AMDGPU/InsertWaitcnts: Remove the dependence on MachineLoopInfo

[nhaehnle]

Summary

MachineLoopInfo cannot be relied on for correctness, because it cannot
properly recognize loops in irreducible control flow which can be
introduced by late machine basic block optimization passes. See the new
test case for the reduced form of an example that occurred in practice.

Use a simple fixpoint iteration instead.

In order to facilitate this change, refactor WaitcntBrackets so that it
only tracks pending events and registers, rather than also maintaining
state that is relevant for the high-level algorithm. Various accessor
methods can be removed or made private as a consequence.

Affects (in radv):

• dEQP-VK.glsl.loops.special.{for,while}_uniform_iterations.select_iteration_count_{fragment,vertex}

Fixes: r345719 ("AMDGPU: Rewrite SILowerI1Copies to always stay on SALU")

[nhaehnle]

ping

[msearles]

LGTM

[hakzsam]

Hi Nicolai,

This change introduced a regression in The Witcher 3, see https://bugs.freedesktop.org/show_bug.cgi?id=108935

Thanks.

[hakzsam]

Did you get a chance to look into this?

Thanks!

[critson]

In D54231#1318253, @hakzsam wrote:

This change introduced a regression in The Witcher 3, see https://bugs.freedesktop.org/show_bug.cgi?id=108935

Hi Samuel,

Would you be able to test https://reviews.llvm.org/D55602 to see if it resolves the issue?

Thanks,
Carl

[hakzsam]

Hi,

Yes, it does fix the issue. Thanks!

lib/Target/AMDGPU/SIInsertWaitcnts.cpp

 #include "llvm/ADT/PostOrderIterator.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/CodeGen/MachineBasicBlock.h"
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
 #include "llvm/CodeGen/MachineInstr.h"
 #include "llvm/CodeGen/MachineInstrBuilder.h"
-#include "llvm/CodeGen/MachineLoopInfo.h"
 #include "llvm/CodeGen/MachineMemOperand.h"
 #include "llvm/CodeGen/MachineOperand.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
 #include "llvm/IR/DebugLoc.h"
 #include "llvm/Pass.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/DebugCounter.h"
 #include "llvm/Support/ErrorHandling.h"
void addWait(AMDGPU::Waitcnt &Wait, InstCounterType T, unsigned Count) {
   case LGKM_CNT:
     Wait.LgkmCnt = std::min(Wait.LgkmCnt, Count);
     break;
   default:
     llvm_unreachable("bad InstCounterType");
   }
 }
 
-// This is a per-basic-block object that maintains current score brackets
-// of each wait counter, and a per-register scoreboard for each wait counter.
+// This objects maintains the current score brackets of each wait counter, and
+// a per-register scoreboard for each wait counter.
+//
 // We also maintain the latest score for every event type that can change the
 // waitcnt in order to know if there are multiple types of events within
 // the brackets. When multiple types of event happen in the bracket,
 // wait count may get decreased out of order, therefore we need to put in
 // "s_waitcnt 0" before use.
-class BlockWaitcntBrackets {
+class WaitcntBrackets {
 public:
-  BlockWaitcntBrackets(const GCNSubtarget *SubTarget) : ST(SubTarget) {
+  WaitcntBrackets(const GCNSubtarget *SubTarget) : ST(SubTarget) {
     for (auto T : inst_counter_types())
       memset(VgprScores[T], 0, sizeof(VgprScores[T]));
   }
 
-  ~BlockWaitcntBrackets() = default;
-
   static uint32_t getWaitCountMax(InstCounterType T) {
     switch (T) {
     case VM_CNT:
       return HardwareLimits.VmcntMax;
     case LGKM_CNT:
       return HardwareLimits.LgkmcntMax;
     case EXP_CNT:
       return HardwareLimits.ExpcntMax;
     default:
       break;
     }
     return 0;
   }
 
-  void setScoreLB(InstCounterType T, uint32_t Val) {
-    assert(T < NUM_INST_CNTS);
-    if (T >= NUM_INST_CNTS)
-      return;
-    ScoreLBs[T] = Val;
-  }
-
-  void setScoreUB(InstCounterType T, uint32_t Val) {
-    assert(T < NUM_INST_CNTS);
-    if (T >= NUM_INST_CNTS)
-      return;
-    ScoreUBs[T] = Val;
-    if (T == EXP_CNT) {
-      uint32_t UB = ScoreUBs[T] - getWaitCountMax(EXP_CNT);
-      if (ScoreLBs[T] < UB && UB < ScoreUBs[T])
-        ScoreLBs[T] = UB;
-    }
-  }
-
   uint32_t getScoreLB(InstCounterType T) const {
     assert(T < NUM_INST_CNTS);
     if (T >= NUM_INST_CNTS)
       return 0;
     return ScoreLBs[T];
   }
 
   uint32_t getScoreUB(InstCounterType T) const {
     assert(T < NUM_INST_CNTS);
     if (T >= NUM_INST_CNTS)
       return 0;
     return ScoreUBs[T];
   }
 
   // Mapping from event to counter.
   InstCounterType eventCounter(WaitEventType E) {
     if (E == VMEM_ACCESS)
       return VM_CNT;
     if (WaitEventMaskForInst[LGKM_CNT] & (1 << E))
       return LGKM_CNT;
     assert(WaitEventMaskForInst[EXP_CNT] & (1 << E));
     return EXP_CNT;
   }
 
-  void setRegScore(int GprNo, InstCounterType T, uint32_t Val) {
-    if (GprNo < NUM_ALL_VGPRS) {
-      if (GprNo > VgprUB) {
-        VgprUB = GprNo;
-      }
-      VgprScores[T][GprNo] = Val;
-    } else {
-      assert(T == LGKM_CNT);
-      if (GprNo - NUM_ALL_VGPRS > SgprUB) {
-        SgprUB = GprNo - NUM_ALL_VGPRS;
-      }
-      SgprScores[GprNo - NUM_ALL_VGPRS] = Val;
-    }
-  }
-
   uint32_t getRegScore(int GprNo, InstCounterType T) {
     if (GprNo < NUM_ALL_VGPRS) {
       return VgprScores[T][GprNo];
     }
     assert(T == LGKM_CNT);
     return SgprScores[GprNo - NUM_ALL_VGPRS];
   }
 
   void clear() {
     memset(ScoreLBs, 0, sizeof(ScoreLBs));
     memset(ScoreUBs, 0, sizeof(ScoreUBs));
     PendingEvents = 0;
     memset(MixedPendingEvents, 0, sizeof(MixedPendingEvents));
     for (auto T : inst_counter_types())
       memset(VgprScores[T], 0, sizeof(VgprScores[T]));
     memset(SgprScores, 0, sizeof(SgprScores));
   }
 
+  bool merge(const WaitcntBrackets &Other);
+
   RegInterval getRegInterval(const MachineInstr *MI, const SIInstrInfo *TII,
                              const MachineRegisterInfo *MRI,
                              const SIRegisterInfo *TRI, unsigned OpNo,
                              bool Def) const;
 
-  void setExpScore(const MachineInstr *MI, const SIInstrInfo *TII,
-                   const SIRegisterInfo *TRI, const MachineRegisterInfo *MRI,
-                   unsigned OpNo, uint32_t Val);
-
   int32_t getMaxVGPR() const { return VgprUB; }
   int32_t getMaxSGPR() const { return SgprUB; }
 
   bool counterOutOfOrder(InstCounterType T) const;
   bool simplifyWaitcnt(AMDGPU::Waitcnt &Wait) const;
   bool simplifyWaitcnt(InstCounterType T, unsigned &Count) const;
   void determineWait(InstCounterType T, uint32_t ScoreToWait,
                      AMDGPU::Waitcnt &Wait) const;
   void applyWaitcnt(const AMDGPU::Waitcnt &Wait);
   void applyWaitcnt(InstCounterType T, unsigned Count);
   void updateByEvent(const SIInstrInfo *TII, const SIRegisterInfo *TRI,
                      const MachineRegisterInfo *MRI, WaitEventType E,
                      MachineInstr &MI);
 
+  bool hasPending() const { return PendingEvents != 0; }
   bool hasPendingEvent(WaitEventType E) const {
     return PendingEvents & (1 << E);
   }
 
-  void mergePendingEvents(const BlockWaitcntBrackets &Other);
-
   bool hasPendingFlat() const {
     return ((LastFlat[LGKM_CNT] > ScoreLBs[LGKM_CNT] &&
              LastFlat[LGKM_CNT] <= ScoreUBs[LGKM_CNT]) ||
             (LastFlat[VM_CNT] > ScoreLBs[VM_CNT] &&
              LastFlat[VM_CNT] <= ScoreUBs[VM_CNT]));
   }
 
   void setPendingFlat() {
     LastFlat[VM_CNT] = ScoreUBs[VM_CNT];
     LastFlat[LGKM_CNT] = ScoreUBs[LGKM_CNT];
   }
 
