount. We may
+    // add a separate intrinsic in future, but for now we'll try this.
+    if (Opcode == Instruction::Sub)
+      IncAmt = Builder.CreateNeg(IncAmt);
+    else
+      assert(Opcode == Instruction::Add);
----------------
fhahn wrote:

```suggestion
      assert(Opcode == Instruction::Add && "only add or sub supported for now");
```

https://github.com/llvm/llvm-project/pull/99851