[llvm] [IR][LangRef] Add partial reduction add intrinsic (PR #94499)

[Graham Hunter via llvm-commits]

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@@ -7914,6 +7914,28 @@ void SelectionDAGBuilder::visitIntrinsicCall(const CallInst &I,
     setValue(&I, Trunc);
     return;
   }
+  case Intrinsic::experimental_vector_partial_reduce_add: {
+    auto DL = getCurSDLoc();
+    auto ReducedTy = EVT::getEVT(I.getType());
+    auto OpNode = getValue(I.getOperand(1));
+    auto FullTy = OpNode.getValueType();
+
+    auto Accumulator = getValue(I.getOperand(0));
+    unsigned ScaleFactor = FullTy.getVectorMinNumElements() / ReducedTy.getVectorMinNumElements();
+
+    for(unsigned i = 0; i < ScaleFactor; i++) {
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huntergr-arm wrote:

Ok, I was thrown off by the langref description. I guess then I'd like to see the default lowering changed to just extract the subvectors from the second operand and perform a vector add on to the first operand, instead of reducing the subvectors and adding the result to individual lanes. It technically meets the defined semantics (target-defined order of reduction operations), but the current codegen is pretty awful compared to a series of vector adds.

https://github.com/llvm/llvm-project/pull/94499