-  int pendingFlat(InstCounterType Ct) const { return LastFlat[Ct]; }
+  void print(raw_ostream &);
+  void dump() { print(dbgs()); }
 
-  void setLastFlat(InstCounterType Ct, int Val) { LastFlat[Ct] = Val; }
+private:
+  struct MergeInfo {
+    uint32_t OldLB;
+    uint32_t OtherLB;
+    uint32_t MyShift;
+    uint32_t OtherShift;
+  };
+  static bool mergeScore(const MergeInfo &M, uint32_t &Score,
+                         uint32_t OtherScore);
 
-  bool getRevisitLoop() const { return RevisitLoop; }
-  void setRevisitLoop(bool RevisitLoopIn) { RevisitLoop = RevisitLoopIn; }
+  void setScoreLB(InstCounterType T, uint32_t Val) {
+    assert(T < NUM_INST_CNTS);
+    if (T >= NUM_INST_CNTS)
+      return;
+    ScoreLBs[T] = Val;
+  }
 
-  void setPostOrder(int32_t PostOrderIn) { PostOrder = PostOrderIn; }
-  int32_t getPostOrder() const { return PostOrder; }
+  void setScoreUB(InstCounterType T, uint32_t Val) {
+    assert(T < NUM_INST_CNTS);
+    if (T >= NUM_INST_CNTS)
+      return;
+    ScoreUBs[T] = Val;
+    if (T == EXP_CNT) {
+      uint32_t UB = ScoreUBs[T] - getWaitCountMax(EXP_CNT);
+      if (ScoreLBs[T] < UB && UB < ScoreUBs[T])
+        ScoreLBs[T] = UB;
+    }
+  }
 
-  void print(raw_ostream &);
-  void dump() { print(dbgs()); }
+  void setRegScore(int GprNo, InstCounterType T, uint32_t Val) {
+    if (GprNo < NUM_ALL_VGPRS) {
+      if (GprNo > VgprUB) {
+        VgprUB = GprNo;
+      }
+      VgprScores[T][GprNo] = Val;
+    } else {
+      assert(T == LGKM_CNT);
+      if (GprNo - NUM_ALL_VGPRS > SgprUB) {
+        SgprUB = GprNo - NUM_ALL_VGPRS;
+      }
+      SgprScores[GprNo - NUM_ALL_VGPRS] = Val;
+    }
+  }
+
+  void setExpScore(const MachineInstr *MI, const SIInstrInfo *TII,
+                   const SIRegisterInfo *TRI, const MachineRegisterInfo *MRI,
+                   unsigned OpNo, uint32_t Val);
 
-private:
   const GCNSubtarget *ST = nullptr;
-  bool RevisitLoop = false;
-  int32_t PostOrder = 0;
   uint32_t ScoreLBs[NUM_INST_CNTS] = {0};
   uint32_t ScoreUBs[NUM_INST_CNTS] = {0};
   uint32_t PendingEvents = 0;
   bool MixedPendingEvents[NUM_INST_CNTS] = {false};
   // Remember the last flat memory operation.
   uint32_t LastFlat[NUM_INST_CNTS] = {0};
   // wait_cnt scores for every vgpr.
   // Keep track of the VgprUB and SgprUB to make merge at join efficient.
   int32_t VgprUB = 0;
   int32_t SgprUB = 0;
   uint32_t VgprScores[NUM_INST_CNTS][NUM_ALL_VGPRS];
   // Wait cnt scores for every sgpr, only lgkmcnt is relevant.
   uint32_t SgprScores[SQ_MAX_PGM_SGPRS] = {0};
 };
 
-// This is a per-loop-region object that records waitcnt status at the end of
-// loop footer from the previous iteration. We also maintain an iteration
-// count to track the number of times the loop has been visited. When it
-// doesn't converge naturally, we force convergence by inserting s_waitcnt 0
-// at the end of the loop footer.
-class LoopWaitcntData {
-public:
-  LoopWaitcntData() = default;
-  ~LoopWaitcntData() = default;
-
-  void incIterCnt() { IterCnt++; }
-  void resetIterCnt() { IterCnt = 0; }
-  unsigned getIterCnt() { return IterCnt; }
-
-  void setWaitcnt(MachineInstr *WaitcntIn) { LfWaitcnt = WaitcntIn; }
-  MachineInstr *getWaitcnt() const { return LfWaitcnt; }
-
-  void print() { LLVM_DEBUG(dbgs() << "  iteration " << IterCnt << '\n';); }
-
-private:
-  // s_waitcnt added at the end of loop footer to stablize wait scores
-  // at the end of the loop footer.
-  MachineInstr *LfWaitcnt = nullptr;
-  // Number of iterations the loop has been visited, not including the initial
-  // walk over.
-  int32_t IterCnt = 0;
-};
-
 class SIInsertWaitcnts : public MachineFunctionPass {
 private:
   const GCNSubtarget *ST = nullptr;
   const SIInstrInfo *TII = nullptr;
   const SIRegisterInfo *TRI = nullptr;
   const MachineRegisterInfo *MRI = nullptr;
-  const MachineLoopInfo *MLI = nullptr;
   AMDGPU::IsaVersion IV;
 
-  DenseSet<MachineBasicBlock *> BlockVisitedSet;
   DenseSet<MachineInstr *> TrackedWaitcntSet;
   DenseSet<MachineInstr *> VCCZBugHandledSet;
 
-  DenseMap<MachineBasicBlock *, std::unique_ptr<BlockWaitcntBrackets>>
-      BlockWaitcntBracketsMap;
+  struct BlockInfo {
+    MachineBasicBlock *MBB;
+    std::unique_ptr<WaitcntBrackets> Incoming;
+    bool Dirty = true;
 
-  std::vector<MachineBasicBlock *> BlockWaitcntProcessedSet;
+    explicit BlockInfo(MachineBasicBlock *MBB) : MBB(MBB) {}
+  };
 
-  DenseMap<MachineLoop *, std::unique_ptr<LoopWaitcntData>> LoopWaitcntDataMap;
+  std::vector<BlockInfo> BlockInfos; // by reverse post-order traversal index
+  DenseMap<MachineBasicBlock *, unsigned> RpotIdxMap;
 
   // ForceEmitZeroWaitcnts: force all waitcnts insts to be s_waitcnt 0
   // because of amdgpu-waitcnt-forcezero flag
   bool ForceEmitZeroWaitcnts;
   bool ForceEmitWaitcnt[NUM_INST_CNTS];
 
 public:
   static char ID;
 
   SIInsertWaitcnts() : MachineFunctionPass(ID) {
     (void)ForceExpCounter;
     (void)ForceLgkmCounter;
     (void)ForceVMCounter;
   }
 
   bool runOnMachineFunction(MachineFunction &MF) override;
 
   StringRef getPassName() const override {
     return "SI insert wait instructions";
   }
 
   void getAnalysisUsage(AnalysisUsage &AU) const override {
     AU.setPreservesCFG();
-    AU.addRequired<MachineLoopInfo>();
     MachineFunctionPass::getAnalysisUsage(AU);
   }
 
   bool isForceEmitWaitcnt() const {
     for (auto T : inst_counter_types())
       if (ForceEmitWaitcnt[T])
         return true;
     return false;
if (DebugCounter::isCounterSet(ForceVMCounter) &&
       ForceEmitWaitcnt[VM_CNT] = true;
     } else {
       ForceEmitWaitcnt[VM_CNT] = false;
     }
 #endif // NDEBUG
   }
 
   bool mayAccessLDSThroughFlat(const MachineInstr &MI) const;
-  void generateWaitcntInstBefore(MachineInstr &MI,
-                                 BlockWaitcntBrackets *ScoreBrackets,
+  bool generateWaitcntInstBefore(MachineInstr &MI,
+                                 WaitcntBrackets &ScoreBrackets,
                                  MachineInstr *OldWaitcntInstr);
   void updateEventWaitcntAfter(MachineInstr &Inst,
-                               BlockWaitcntBrackets *ScoreBrackets);
-  void mergeInputScoreBrackets(MachineBasicBlock &Block);
-  bool isLoopBottom(const MachineLoop *Loop, const MachineBasicBlock *Block);
-  unsigned countNumBottomBlocks(const MachineLoop *Loop);
-  void insertWaitcntInBlock(MachineFunction &MF, MachineBasicBlock &Block);
-  void insertWaitcntBeforeCF(MachineBasicBlock &Block, MachineInstr *Inst);
+                               WaitcntBrackets *ScoreBrackets);
+  bool insertWaitcntInBlock(MachineFunction &MF, MachineBasicBlock &Block,
+                            WaitcntBrackets &ScoreBrackets);
 };
 
