[llvm] [AArch64] Fold four and eight way partial reductions with [SU]ADDLP (PR #214636)

[Paul Walker via llvm-commits]

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@@ -1755,11 +1755,41 @@ class LLVM_ABI TargetLoweringBase {
   }
 
   /// Return true if a PARTIAL_REDUCE_U/SMLA node with the specified types is
-  /// legal or custom for this target.
+  /// legal or custom for this target. A target that does not take the fold
+  /// directly can still build it from a ladder of narrower steps, each halving
+  /// the element count and doubling the width.
   bool isPartialReduceMLALegalOrCustom(unsigned Opc, EVT AccVT,
                                        EVT InputVT) const {
-    LegalizeAction Action = getPartialReduceMLAAction(Opc, AccVT, InputVT);
-    return Action == Legal || Action == Custom;
+    auto Supported = [&](EVT Acc, EVT In) {
+      LegalizeAction Action = getPartialReduceMLAAction(Opc, Acc, In);
+      return Action == Legal || Action == Custom;
+    };
+    if (Supported(AccVT, InputVT))
+      return true;
+    // The rungs below widen an integer element type.
+    if (Opc == ISD::PARTIAL_REDUCE_FMLA)
+      return false;
+
+    ElementCount AccEC = AccVT.getVectorElementCount();
+    ElementCount InEC = InputVT.getVectorElementCount();
+    // Scalable and fixed lengths are not comparable below.
+    if (AccEC.isScalable() != InEC.isScalable())
+      return false;
+
+    // Step down a rung at a time, taking each step the target supports, until
+    // one of them lands on the accumulator.
+    EVT In = InputVT;
+    while (InEC.getKnownMinValue() > 2 * AccEC.getKnownMinValue()) {
+      InEC = InEC.divideCoefficientBy(2);
+      MVT Next = MVT::getVectorVT(
+          MVT::getIntegerVT(In.getScalarSizeInBits() * 2), InEC);
+      if (!Next.isValid() || !Supported(Next, In))
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paulwalker-arm wrote:

I would not expect this function to change.  isPartialReduceMLALegalOrCustom should not be second guessing how legalisation will happen. It should just state facts based on what the target opts in to.  It's these facts that then guide how the legalisation happens.

https://github.com/llvm/llvm-project/pull/214636