[ExecutionDepsFix] Don't recurse over the CFG

[loladiro]

Summary

Use an explicit std::vector instead, to avoid accidentally causing stack overflows
for input with very large CFGs.

[mehdi_amini]

lib/CodeGen/ExecutionDepsFix.cpp

710 ↗

(On Diff #94129)

SmallVector?
Worklist or Workqueue seems a more common name for iterative algorithm I think.

719 ↗

(On Diff #94129)

Seed the workqueue instead to remove this call.

724 ↗

(On Diff #94129)

Is it the only use of updateSuccessors? Could be inlined if so, the code seems simple enough.

llvm/trunk/include/llvm/CodeGen/ExecutionDepsFix.h

private:
   void force(int rx, unsigned domain);
   void collapse(DomainValue *dv, unsigned domain);
   bool merge(DomainValue *A, DomainValue *B);
 
   void enterBasicBlock(MachineBasicBlock*);
   void leaveBasicBlock(MachineBasicBlock*);
   bool isBlockDone(MachineBasicBlock *);
   void processBasicBlock(MachineBasicBlock *MBB, bool PrimaryPass);
-  void updateSuccessors(MachineBasicBlock *MBB, bool PrimaryPass);
   bool visitInstr(MachineInstr *);
   void processDefs(MachineInstr *, bool breakDependency, bool Kill);
   void visitSoftInstr(MachineInstr*, unsigned mask);
   void visitHardInstr(MachineInstr*, unsigned domain);
   bool pickBestRegisterForUndef(MachineInstr *MI, unsigned OpIdx,
                                 unsigned Pref);
   bool shouldBreakDependence(MachineInstr*, unsigned OpIdx, unsigned Pref);
   void processUndefReads(MachineBasicBlock*);
 };
 
 } // end namepsace llvm
 
 #endif

llvm/trunk/lib/CodeGen/ExecutionDepsFix.cpp

 }
 
 bool ExecutionDepsFix::isBlockDone(MachineBasicBlock *MBB) {
   return MBBInfos[MBB].PrimaryCompleted &&
          MBBInfos[MBB].IncomingCompleted == MBBInfos[MBB].PrimaryIncoming &&
          MBBInfos[MBB].IncomingProcessed == MBB->pred_size();
 }
 
-void ExecutionDepsFix::updateSuccessors(MachineBasicBlock *MBB, bool Primary) {
-  bool Done = isBlockDone(MBB);
-  for (auto *Succ : MBB->successors()) {
-    if (!isBlockDone(Succ)) {
-      if (Primary) {
-        MBBInfos[Succ].IncomingProcessed++;
-      }
-      if (Done) {
-        MBBInfos[Succ].IncomingCompleted++;
-      }
-      if (isBlockDone(Succ)) {
-        // Perform secondary processing for this successor. See the big comment
-        // in runOnMachineFunction, for an explanation of the iteration order.
-        processBasicBlock(Succ, false);
-        updateSuccessors(Succ, false);
-      }
-    }
-  }
-}
-
 bool ExecutionDepsFix::runOnMachineFunction(MachineFunction &mf) {
   if (skipFunction(*mf.getFunction()))
     return false;
   MF = &mf;
   TII = MF->getSubtarget().getInstrInfo();
   TRI = MF->getSubtarget().getRegisterInfo();
   RegClassInfo.runOnMachineFunction(mf);
   LiveRegs = nullptr;
bool ExecutionDepsFix::runOnMachineFunction(MachineFunction &mf) {
    *  the predecessors before getting to D. We call a block that is ready
    *  for its second round of processing `done` (isBlockDone). Once we finish
    *  processing some block, we update the counters in MBBInfos and re-process
    *  any successors that are now done.
    */
 
   MachineBasicBlock *Entry = &*MF->begin();
   ReversePostOrderTraversal<MachineBasicBlock*> RPOT(Entry);
+  SmallVector<MachineBasicBlock *, 4> Workqueue;
   for (ReversePostOrderTraversal<MachineBasicBlock*>::rpo_iterator
          MBBI = RPOT.begin(), MBBE = RPOT.end(); MBBI != MBBE; ++MBBI) {
     MachineBasicBlock *MBB = *MBBI;
     // N.B: IncomingProcessed and IncomingCompleted were already updated while
     // processing this block's predecessors.
     MBBInfos[MBB].PrimaryCompleted = true;
     MBBInfos[MBB].PrimaryIncoming = MBBInfos[MBB].IncomingProcessed;
-    processBasicBlock(MBB, true);
-    updateSuccessors(MBB, true);
+    bool Primary = true;
+    Workqueue.push_back(MBB);
+    while (!Workqueue.empty()) {
+      MachineBasicBlock *ActiveMBB = &*Workqueue.back();
+      Workqueue.pop_back();
+      processBasicBlock(ActiveMBB, Primary);
+      bool Done = isBlockDone(ActiveMBB);
+      for (auto *Succ : ActiveMBB->successors()) {
+        if (!isBlockDone(Succ)) {
+          if (Primary) {
+            MBBInfos[Succ].IncomingProcessed++;
+          }
+          if (Done) {
+            MBBInfos[Succ].IncomingCompleted++;
+          }
+          if (isBlockDone(Succ)) {
+            Workqueue.push_back(Succ);
+          }
+        }
+      }
+      Primary = false;
+    }
   }
 
   // We need to go through again and finalize any blocks that are not done yet.
   // This is possible if blocks have dead predecessors, so we didn't visit them
   // above.
   for (ReversePostOrderTraversal<MachineBasicBlock *>::rpo_iterator
            MBBI = RPOT.begin(),
            MBBE = RPOT.end();