 } // end anonymous namespace
 
-RegInterval BlockWaitcntBrackets::getRegInterval(const MachineInstr *MI,
-                                                 const SIInstrInfo *TII,
-                                                 const MachineRegisterInfo *MRI,
-                                                 const SIRegisterInfo *TRI,
-                                                 unsigned OpNo,
-                                                 bool Def) const {
+RegInterval WaitcntBrackets::getRegInterval(const MachineInstr *MI,
+                                            unsigned OpNo, bool Def) const {
   const MachineOperand &Op = MI->getOperand(OpNo);
   if (!Op.isReg() || !TRI->isInAllocatableClass(Op.getReg()) ||
       (Def && !Op.isDef()))
     return {-1, -1};
 
   // A use via a PW operand does not need a waitcnt.
   // A partial write is not a WAW.
   assert(!Op.getSubReg() || !Op.isUndef());
RegInterval WaitcntBrackets::getRegInterval(const MachineInstr *MI,
   const MachineInstr &MIA = *MI;
   const TargetRegisterClass *RC = TII->getOpRegClass(MIA, OpNo);
   unsigned Size = TRI->getRegSizeInBits(*RC);
   Result.second = Result.first + (Size / 32);
 
   return Result;
 }
 
-void BlockWaitcntBrackets::setExpScore(const MachineInstr *MI,
-                                       const SIInstrInfo *TII,
-                                       const SIRegisterInfo *TRI,
-                                       const MachineRegisterInfo *MRI,
-                                       unsigned OpNo, uint32_t Val) {
+void WaitcntBrackets::setExpScore(const MachineInstr *MI,
+                                  const MachineRegisterInfo *MRI, unsigned OpNo,
+                                  uint32_t Val) {
   RegInterval Interval = getRegInterval(MI, TII, MRI, TRI, OpNo, false);
   LLVM_DEBUG({
     const MachineOperand &Opnd = MI->getOperand(OpNo);
     assert(TRI->isVGPR(*MRI, Opnd.getReg()));
   });
   for (signed RegNo = Interval.first; RegNo < Interval.second; ++RegNo) {
     setRegScore(RegNo, EXP_CNT, Val);
   }
 }
 
-void BlockWaitcntBrackets::updateByEvent(const SIInstrInfo *TII,
-                                         const SIRegisterInfo *TRI,
-                                         const MachineRegisterInfo *MRI,
-                                         WaitEventType E, MachineInstr &Inst) {
+void WaitcntBrackets::updateByEvent(const SIInstrInfo *TII,
   const MachineRegisterInfo &MRIA = *MRI;
   InstCounterType T = eventCounter(E);
   uint32_t CurrScore = getScoreUB(T) + 1;
   if (CurrScore == 0)
     report_fatal_error("InsertWaitcnt score wraparound");
   // PendingEvents and ScoreUB need to be update regardless if this event
   // changes the score of a register or not.
   // Examples including vm_cnt when buffer-store or lgkm_cnt when send-message.
for (unsigned I = 0, E = Inst.getNumOperands(); I != E; ++I) {
       }
     }
     if (TII->isDS(Inst) && Inst.mayStore()) {
       setRegScore(SQ_MAX_PGM_VGPRS + EXTRA_VGPR_LDS, T, CurrScore);
     }
   }
 }
 
-void BlockWaitcntBrackets::print(raw_ostream &OS) {
+void WaitcntBrackets::print(raw_ostream &OS) {
   OS << '\n';
   for (auto T : inst_counter_types()) {
     uint32_t LB = getScoreLB(T);
     uint32_t UB = getScoreUB(T);
 
     switch (T) {
     case VM_CNT:
       OS << "    VM_CNT(" << UB - LB << "): ";
for (auto T : inst_counter_types()) {
     }
     OS << '\n';
   }
   OS << '\n';
 }
 
 /// Simplify the waitcnt, in the sense of removing redundant counts, and return
 /// whether a waitcnt instruction is needed at all.
-bool BlockWaitcntBrackets::simplifyWaitcnt(AMDGPU::Waitcnt &Wait) const {
+bool WaitcntBrackets::simplifyWaitcnt(AMDGPU::Waitcnt &Wait) const {
   return simplifyWaitcnt(VM_CNT, Wait.VmCnt) |
          simplifyWaitcnt(EXP_CNT, Wait.ExpCnt) |
          simplifyWaitcnt(LGKM_CNT, Wait.LgkmCnt);
 }
 
-bool BlockWaitcntBrackets::simplifyWaitcnt(InstCounterType T,
-                                           unsigned &Count) const {
+bool WaitcntBrackets::simplifyWaitcnt(InstCounterType T,
   const uint32_t LB = getScoreLB(T);
   const uint32_t UB = getScoreUB(T);
   if (Count < UB && UB - Count > LB)
     return true;
 
   Count = ~0u;
   return false;
 }
 
-void BlockWaitcntBrackets::determineWait(InstCounterType T,
-                                         uint32_t ScoreToWait,
-                                         AMDGPU::Waitcnt &Wait) const {
+void WaitcntBrackets::determineWait(InstCounterType T, uint32_t ScoreToWait,
   // If the score of src_operand falls within the bracket, we need an
   // s_waitcnt instruction.
   const uint32_t LB = getScoreLB(T);
   const uint32_t UB = getScoreUB(T);
   if ((UB >= ScoreToWait) && (ScoreToWait > LB)) {
     if ((T == VM_CNT || T == LGKM_CNT) &&
         hasPendingFlat() &&
         !ST->hasFlatLgkmVMemCountInOrder()) {
       // If there is a pending FLAT operation, and this is a VMem or LGKM
       // waitcnt and the target can report early completion, then we need
       // to force a waitcnt 0.
       addWait(Wait, T, 0);
     } else if (counterOutOfOrder(T)) {
       // Counter can get decremented out-of-order when there
       // are multiple types event in the bracket. Also emit an s_wait counter
       // with a conservative value of 0 for the counter.
       addWait(Wait, T, 0);
     } else {
       addWait(Wait, T, UB - ScoreToWait);
     }
   }
 }
 
-void BlockWaitcntBrackets::applyWaitcnt(const AMDGPU::Waitcnt &Wait) {
+void WaitcntBrackets::applyWaitcnt(const AMDGPU::Waitcnt &Wait) {
   applyWaitcnt(VM_CNT, Wait.VmCnt);
   applyWaitcnt(EXP_CNT, Wait.ExpCnt);
   applyWaitcnt(LGKM_CNT, Wait.LgkmCnt);
 }
 
-void BlockWaitcntBrackets::applyWaitcnt(InstCounterType T, unsigned Count) {
+void WaitcntBrackets::applyWaitcnt(InstCounterType T, unsigned Count) {
   const uint32_t UB = getScoreUB(T);
   if (Count >= UB)
     return;
   if (Count != 0) {
     if (counterOutOfOrder(T))
       return;
     setScoreLB(T, std::max(getScoreLB(T), UB - Count));
   } else {
     setScoreLB(T, UB);
     MixedPendingEvents[T] = false;
     PendingEvents &= ~WaitEventMaskForInst[T];
   }
 }
 
-void BlockWaitcntBrackets::mergePendingEvents(const BlockWaitcntBrackets &Other) {
-  for (auto T : inst_counter_types()) {
-    uint32_t Old = PendingEvents & WaitEventMaskForInst[T];
-    uint32_t New = Other.PendingEvents & WaitEventMaskForInst[T];
-    if (Other.MixedPendingEvents[T] || (Old && New && Old != New))
-      MixedPendingEvents[T] = true;
-    PendingEvents |= New;
-  }
-}
-
 // Where there are multiple types of event in the bracket of a counter,
 // the decrement may go out of order.
-bool BlockWaitcntBrackets::counterOutOfOrder(InstCounterType T) const {
+bool WaitcntBrackets::counterOutOfOrder(InstCounterType T) const {
   // Scalar memory read always can go out of order.
   if (T == LGKM_CNT && hasPendingEvent(SMEM_ACCESS))
     return true;
   return MixedPendingEvents[T];
 }
 
 INITIALIZE_PASS_BEGIN(SIInsertWaitcnts, DEBUG_TYPE, "SI Insert Waitcnts", false,
                       false)

