[llvm] [RISCV] Handle recurrences in RISCVVLOptimizer (PR #151285)

[Pengcheng Wang via llvm-commits]

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@@ -30,6 +40,27 @@ using namespace llvm;
 
 namespace {
 
+/// Wrapper around MachineOperand that defaults to immediate 0.
+struct DemandedVL {
+  MachineOperand VL;
+  DemandedVL() : VL(MachineOperand::CreateImm(0)) {}
+  DemandedVL(MachineOperand VL) : VL(VL) {}
+  static DemandedVL vlmax() {
+    return DemandedVL(MachineOperand::CreateImm(RISCV::VLMaxSentinel));
+  }
+  bool operator!=(const DemandedVL &Other) const {
+    return !VL.isIdenticalTo(Other.VL);
+  }
+};
+
+static DemandedVL max(const DemandedVL &LHS, const DemandedVL &RHS) {
+  if (RISCV::isVLKnownLE(LHS.VL, RHS.VL))
+    return RHS;
+  if (RISCV::isVLKnownLE(RHS.VL, LHS.VL))
+    return LHS;
+  return DemandedVL::vlmax();
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wangpc-pp wrote:

In which case we will go into this code path?

https://github.com/llvm/llvm-project/pull/151285