[llvm] [RISCV] Improve cleanup phase of RISCV Insert VSETVLI pass (PR #67144)

[Yeting Kuo via llvm-commits]

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@@ -1426,85 +1426,96 @@ static bool isNonZeroAVL(const MachineOperand &MO) {
   return 0 != MO.getImm();
 }
 
-// Return true if we can mutate PrevMI to match MI without changing any the
-// fields which would be observed.
-static bool canMutatePriorConfig(const MachineInstr &PrevMI,
-                                 const MachineInstr &MI,
-                                 const DemandedFields &Used) {
-  // If the VL values aren't equal, return false if either a) the former is
-  // demanded, or b) we can't rewrite the former to be the later for
-  // implementation reasons.
-  if (!isVLPreservingConfig(MI)) {
-    if (Used.VLAny)
-      return false;
-
-    // We don't bother to handle the equally zero case here as it's largely
-    // uninteresting.
-    if (Used.VLZeroness) {
-      if (isVLPreservingConfig(PrevMI))
-        return false;
-      if (!isNonZeroAVL(MI.getOperand(1)) ||
-          !isNonZeroAVL(PrevMI.getOperand(1)))
-        return false;
-    }
-
-    // TODO: Track whether the register is defined between
-    // PrevMI and MI.
-    if (MI.getOperand(1).isReg() &&
-        RISCV::X0 != MI.getOperand(1).getReg())
-      return false;
-  }
-
-  if (!PrevMI.getOperand(2).isImm() || !MI.getOperand(2).isImm())
-    return false;
-
-  auto PriorVType = PrevMI.getOperand(2).getImm();
-  auto VType = MI.getOperand(2).getImm();
-  return areCompatibleVTYPEs(PriorVType, VType, Used);
-}
-
 void RISCVInsertVSETVLI::doLocalPostpass(MachineBasicBlock &MBB) {
   MachineInstr *NextMI = nullptr;
   // We can have arbitrary code in successors, so VL and VTYPE
   // must be considered demanded.
   DemandedFields Used;
   Used.demandVL();
   Used.demandVTYPE();
-  SmallVector<MachineInstr*> ToDelete;
+  SmallPtrSet<MachineInstr *, 8> ToDelete;
   for (MachineInstr &MI : make_range(MBB.rbegin(), MBB.rend())) {
 
     if (!isVectorConfigInstr(MI)) {
       doUnion(Used, getDemanded(MI, MRI));
+
+      // We can't handle the case when the source AVL register of *NextMI is
+      // defined after MI
+      if (NextMI && NextMI->getOperand(1).isReg()) {
+        for (const MachineOperand &MO : MI.operands()) {
+          if (MO.isReg() && MO.isDef() &&
+              MO.getReg() == NextMI->getOperand(1).getReg()) {
+            Used.demandVL();
+            break;
+          }
+        }
+      }
+
       continue;
     }
 
     Register VRegDef = MI.getOperand(0).getReg();
     if (VRegDef != RISCV::X0 &&
-        !(VRegDef.isVirtual() && MRI->use_nodbg_empty(VRegDef)))
-      Used.demandVL();
+        !(VRegDef.isVirtual() && MRI->use_nodbg_empty(VRegDef))) {
+      for (MachineInstr &UserMI : MRI->use_nodbg_instructions(VRegDef)) {
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yetingk wrote:

Could we simplify the loop by `any_of`?

https://github.com/llvm/llvm-project/pull/67144