 ///  but instructions of different types can complete out of order.
 ///  We rely on this in-order completion
 ///  and simply assign a score to the memory access instructions.
 ///  We keep track of the active "score bracket" to determine
 ///  if an access of a memory read requires an s_waitcnt
 ///  and if so what the value of each counter is.
 ///  The "score bracket" is bound by the lower bound and upper bound
 ///  scores (*_score_LB and *_score_ub respectively).
-void SIInsertWaitcnts::generateWaitcntInstBefore(
-    MachineInstr &MI, BlockWaitcntBrackets *ScoreBrackets,
+bool SIInsertWaitcnts::generateWaitcntInstBefore(
+    MachineInstr &MI, WaitcntBrackets &ScoreBrackets,
     MachineInstr *OldWaitcntInstr) {
   setForceEmitWaitcnt();
   bool IsForceEmitWaitcnt = isForceEmitWaitcnt();
 
   if (MI.isDebugInstr())
-    return;
+    return false;
 
   AMDGPU::Waitcnt Wait;
 
   // See if this instruction has a forced S_WAITCNT VM.
   // TODO: Handle other cases of NeedsWaitcntVmBefore()
   if (MI.getOpcode() == AMDGPU::BUFFER_WBINVL1 ||
-           MI.getOpcode() == AMDGPU::BUFFER_WBINVL1_SC ||
-           MI.getOpcode() == AMDGPU::BUFFER_WBINVL1_VOL) {
     Wait.VmCnt = 0;
   }
 
   // All waits must be resolved at call return.
   // NOTE: this could be improved with knowledge of all call sites or
   //   with knowledge of the called routines.
   if (MI.getOpcode() == AMDGPU::SI_RETURN_TO_EPILOG ||
       MI.getOpcode() == AMDGPU::S_SETPC_B64_return) {
#endif
   // Export & GDS instructions do not read the EXEC mask until after the export
   // is granted (which can occur well after the instruction is issued).
   // The shader program must flush all EXP operations on the export-count
   // before overwriting the EXEC mask.
   else {
     if (MI.modifiesRegister(AMDGPU::EXEC, TRI)) {
       // Export and GDS are tracked individually, either may trigger a waitcnt
       // for EXEC.
-      if (ScoreBrackets->hasPendingEvent(EXP_GPR_LOCK) ||
-          ScoreBrackets->hasPendingEvent(EXP_PARAM_ACCESS) ||
-          ScoreBrackets->hasPendingEvent(EXP_POS_ACCESS) ||
-          ScoreBrackets->hasPendingEvent(GDS_GPR_LOCK)) {
+      if (ScoreBrackets.hasPendingEvent(EXP_GPR_LOCK) ||
+          ScoreBrackets.hasPendingEvent(EXP_PARAM_ACCESS) ||
+          ScoreBrackets.hasPendingEvent(EXP_POS_ACCESS) ||
+          ScoreBrackets.hasPendingEvent(GDS_GPR_LOCK)) {
         Wait.ExpCnt = 0;
       }
     }
 
 #if 0 // TODO: the following code to handle CALL.
     // The argument passing for CALLs should suffice for VM_CNT and LGKM_CNT.
     // However, there is a problem with EXP_CNT, because the call cannot
     // easily tell if a register is used in the function, and if it did, then
#endif
     // If the source operand was defined by a load, add the s_waitcnt
     // instruction.
     for (const MachineMemOperand *Memop : MI.memoperands()) {
       unsigned AS = Memop->getAddrSpace();
       if (AS != AMDGPUAS::LOCAL_ADDRESS)
         continue;
       unsigned RegNo = SQ_MAX_PGM_VGPRS + EXTRA_VGPR_LDS;
       // VM_CNT is only relevant to vgpr or LDS.
-      ScoreBrackets->determineWait(
-          VM_CNT, ScoreBrackets->getRegScore(RegNo, VM_CNT), Wait);
+      ScoreBrackets.determineWait(
+          VM_CNT, ScoreBrackets.getRegScore(RegNo, VM_CNT), Wait);
     }
 
     for (unsigned I = 0, E = MI.getNumOperands(); I != E; ++I) {
       const MachineOperand &Op = MI.getOperand(I);
       const MachineRegisterInfo &MRIA = *MRI;
       RegInterval Interval =
-          ScoreBrackets->getRegInterval(&MI, TII, MRI, TRI, I, false);
+          ScoreBrackets.getRegInterval(&MI, TII, MRI, TRI, I, false);
       for (signed RegNo = Interval.first; RegNo < Interval.second; ++RegNo) {
         if (TRI->isVGPR(MRIA, Op.getReg())) {
           // VM_CNT is only relevant to vgpr or LDS.
-          ScoreBrackets->determineWait(
-              VM_CNT, ScoreBrackets->getRegScore(RegNo, VM_CNT), Wait);
+          ScoreBrackets.determineWait(
+              VM_CNT, ScoreBrackets.getRegScore(RegNo, VM_CNT), Wait);
         }
-        ScoreBrackets->determineWait(
-            LGKM_CNT, ScoreBrackets->getRegScore(RegNo, LGKM_CNT), Wait);
+        ScoreBrackets.determineWait(
+            LGKM_CNT, ScoreBrackets.getRegScore(RegNo, LGKM_CNT), Wait);
       }
     }
     // End of for loop that looks at all source operands to decide vm_wait_cnt
     // and lgk_wait_cnt.
 
     // Two cases are handled for destination operands:
     // 1) If the destination operand was defined by a load, add the s_waitcnt
     // instruction to guarantee the right WAW order.
     // 2) If a destination operand that was used by a recent export/store ins,
     // add s_waitcnt on exp_cnt to guarantee the WAR order.
     if (MI.mayStore()) {
       // FIXME: Should not be relying on memoperands.
       for (const MachineMemOperand *Memop : MI.memoperands()) {
         unsigned AS = Memop->getAddrSpace();
         if (AS != AMDGPUAS::LOCAL_ADDRESS)
           continue;
         unsigned RegNo = SQ_MAX_PGM_VGPRS + EXTRA_VGPR_LDS;
-        ScoreBrackets->determineWait(
-            VM_CNT, ScoreBrackets->getRegScore(RegNo, VM_CNT), Wait);
-        ScoreBrackets->determineWait(
-            EXP_CNT, ScoreBrackets->getRegScore(RegNo, EXP_CNT), Wait);
+        ScoreBrackets.determineWait(
+            VM_CNT, ScoreBrackets.getRegScore(RegNo, VM_CNT), Wait);
+        ScoreBrackets.determineWait(
+            EXP_CNT, ScoreBrackets.getRegScore(RegNo, EXP_CNT), Wait);
       }
     }
     for (unsigned I = 0, E = MI.getNumOperands(); I != E; ++I) {
       MachineOperand &Def = MI.getOperand(I);
       const MachineRegisterInfo &MRIA = *MRI;
       RegInterval Interval =
-          ScoreBrackets->getRegInterval(&MI, TII, MRI, TRI, I, true);
+          ScoreBrackets.getRegInterval(&MI, TII, MRI, TRI, I, true);
       for (signed RegNo = Interval.first; RegNo < Interval.second; ++RegNo) {
         if (TRI->isVGPR(MRIA, Def.getReg())) {
-          ScoreBrackets->determineWait(
-              VM_CNT, ScoreBrackets->getRegScore(RegNo, VM_CNT), Wait);
-          ScoreBrackets->determineWait(
-              EXP_CNT, ScoreBrackets->getRegScore(RegNo, EXP_CNT), Wait);
+          ScoreBrackets.determineWait(
+              VM_CNT, ScoreBrackets.getRegScore(RegNo, VM_CNT), Wait);
+          ScoreBrackets.determineWait(
+              EXP_CNT, ScoreBrackets.getRegScore(RegNo, EXP_CNT), Wait);
         }
-        ScoreBrackets->determineWait(
-            LGKM_CNT, ScoreBrackets->getRegScore(RegNo, LGKM_CNT), Wait);
+        ScoreBrackets.determineWait(
+            LGKM_CNT, ScoreBrackets.getRegScore(RegNo, LGKM_CNT), Wait);
       }
     } // End of for loop that looks at all dest operands.
   }
 
   // Check to see if this is an S_BARRIER, and if an implicit S_WAITCNT 0
   // occurs before the instruction. Doing it here prevents any additional
   // S_WAITCNTs from being emitted if the instruction was marked as
   // requiring a WAITCNT beforehand.
   if (MI.getOpcode() == AMDGPU::S_BARRIER &&
       !ST->hasAutoWaitcntBeforeBarrier()) {
     Wait = AMDGPU::Waitcnt::allZero();
   }
 
