[llvm] [RISCV] Combine trunc (sra sext (x), zext (y)) to sra (x, smin (y, scalarsizeinbits(y) - 1)) (PR #65728)

[Craig Topper via llvm-commits]

================
@@ -13639,6 +13639,64 @@ SDValue RISCVTargetLowering::PerformDAGCombine(SDNode *N,
       }
     }
     return SDValue();
+  case RISCVISD::TRUNCATE_VECTOR_VL: {
+    // trunc (sra sext (X), zext (Y)) -> sra (X, smin (Y, scalarsize(Y) - 1))
+    // This would be benefit for the cases where X and Y are both the same value
+    // type of low precision vectors. Since the truncate would be lowered into
+    // n-levels TRUNCATE_VECTOR_VL to satisfy RVV's SEW*2->SEW truncate
+    // restriction, such pattern would be expanded into a series of "vsetvli"
+    // and "vnsrl" instructions later to reach this point.
+
+    auto IsTruncNode = [](SDValue V) {
+      if (V.getOpcode() != RISCVISD::TRUNCATE_VECTOR_VL)
+        return false;
+
+      SDValue VL = V.getOperand(2);
+      auto *C = dyn_cast<ConstantSDNode>(VL);
+
+      // Assume all TRUNCATE_VECTOR_VL nodes use VLMAX for VMSET_VL operand
+      bool IsVLMAXForVMSET = (C && C->isAllOnes()) ||
+                             (isa<RegisterSDNode>(VL) &&
+                              cast<RegisterSDNode>(VL)->getReg() == RISCV::X0);
+
+      return V.getOperand(1).getOpcode() == RISCVISD::VMSET_VL &&
+             IsVLMAXForVMSET;
+    };
+
+    SDValue Op = N->getOperand(0);
+
+    // We need to first find the inner level of TRUNCATE_VECTOR_VL node
+    // to distinguish such pattern.
+    while (IsTruncNode(Op)) {
+      if (!Op.hasOneUse())
+        return SDValue();
+      Op = Op.getOperand(0);
+    }
+
+    if (Op.getOpcode() == ISD::SRA && Op.hasOneUse()) {
+      SDValue N0 = Op.getOperand(0);
+      SDValue N1 = Op.getOperand(1);
+
+      if (N0.getOpcode() == ISD::SIGN_EXTEND && N0.hasOneUse() &&
+          N1.getOpcode() == ISD::ZERO_EXTEND && N1.hasOneUse()) {
+        SDValue N00 = N0.getOperand(0);
+        SDValue N10 = N1.getOperand(0);
+
+        if (N00.getValueType().isVector() &&
+            N00.getValueType() == N10.getValueType() && N->hasOneUse() &&
----------------
topperc wrote:

Why does N need to have a single use?

https://github.com/llvm/llvm-project/pull/65728