[llvm] [RISCV][WIP] Optimize sum of absolute differences pattern. (PR #82722)

[Craig Topper via llvm-commits]

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@@ -13176,6 +13176,61 @@ static SDValue performXORCombine(SDNode *N, SelectionDAG &DAG,
   return combineSelectAndUseCommutative(N, DAG, /*AllOnes*/ false, Subtarget);
 }
 
+// Look for (abs (sub (zext X), (zext Y))).
+// Rewrite as (zext (sub (zext (max X, Y), (min X, Y)))) if the user is an add
+// or reduction add. The min/max can be done in parallel and with a lower LMUL
+// than the original code. The two zexts can be folded into widening sub and
+// widening add or widening redsum.
+static SDValue performABSCombine(SDNode *N, SelectionDAG &DAG) {
+  EVT VT = N->getValueType(0);
+  const TargetLowering &TLI = DAG.getTargetLoweringInfo();
+
+  if (!VT.isFixedLengthVector() || VT.getVectorElementType() != MVT::i32 ||
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topperc wrote:

Yep it's overfitting a workload.

https://github.com/llvm/llvm-project/pull/82722