   // TODO: Remove this work-around, enable the assert for Bug 457939
   //       after fixing the scheduler. Also, the Shader Compiler code is
   //       independent of target.
   if (readsVCCZ(MI) && ST->getGeneration() <= AMDGPUSubtarget::SEA_ISLANDS) {
-    if (ScoreBrackets->getScoreLB(LGKM_CNT) <
-            ScoreBrackets->getScoreUB(LGKM_CNT) &&
-        ScoreBrackets->hasPendingEvent(SMEM_ACCESS)) {
+    if (ScoreBrackets.getScoreLB(LGKM_CNT) <
+            ScoreBrackets.getScoreUB(LGKM_CNT) &&
+        ScoreBrackets.hasPendingEvent(SMEM_ACCESS)) {
       Wait.LgkmCnt = 0;
     }
   }
 
   // Does this operand processing indicate s_wait counter update?
-  if (ScoreBrackets->simplifyWaitcnt(Wait) || IsForceEmitWaitcnt) {
+  bool Modified = false;
+
+  if (ScoreBrackets.simplifyWaitcnt(Wait) || IsForceEmitWaitcnt) {
     if (ForceEmitZeroWaitcnts)
       Wait = AMDGPU::Waitcnt::allZero();
 
     if (ForceEmitWaitcnt[VM_CNT])
       Wait.VmCnt = 0;
     if (ForceEmitWaitcnt[EXP_CNT])
       Wait.ExpCnt = 0;
     if (ForceEmitWaitcnt[LGKM_CNT])
       Wait.LgkmCnt = 0;
 
-    ScoreBrackets->applyWaitcnt(Wait);
+    ScoreBrackets.applyWaitcnt(Wait);
 
     AMDGPU::Waitcnt OldWait;
     if (OldWaitcntInstr) {
       OldWait =
           AMDGPU::decodeWaitcnt(IV, OldWaitcntInstr->getOperand(0).getImm());
     }
     if (!OldWait.dominates(Wait)) {
-      MachineLoop *ContainingLoop = MLI->getLoopFor(MI.getParent());
-      if (ContainingLoop) {
-        MachineBasicBlock *TBB = ContainingLoop->getHeader();
-        BlockWaitcntBrackets *ScoreBracket = BlockWaitcntBracketsMap[TBB].get();
-        if (!ScoreBracket) {
-          assert(!BlockVisitedSet.count(TBB));
-          BlockWaitcntBracketsMap[TBB] =
-              llvm::make_unique<BlockWaitcntBrackets>(ST);
-          ScoreBracket = BlockWaitcntBracketsMap[TBB].get();
-        }
-        ScoreBracket->setRevisitLoop(true);
-        LLVM_DEBUG(dbgs() << "set-revisit2: Block"
-                          << ContainingLoop->getHeader()->getNumber() << '\n';);
-      }
-
       if (OldWaitcntInstr && !TrackedWaitcntSet.count(OldWaitcntInstr))
         Wait = Wait.combined(OldWait);
 
       unsigned Enc = AMDGPU::encodeWaitcnt(IV, Wait);
       if (OldWaitcntInstr) {
         OldWaitcntInstr->getOperand(0).setImm(Enc);
 
         LLVM_DEBUG(dbgs() << "updateWaitcntInBlock\n"
                           << "Old Instr: " << MI << '\n'
                           << "New Instr: " << *OldWaitcntInstr << '\n');
       } else {
         auto SWaitInst = BuildMI(*MI.getParent(), MI.getIterator(),
                                  MI.getDebugLoc(), TII->get(AMDGPU::S_WAITCNT))
                              .addImm(Enc);
         TrackedWaitcntSet.insert(SWaitInst);
 
         LLVM_DEBUG(dbgs() << "insertWaitcntInBlock\n"
                           << "Old Instr: " << MI << '\n'
                           << "New Instr: " << *SWaitInst << '\n');
       }
+
+      Modified = true;
     }
   } else {
     if (OldWaitcntInstr) {
       if (TrackedWaitcntSet.count(OldWaitcntInstr)) {
         TrackedWaitcntSet.erase(OldWaitcntInstr);
         OldWaitcntInstr->eraseFromParent();
       } else {
         int64_t Imm = OldWaitcntInstr->getOperand(0).getImm();
-        ScoreBrackets->applyWaitcnt(AMDGPU::decodeWaitcnt(IV, Imm));
-      }
-    }
+        ScoreBrackets.applyWaitcnt(AMDGPU::decodeWaitcnt(IV, Imm));
       }
+      Modified = true;
     }
-
-void SIInsertWaitcnts::insertWaitcntBeforeCF(MachineBasicBlock &MBB,
-                                             MachineInstr *Waitcnt) {
-  if (MBB.empty()) {
-    MBB.push_back(Waitcnt);
-    return;
   }
 
-  MachineBasicBlock::iterator It = MBB.end();
-  MachineInstr *MI = &*(--It);
-  if (MI->isBranch()) {
-    MBB.insert(It, Waitcnt);
-  } else {
-    MBB.push_back(Waitcnt);
-  }
+  return Modified;
 }
 
 // This is a flat memory operation. Check to see if it has memory
 // tokens for both LDS and Memory, and if so mark it as a flat.
 bool SIInsertWaitcnts::mayAccessLDSThroughFlat(const MachineInstr &MI) const {
   if (MI.memoperands_empty())
     return true;
 
   for (const MachineMemOperand *Memop : MI.memoperands()) {
     unsigned AS = Memop->getAddrSpace();
     if (AS == AMDGPUAS::LOCAL_ADDRESS || AS == AMDGPUAS::FLAT_ADDRESS)
       return true;
   }
 
   return false;
 }
 
-void SIInsertWaitcnts::updateEventWaitcntAfter(
-    MachineInstr &Inst, BlockWaitcntBrackets *ScoreBrackets) {
+void SIInsertWaitcnts::updateEventWaitcntAfter(MachineInstr &Inst,
+                                               WaitcntBrackets *ScoreBrackets) {
   // Now look at the instruction opcode. If it is a memory access
   // instruction, update the upper-bound of the appropriate counter's
   // bracket and the destination operand scores.
   // TODO: Use the (TSFlags & SIInstrFlags::LGKM_CNT) property everywhere.
   if (TII->isDS(Inst) && TII->usesLGKM_CNT(Inst)) {
     if (TII->hasModifiersSet(Inst, AMDGPU::OpName::gds)) {
       ScoreBrackets->updateByEvent(TII, TRI, MRI, GDS_ACCESS, Inst);
       ScoreBrackets->updateByEvent(TII, TRI, MRI, GDS_GPR_LOCK, Inst);
case AMDGPU::S_MEMREALTIME:
       ScoreBrackets->updateByEvent(TII, TRI, MRI, SMEM_ACCESS, Inst);
       break;
     default:
       break;
     }
   }
 }
 
-// Merge the score brackets of the Block's predecessors;
-// this merged score bracket is used when adding waitcnts to the Block
-void SIInsertWaitcnts::mergeInputScoreBrackets(MachineBasicBlock &Block) {
-  BlockWaitcntBrackets *ScoreBrackets = BlockWaitcntBracketsMap[&Block].get();
-  uint32_t MaxPending[NUM_INST_CNTS] = {0};
-  uint32_t MaxFlat[NUM_INST_CNTS] = {0};
-
-  // For single basic block loops, we need to retain the Block's
-  // score bracket to have accurate Pred info. So, make a copy of Block's
-  // score bracket, clear() it (which retains several important bits of info),
-  // populate, and then replace en masse. For non-single basic block loops,
-  // just clear Block's current score bracket and repopulate in-place.
-  bool IsSelfPred;
-  std::unique_ptr<BlockWaitcntBrackets> S;
-
-  IsSelfPred = (std::find(Block.pred_begin(), Block.pred_end(), &Block))
-    != Block.pred_end();
-  if (IsSelfPred) {
-    S = llvm::make_unique<BlockWaitcntBrackets>(*ScoreBrackets);
-    ScoreBrackets = S.get();
-  }
-
-  ScoreBrackets->clear();
-
-  // See if there are any uninitialized predecessors. If so, emit an
-  // s_waitcnt 0 at the beginning of the block.
-  for (MachineBasicBlock *Pred : Block.predecessors()) {
-    BlockWaitcntBrackets *PredScoreBrackets =
-        BlockWaitcntBracketsMap[Pred].get();
-    bool Visited = BlockVisitedSet.count(Pred);
-    if (!Visited)
-      continue;
-    for (auto T : inst_counter_types()) {
-      uint32_t span =
-          PredScoreBrackets->getScoreUB(T) - PredScoreBrackets->getScoreLB(T);
-      MaxPending[T] = std::max(MaxPending[T], span);
-      span =
-          PredScoreBrackets->pendingFlat(T) - PredScoreBrackets->getScoreLB(T);
-      MaxFlat[T] = std::max(MaxFlat[T], span);
-    }
-  }
+bool WaitcntBrackets::mergeScore(const MergeInfo &M, uint32_t &Score,
+                                 uint32_t OtherScore) {
+  uint32_t MyShifted = Score <= M.OldLB ? 0 : Score + M.MyShift;
+  uint32_t OtherShifted =
+      OtherScore <= M.OtherLB ? 0 : OtherScore + M.OtherShift;
+  Score = std::max(MyShifted, OtherShifted);
+  return OtherShifted > MyShifted;
 
