[RISCV] Reverse iteration/deletion structure in vsetvli coalescing [NFC] (llvm#98936)

The code previously deferred deleting the vsetvli to avoid invalidating
iterators, but eagerly deleted any ADDIs feeding the AVL register
operand. This was safe because the iterator was known to point to a
non-ADDI instruction (the vsetvli which was the previous user.) This
change switches to using an early_inc_range so that we can eagerly
delete the vsetvlis, but have to track ADDIs for later deletion.

This is purely stylistic, but IMO makes the code easier to follow. It
will also simplify a future change to support recursive deletion of
trivially dead instructions (i.e. LUI/ADDI pairs.)

Author: preames

llvm/lib/Target/RISCV/RISCVInsertVSETVLI.cpp

@@ -1645,24 +1645,23 @@ void RISCVInsertVSETVLI::coalesceVSETVLIs(MachineBasicBlock &MBB) const {
   Used.demandVTYPE();
   SmallVector<MachineInstr*> ToDelete;
 
-  // Update LIS and cleanup dead AVLs given a value which has
-  // has had one use (as an AVL) removed.
-  auto afterDroppedAVLUse = [&](Register OldVLReg) {
+  auto dropAVLUse = [&](MachineOperand &MO) {
+    if (!MO.isReg() || !MO.getReg().isVirtual())
+      return;
+    Register OldVLReg = MO.getReg();
+    MO.setReg(RISCV::NoRegister);
+
     if (LIS)
       LIS->shrinkToUses(&LIS->getInterval(OldVLReg));
 
     MachineInstr *VLOpDef = MRI->getUniqueVRegDef(OldVLReg);
     if (VLOpDef && TII->isAddImmediate(*VLOpDef, OldVLReg) &&
-        MRI->use_nodbg_empty(OldVLReg)) {
-      if (LIS) {
-        LIS->removeInterval(OldVLReg);
-        LIS->RemoveMachineInstrFromMaps(*VLOpDef);
-      }
-      VLOpDef->eraseFromParent();
-    }
+        MRI->use_nodbg_empty(OldVLReg))
+      ToDelete.push_back(VLOpDef);
   };
 
-  for (MachineInstr &MI : make_range(MBB.rbegin(), MBB.rend())) {
+  for (MachineInstr &MI :
+       make_early_inc_range(make_range(MBB.rbegin(), MBB.rend()))) {
 
     if (!isVectorConfigInstr(MI)) {
       Used.doUnion(getDemanded(MI, ST));
@@ -1678,7 +1677,11 @@ void RISCVInsertVSETVLI::coalesceVSETVLIs(MachineBasicBlock &MBB) const {
 
     if (NextMI) {
       if (!Used.usedVL() && !Used.usedVTYPE()) {
-        ToDelete.push_back(&MI);
+        dropAVLUse(MI.getOperand(1));
+        if (LIS)
+          LIS->RemoveMachineInstrFromMaps(MI);
+        MI.eraseFromParent();
+        NumCoalescedVSETVL++;
         // Leave NextMI unchanged
         continue;
       }
@@ -1707,37 +1710,37 @@ void RISCVInsertVSETVLI::coalesceVSETVLIs(MachineBasicBlock &MBB) const {
             LIS->shrinkToUses(&DefLI);
           }
 
-          Register OldVLReg;
-          if (MI.getOperand(1).isReg())
-            OldVLReg = MI.getOperand(1).getReg();
+          dropAVLUse(MI.getOperand(1));
           if (NextMI->getOperand(1).isImm())
             MI.getOperand(1).ChangeToImmediate(NextMI->getOperand(1).getImm());
           else
-            MI.getOperand(1).ChangeToRegister(NextMI->getOperand(1).getReg(), false);
-          if (OldVLReg && OldVLReg.isVirtual())
-            afterDroppedAVLUse(OldVLReg);
+            MI.getOperand(1).ChangeToRegister(NextMI->getOperand(1).getReg(),
+                                              false);
 
           MI.setDesc(NextMI->getDesc());
         }
         MI.getOperand(2).setImm(NextMI->getOperand(2).getImm());
-        ToDelete.push_back(NextMI);
+
+        dropAVLUse(NextMI->getOperand(1));
+        if (LIS)
+          LIS->RemoveMachineInstrFromMaps(*NextMI);
+        NextMI->eraseFromParent();
+        NumCoalescedVSETVL++;
         // fallthrough
       }
     }
     NextMI = &MI;
     Used = getDemanded(MI, ST);
   }
 
-  NumCoalescedVSETVL += ToDelete.size();
+  // Loop over the dead AVL values, and delete them now.  This has
+  // to be outside the above loop to avoid invalidating iterators.
   for (auto *MI : ToDelete) {
-    if (LIS)
+    if (LIS) {
+      LIS->removeInterval(MI->getOperand(0).getReg());
       LIS->RemoveMachineInstrFromMaps(*MI);
-    Register OldAVLReg;
-    if (MI->getOperand(1).isReg())
-      OldAVLReg = MI->getOperand(1).getReg();
+    }
     MI->eraseFromParent();
-    if (OldAVLReg && OldAVLReg.isVirtual())
-      afterDroppedAVLUse(OldAVLReg);
   }
 }