-  // Now set the current Block's brackets to the largest ending bracket.
-  for (auto T : inst_counter_types()) {
-    ScoreBrackets->setScoreUB(T, MaxPending[T]);
-    ScoreBrackets->setScoreLB(T, 0);
-    ScoreBrackets->setLastFlat(T, MaxFlat[T]);
-  }
-
-  // Set the register scoreboard.
-  for (MachineBasicBlock *Pred : Block.predecessors()) {
-    if (!BlockVisitedSet.count(Pred)) {
-      continue;
-    }
-
-    BlockWaitcntBrackets *PredScoreBrackets =
-        BlockWaitcntBracketsMap[Pred].get();
+/// Merge the pending events and associater score brackets of \p Other into
+/// this brackets status.
+///
+/// Returns whether the merge resulted in a change that requires tighter waits
+/// (i.e. the merged brackets strictly dominate the original brackets).
+bool WaitcntBrackets::merge(const WaitcntBrackets &Other) {
+  bool StrictDom = false;
 
-    // Now merge the gpr_reg_score information
-    for (auto T : inst_counter_types()) {
-      uint32_t PredLB = PredScoreBrackets->getScoreLB(T);
-      uint32_t PredUB = PredScoreBrackets->getScoreUB(T);
-      if (PredLB < PredUB) {
-        uint32_t PredScale = MaxPending[T] - PredUB;
-        // Merge vgpr scores.
-        for (int J = 0; J <= PredScoreBrackets->getMaxVGPR(); J++) {
-          uint32_t PredRegScore = PredScoreBrackets->getRegScore(J, T);
-          if (PredRegScore <= PredLB)
-            continue;
-          uint32_t NewRegScore = PredScale + PredRegScore;
-          ScoreBrackets->setRegScore(
-              J, T, std::max(ScoreBrackets->getRegScore(J, T), NewRegScore));
-        }
-        // Also need to merge sgpr scores for lgkm_cnt.
-        if (T == LGKM_CNT) {
-          for (int J = 0; J <= PredScoreBrackets->getMaxSGPR(); J++) {
-            uint32_t PredRegScore =
-                PredScoreBrackets->getRegScore(J + NUM_ALL_VGPRS, LGKM_CNT);
-            if (PredRegScore <= PredLB)
-              continue;
-            uint32_t NewRegScore = PredScale + PredRegScore;
-            ScoreBrackets->setRegScore(
-                J + NUM_ALL_VGPRS, LGKM_CNT,
-                std::max(
-                    ScoreBrackets->getRegScore(J + NUM_ALL_VGPRS, LGKM_CNT),
-                    NewRegScore));
-          }
-        }
-      }
-    }
+    // Merge event flags for this counter
+    const bool OldOutOfOrder = counterOutOfOrder(T);
+    const uint32_t OldEvents = PendingEvents & WaitEventMaskForInst[T];
+    const uint32_t OtherEvents = Other.PendingEvents & WaitEventMaskForInst[T];
+    if (OtherEvents & ~OldEvents)
+      StrictDom = true;
+    if (Other.MixedPendingEvents[T] ||
+        (OldEvents && OtherEvents && OldEvents != OtherEvents))
+      MixedPendingEvents[T] = true;
+    PendingEvents |= OtherEvents;
 
-    ScoreBrackets->mergePendingEvents(*PredScoreBrackets);
+    // Merge scores for this counter
+    const uint32_t MyPending = ScoreUBs[T] - ScoreLBs[T];
+    const uint32_t OtherPending = Other.ScoreUBs[T] - Other.ScoreLBs[T];
+    MergeInfo M;
+    M.OldLB = ScoreLBs[T];
+    M.OtherLB = Other.ScoreLBs[T];
+    M.MyShift = OtherPending > MyPending ? OtherPending - MyPending : 0;
+    M.OtherShift = ScoreUBs[T] - Other.ScoreUBs[T] + M.MyShift;
+
+    const uint32_t NewUB = ScoreUBs[T] + M.MyShift;
+    if (NewUB < ScoreUBs[T])
+      report_fatal_error("waitcnt score overflow");
+    ScoreUBs[T] = NewUB;
+    ScoreLBs[T] = std::min(M.OldLB + M.MyShift, M.OtherLB + M.OtherShift);
+
+    StrictDom |= mergeScore(M, LastFlat[T], Other.LastFlat[T]);
+
+    bool RegStrictDom = false;
+    for (int J = 0, E = std::max(getMaxVGPR(), Other.getMaxVGPR()) + 1; J != E;
+         J++) {
+      RegStrictDom |= mergeScore(M, VgprScores[T][J], Other.VgprScores[T][J]);
     }
 
-  // if a single block loop, update the score brackets. Not needed for other
-  // blocks, as we did this in-place
-  if (IsSelfPred) {
-    BlockWaitcntBracketsMap[&Block] = llvm::make_unique<BlockWaitcntBrackets>(*ScoreBrackets);
+    if (T == LGKM_CNT) {
+      for (int J = 0, E = std::max(getMaxSGPR(), Other.getMaxSGPR()) + 1;
+           J != E; J++) {
+        RegStrictDom |= mergeScore(M, SgprScores[J], Other.SgprScores[J]);
       }
     }
 
-/// Return true if the given basic block is a "bottom" block of a loop.
-/// This works even if the loop is discontiguous. This also handles
-/// multiple back-edges for the same "header" block of a loop.
-bool SIInsertWaitcnts::isLoopBottom(const MachineLoop *Loop,
-                                    const MachineBasicBlock *Block) {
-  for (MachineBasicBlock *MBB : Loop->blocks()) {
-    if (MBB == Block && MBB->isSuccessor(Loop->getHeader())) {
-      return true;
-    }
-  }
-  return false;
+    if (RegStrictDom && !OldOutOfOrder)
+      StrictDom = true;
   }
 
-/// Count the number of "bottom" basic blocks of a loop.
-unsigned SIInsertWaitcnts::countNumBottomBlocks(const MachineLoop *Loop) {
-  unsigned Count = 0;
-  for (MachineBasicBlock *MBB : Loop->blocks()) {
-    if (MBB->isSuccessor(Loop->getHeader())) {
-      Count++;
-    }
-  }
-  return Count;
+  return StrictDom;
 }
 
 // Generate s_waitcnt instructions where needed.
-void SIInsertWaitcnts::insertWaitcntInBlock(MachineFunction &MF,
-                                            MachineBasicBlock &Block) {
-  // Initialize the state information.
-  mergeInputScoreBrackets(Block);
-
-  BlockWaitcntBrackets *ScoreBrackets = BlockWaitcntBracketsMap[&Block].get();
+bool SIInsertWaitcnts::insertWaitcntInBlock(MachineFunction &MF,
+                                            MachineBasicBlock &Block,
+                                            WaitcntBrackets &ScoreBrackets) {
+  bool Modified = false;
 
   LLVM_DEBUG({
     dbgs() << "*** Block" << Block.getNumber() << " ***";
-    ScoreBrackets->dump();
+    ScoreBrackets.dump();
   });
 
   // Walk over the instructions.
   MachineInstr *OldWaitcntInstr = nullptr;
 
   for (MachineBasicBlock::iterator Iter = Block.begin(), E = Block.end();
        Iter != E;) {
     MachineInstr &Inst = *Iter;
 
     // Remove any previously existing waitcnts.
     if (Inst.getOpcode() == AMDGPU::S_WAITCNT) {
       if (OldWaitcntInstr) {
         if (TrackedWaitcntSet.count(OldWaitcntInstr)) {
           TrackedWaitcntSet.erase(OldWaitcntInstr);
           OldWaitcntInstr->eraseFromParent();
           OldWaitcntInstr = nullptr;
         } else if (!TrackedWaitcntSet.count(&Inst)) {
           // Two successive s_waitcnt's, both of which are pre-existing and
           // are therefore preserved.
           int64_t Imm = OldWaitcntInstr->getOperand(0).getImm();
-          ScoreBrackets->applyWaitcnt(AMDGPU::decodeWaitcnt(IV, Imm));
+          ScoreBrackets.applyWaitcnt(AMDGPU::decodeWaitcnt(IV, Imm));
         } else {
           ++Iter;
           Inst.eraseFromParent();
+          Modified = true;
           continue;
         }
       }
 
       OldWaitcntInstr = &Inst;
       ++Iter;
       continue;
     }
 
     bool VCCZBugWorkAround = false;
     if (readsVCCZ(Inst) &&
         (!VCCZBugHandledSet.count(&Inst))) {
-      if (ScoreBrackets->getScoreLB(LGKM_CNT) <
-              ScoreBrackets->getScoreUB(LGKM_CNT) &&
-          ScoreBrackets->hasPendingEvent(SMEM_ACCESS)) {
+      if (ScoreBrackets.getScoreLB(LGKM_CNT) <
+              ScoreBrackets.getScoreUB(LGKM_CNT) &&
+          ScoreBrackets.hasPendingEvent(SMEM_ACCESS)) {
         if (ST->getGeneration() <= AMDGPUSubtarget::SEA_ISLANDS)
           VCCZBugWorkAround = true;
       }
     }
 
     // Generate an s_waitcnt instruction to be placed before
     // cur_Inst, if needed.
-    generateWaitcntInstBefore(Inst, ScoreBrackets, OldWaitcntInstr);
+    Modified |= generateWaitcntInstBefore(Inst, ScoreBrackets, OldWaitcntInstr);
     OldWaitcntInstr = nullptr;
 
-    updateEventWaitcntAfter(Inst, ScoreBrackets);
+    updateEventWaitcntAfter(Inst, &ScoreBrackets);
 
 #if 0 // TODO: implement resource type check controlled by options with ub = LB.
     // If this instruction generates a S_SETVSKIP because it is an
     // indexed resource, and we are on Tahiti, then it will also force
     // an S_WAITCNT vmcnt(0)
     if (RequireCheckResourceType(Inst, context)) {
       // Force the score to as if an S_WAITCNT vmcnt(0) is emitted.
       ScoreBrackets->setScoreLB(VM_CNT,
       ScoreBrackets->getScoreUB(VM_CNT));
     }
 #endif
 
     LLVM_DEBUG({
       Inst.print(dbgs());
-      ScoreBrackets->dump();
+      ScoreBrackets.dump();
     });
 
     // Check to see if this is a GWS instruction. If so, and if this is CI or
     // VI, then the generated code sequence will include an S_WAITCNT 0.
     // TODO: Are these the only GWS instructions?
     if (Inst.getOpcode() == AMDGPU::DS_GWS_INIT ||
         Inst.getOpcode() == AMDGPU::DS_GWS_SEMA_V ||
         Inst.getOpcode() == AMDGPU::DS_GWS_SEMA_BR ||
         Inst.getOpcode() == AMDGPU::DS_GWS_SEMA_P ||
         Inst.getOpcode() == AMDGPU::DS_GWS_BARRIER) {
       // TODO: && context->target_info->GwsRequiresMemViolTest() ) {
-      ScoreBrackets->applyWaitcnt(AMDGPU::Waitcnt::allZero());
+      ScoreBrackets.applyWaitcnt(AMDGPU::Waitcnt::allZero());
     }
 
     // TODO: Remove this work-around after fixing the scheduler and enable the
     // assert above.
     if (VCCZBugWorkAround) {
       // Restore the vccz bit.  Any time a value is written to vcc, the vcc
       // bit is updated, so we can restore the bit by reading the value of
       // vcc and then writing it back to the register.
       BuildMI(Block, Inst, Inst.getDebugLoc(), TII->get(AMDGPU::S_MOV_B64),
               AMDGPU::VCC)
           .addReg(AMDGPU::VCC);
       VCCZBugHandledSet.insert(&Inst);
+      Modified = true;
     }
 
     ++Iter;
   }
 
-  // Check if we need to force convergence at loop footer.
-  MachineLoop *ContainingLoop = MLI->getLoopFor(&Block);
-  if (ContainingLoop && isLoopBottom(ContainingLoop, &Block)) {
-    LoopWaitcntData *WaitcntData = LoopWaitcntDataMap[ContainingLoop].get();
-    WaitcntData->print();
-    LLVM_DEBUG(dbgs() << '\n';);
-
-    // The iterative waitcnt insertion algorithm aims for optimal waitcnt
-    // placement, but doesn't guarantee convergence for a loop. Each
-    // loop should take at most (n+1) iterations for it to converge naturally,
-    // where n is the number of bottom blocks. If this threshold is reached and
-    // the result hasn't converged, then we force convergence by inserting
-    // a s_waitcnt at the end of loop footer.
-    if (WaitcntData->getIterCnt() > (countNumBottomBlocks(ContainingLoop) + 1)) {
-      // To ensure convergence, need to make wait events at loop footer be no
-      // more than those from the previous iteration.
-      // As a simplification, instead of tracking individual scores and
-      // generating the precise wait count, just wait on 0.
-      bool HasPending = false;
-      MachineInstr *SWaitInst = WaitcntData->getWaitcnt();
-      for (auto T : inst_counter_types()) {
-        if (ScoreBrackets->getScoreUB(T) > ScoreBrackets->getScoreLB(T)) {
-          ScoreBrackets->setScoreLB(T, ScoreBrackets->getScoreUB(T));
-          HasPending = true;
-          break;
-        }
-      }
-
-      if (HasPending) {
-        if (!SWaitInst) {
-          SWaitInst = BuildMI(Block, Block.getFirstNonPHI(),
-                              DebugLoc(), TII->get(AMDGPU::S_WAITCNT))
-                              .addImm(0);
-          TrackedWaitcntSet.insert(SWaitInst);
-#if 0 // TODO: Format the debug output
-          OutputTransformBanner("insertWaitcntInBlock",0,"Create:",context);
-          OutputTransformAdd(SWaitInst, context);
-#endif
-        }
-#if 0 // TODO: ??
-        _DEV( REPORTED_STATS->force_waitcnt_converge = 1; )
-#endif
-      }
-
-      if (SWaitInst) {
-        LLVM_DEBUG({
-          SWaitInst->print(dbgs());
-          dbgs() << "\nAdjusted score board:";
-          ScoreBrackets->dump();
-        });
-
-        // Add this waitcnt to the block. It is either newly created or
-        // created in previous iterations and added back since block traversal
-        // always removes waitcnts.
-        insertWaitcntBeforeCF(Block, SWaitInst);
-        WaitcntData->setWaitcnt(SWaitInst);
-      }
-    }
-  }
+  return Modified;
 }
 
 bool SIInsertWaitcnts::runOnMachineFunction(MachineFunction &MF) {
   ST = &MF.getSubtarget<GCNSubtarget>();
   TII = ST->getInstrInfo();
   TRI = &TII->getRegisterInfo();
   MRI = &MF.getRegInfo();
-  MLI = &getAnalysis<MachineLoopInfo>();
   IV = AMDGPU::getIsaVersion(ST->getCPU());
   const SIMachineFunctionInfo *MFI = MF.getInfo<SIMachineFunctionInfo>();
 
   ForceEmitZeroWaitcnts = ForceEmitZeroFlag;
   for (auto T : inst_counter_types())
     ForceEmitWaitcnt[T] = false;
 
   HardwareLimits.VmcntMax = AMDGPU::getVmcntBitMask(IV);
   HardwareLimits.ExpcntMax = AMDGPU::getExpcntBitMask(IV);
   HardwareLimits.LgkmcntMax = AMDGPU::getLgkmcntBitMask(IV);
 
   HardwareLimits.NumVGPRsMax = ST->getAddressableNumVGPRs();
   HardwareLimits.NumSGPRsMax = ST->getAddressableNumSGPRs();
   assert(HardwareLimits.NumVGPRsMax <= SQ_MAX_PGM_VGPRS);
   assert(HardwareLimits.NumSGPRsMax <= SQ_MAX_PGM_SGPRS);
 
   RegisterEncoding.VGPR0 = TRI->getEncodingValue(AMDGPU::VGPR0);
   RegisterEncoding.VGPRL =
       RegisterEncoding.VGPR0 + HardwareLimits.NumVGPRsMax - 1;
   RegisterEncoding.SGPR0 = TRI->getEncodingValue(AMDGPU::SGPR0);
   RegisterEncoding.SGPRL =
       RegisterEncoding.SGPR0 + HardwareLimits.NumSGPRsMax - 1;
 
   TrackedWaitcntSet.clear();
-  BlockVisitedSet.clear();
   VCCZBugHandledSet.clear();
-  LoopWaitcntDataMap.clear();
-  BlockWaitcntProcessedSet.clear();
+  RpotIdxMap.clear();
+  BlockInfos.clear();
 
-  // Walk over the blocks in reverse post order, inserting
-  // s_waitcnt where needed.
-  ReversePostOrderTraversal<MachineFunction *> RPOT(&MF);
-  bool Modified = false;
-  for (ReversePostOrderTraversal<MachineFunction *>::rpo_iterator
-           I = RPOT.begin(),
-           E = RPOT.end(), J = RPOT.begin();
-       I != E;) {
-    MachineBasicBlock &MBB = **I;
-
-    BlockVisitedSet.insert(&MBB);
-
-    BlockWaitcntBrackets *ScoreBrackets = BlockWaitcntBracketsMap[&MBB].get();
-    if (!ScoreBrackets) {
-      BlockWaitcntBracketsMap[&MBB] = llvm::make_unique<BlockWaitcntBrackets>(ST);
-      ScoreBrackets = BlockWaitcntBracketsMap[&MBB].get();
-    }
-    ScoreBrackets->setPostOrder(MBB.getNumber());
-    MachineLoop *ContainingLoop = MLI->getLoopFor(&MBB);
-    if (ContainingLoop && LoopWaitcntDataMap[ContainingLoop] == nullptr)
-      LoopWaitcntDataMap[ContainingLoop] = llvm::make_unique<LoopWaitcntData>();
-
-    // If we are walking into the block from before the loop, then guarantee
-    // at least 1 re-walk over the loop to propagate the information, even if
-    // no S_WAITCNT instructions were generated.
-    if (ContainingLoop && ContainingLoop->getHeader() == &MBB) {
-      unsigned Count = countNumBottomBlocks(ContainingLoop);
-
-      // If the loop has multiple back-edges, and so more than one "bottom"
-      // basic block, we have to guarantee a re-walk over every blocks.
-      if ((std::count(BlockWaitcntProcessedSet.begin(),
-                      BlockWaitcntProcessedSet.end(), &MBB) < (int)Count)) {
-        BlockWaitcntBracketsMap[&MBB]->setRevisitLoop(true);
-        LLVM_DEBUG(dbgs() << "set-revisit1: Block"
-                          << ContainingLoop->getHeader()->getNumber() << '\n';);
-      }
-    }
+  // Keep iterating over the blocks in reverse post order, inserting and
+  // updating s_waitcnt where needed, until a fix point is reached.
+  for (MachineBasicBlock *MBB :
+       ReversePostOrderTraversal<MachineFunction *>(&MF)) {
+    RpotIdxMap[MBB] = BlockInfos.size();
+    BlockInfos.emplace_back(MBB);
 
-    // Walk over the instructions.
-    insertWaitcntInBlock(MF, MBB);
+  std::unique_ptr<WaitcntBrackets> Brackets;
+  bool Modified = false;
+  bool Repeat;
+  do {
+    Repeat = false;
 
-    // Record that waitcnts have been processed at least once for this block.
-    BlockWaitcntProcessedSet.push_back(&MBB);
+    for (BlockInfo &BI : BlockInfos) {
+      if (!BI.Dirty)
+        continue;
 
-    // See if we want to revisit the loop. If a loop has multiple back-edges,
-    // we shouldn't revisit the same "bottom" basic block.
-    if (ContainingLoop && isLoopBottom(ContainingLoop, &MBB) &&
-        std::count(BlockWaitcntProcessedSet.begin(),
-                   BlockWaitcntProcessedSet.end(), &MBB) == 1) {
-      MachineBasicBlock *EntryBB = ContainingLoop->getHeader();
-      BlockWaitcntBrackets *EntrySB = BlockWaitcntBracketsMap[EntryBB].get();
-      if (EntrySB && EntrySB->getRevisitLoop()) {
-        EntrySB->setRevisitLoop(false);
-        J = I;
-        int32_t PostOrder = EntrySB->getPostOrder();
-        // TODO: Avoid this loop. Find another way to set I.
-        for (ReversePostOrderTraversal<MachineFunction *>::rpo_iterator
-                 X = RPOT.begin(),
-                 Y = RPOT.end();
-             X != Y; ++X) {
-          MachineBasicBlock &MBBX = **X;
-          if (MBBX.getNumber() == PostOrder) {
-            I = X;
-            break;
-          }
-        }
-        LoopWaitcntData *WaitcntData = LoopWaitcntDataMap[ContainingLoop].get();
-        WaitcntData->incIterCnt();
-        LLVM_DEBUG(dbgs() << "revisit: Block" << EntryBB->getNumber() << '\n';);
-        continue;
+      unsigned Idx = std::distance(&*BlockInfos.begin(), &BI);
+
+      if (BI.Incoming) {
+        if (!Brackets)
+          Brackets = llvm::make_unique<WaitcntBrackets>(*BI.Incoming);
+        else
+          *Brackets = *BI.Incoming;
+      } else {
+        if (!Brackets)
+          Brackets = llvm::make_unique<WaitcntBrackets>(ST);
+        else
+          Brackets->clear();
+
+      Modified |= insertWaitcntInBlock(MF, *BI.MBB, *Brackets);
+      BI.Dirty = false;
+
+      if (Brackets->hasPending()) {
+        BlockInfo *MoveBracketsToSucc = nullptr;
+        for (MachineBasicBlock *Succ : BI.MBB->successors()) {
+          unsigned SuccIdx = RpotIdxMap[Succ];
+          BlockInfo &SuccBI = BlockInfos[SuccIdx];
+          if (!SuccBI.Incoming) {
+            SuccBI.Dirty = true;
+            if (SuccIdx <= Idx)
+              Repeat = true;
+            if (!MoveBracketsToSucc) {
+              MoveBracketsToSucc = &SuccBI;
             } else {
-        LoopWaitcntData *WaitcntData = LoopWaitcntDataMap[ContainingLoop].get();
-        // Loop converged, reset iteration count. If this loop gets revisited,
-        // it must be from an outer loop, the counter will restart, this will
-        // ensure we don't force convergence on such revisits.
-        WaitcntData->resetIterCnt();
+              SuccBI.Incoming = llvm::make_unique<WaitcntBrackets>(*Brackets);
             }
+          } else if (SuccBI.Incoming->merge(*Brackets)) {
+            SuccBI.Dirty = true;
+            if (SuccIdx <= Idx)
+              Repeat = true;
+          }
+        }
+        if (MoveBracketsToSucc)
+          MoveBracketsToSucc->Incoming = std::move(Brackets);
       }
-
-    J = I;
-    ++I;
     }
+  } while (Repeat);
 
   SmallVector<MachineBasicBlock *, 4> EndPgmBlocks;
 
   bool HaveScalarStores = false;
 
   for (MachineFunction::iterator BI = MF.begin(), BE = MF.end(); BI != BE;
        ++BI) {
     MachineBasicBlock &MBB = *BI;

test/CodeGen/AMDGPU/waitcnt-loop-irreducible.mir

+# RUN: llc -march=amdgcn -verify-machineinstrs -run-pass si-insert-waitcnts -o - %s | FileCheck -check-prefixes=GCN %s
+
+# GCN-LABEL: name: irreducible_loop{{$}}
+# GCN: S_LOAD_DWORDX4_IMM
+# GCN: S_WAITCNT 127{{$}}
+# GCN: S_BUFFER_LOAD_DWORD_IMM
+# GCN: S_WAITCNT 127{{$}}
+# GCN: S_CMP_GE_I32
+--- |
+
+  define amdgpu_ps void @irreducible_loop() {
+  main:
+    ret void
+  }
+
+...
+---
+name:            irreducible_loop
+body:             |
+  bb.0:
+    successors: %bb.3, %bb.2
+
+    S_CBRANCH_VCCZ %bb.2, implicit $vcc
+    S_BRANCH %bb.3
+
+  bb.1:
+    successors: %bb.3, %bb.2
+
+    S_CBRANCH_VCCNZ %bb.3, implicit $vcc
+
+  bb.2:
+    successors: %bb.3
+
+    renamable $sgpr4_sgpr5_sgpr6_sgpr7 = S_LOAD_DWORDX4_IMM renamable $sgpr0_sgpr1, 0, 0
+    renamable $sgpr3 = S_BUFFER_LOAD_DWORD_IMM killed renamable $sgpr4_sgpr5_sgpr6_sgpr7, 0, 0
+
+  bb.3:
+    successors: %bb.1, %bb.4
+
+    S_CMP_GE_I32 renamable $sgpr2, renamable $sgpr3, implicit-def $scc
+    S_CBRANCH_SCC0 %bb.1, implicit killed $scc
+
+  bb.4:
+
+    S_ENDPGM
+
+...