[AMDGPU] Add second pass of the scheduler

If during scheduling we have identified that we cannot keep optimistic
occupancy increase critical register pressure limit and try scheduling
of the whole function again. In this case blocks with smaller pressure
will have a chance for better scheduling.

Differential Revision: https://reviews.llvm.org/D30442

llvm-svn: 296506

llvm/lib/Target/AMDGPU/GCNSchedStrategy.cpp

@@ -26,7 +26,7 @@ using namespace llvm;
 
 GCNMaxOccupancySchedStrategy::GCNMaxOccupancySchedStrategy(
     const MachineSchedContext *C) :
-    GenericScheduler(C) { }
+    GenericScheduler(C), TargetOccupancy(0), MF(nullptr) { }
 
 static unsigned getMaxWaves(unsigned SGPRs, unsigned VGPRs,
                             const MachineFunction &MF) {
@@ -45,6 +45,12 @@ void GCNMaxOccupancySchedStrategy::initialize(ScheduleDAGMI *DAG) {
 
   const SIRegisterInfo *SRI = static_cast<const SIRegisterInfo*>(TRI);
 
+  if (MF != &DAG->MF)
+    TargetOccupancy = 0;
+  MF = &DAG->MF;
+
+  const SISubtarget &ST = MF->getSubtarget<SISubtarget>();
+
   // FIXME: This is also necessary, because some passes that run after
   // scheduling and before regalloc increase register pressure.
   const int ErrorMargin = 3;
@@ -53,10 +59,18 @@ void GCNMaxOccupancySchedStrategy::initialize(ScheduleDAGMI *DAG) {
     ->getNumAllocatableRegs(&AMDGPU::SGPR_32RegClass) - ErrorMargin;
   VGPRExcessLimit = Context->RegClassInfo
     ->getNumAllocatableRegs(&AMDGPU::VGPR_32RegClass) - ErrorMargin;
-  SGPRCriticalLimit = SRI->getRegPressureSetLimit(DAG->MF,
-                        SRI->getSGPRPressureSet()) - ErrorMargin;
-  VGPRCriticalLimit = SRI->getRegPressureSetLimit(DAG->MF,
-                        SRI->getVGPRPressureSet()) - ErrorMargin;
+  if (TargetOccupancy) {
+    SGPRCriticalLimit = ST.getMaxNumSGPRs(TargetOccupancy, true);
+    VGPRCriticalLimit = ST.getMaxNumVGPRs(TargetOccupancy);
+  } else {
+    SGPRCriticalLimit = SRI->getRegPressureSetLimit(DAG->MF,
+                                                    SRI->getSGPRPressureSet());
+    VGPRCriticalLimit = SRI->getRegPressureSetLimit(DAG->MF,
+                                                    SRI->getVGPRPressureSet());
+  }
+
+  SGPRCriticalLimit -= ErrorMargin;
+  VGPRCriticalLimit -= ErrorMargin;
 }
 
 void GCNMaxOccupancySchedStrategy::initCandidate(SchedCandidate &Cand, SUnit *SU,
@@ -309,6 +323,28 @@ SUnit *GCNMaxOccupancySchedStrategy::pickNode(bool &IsTopNode) {
   return SU;
 }
 
+GCNScheduleDAGMILive::GCNScheduleDAGMILive(MachineSchedContext *C,
+                        std::unique_ptr<MachineSchedStrategy> S) :
+  ScheduleDAGMILive(C, std::move(S)),
+  ST(MF.getSubtarget<SISubtarget>()),
+  MFI(*MF.getInfo<SIMachineFunctionInfo>()),
+  StartingOccupancy(ST.getOccupancyWithLocalMemSize(MFI.getLDSSize(),
+                                                    *MF.getFunction())),
+  MinOccupancy(StartingOccupancy), Stage(0) {
+
+  DEBUG(dbgs() << "Starting occupancy is " << StartingOccupancy << ".\n");
+}
+
+void GCNScheduleDAGMILive::enterRegion(MachineBasicBlock *bb,
+                                       MachineBasicBlock::iterator begin,
+                                       MachineBasicBlock::iterator end,
+                                       unsigned regioninstrs) {
+  ScheduleDAGMILive::enterRegion(bb, begin, end, regioninstrs);
+
+  if (Stage == 0)
+    Regions.push_back(std::make_pair(begin, end));
+}
+
 void GCNScheduleDAGMILive::schedule() {
   std::vector<MachineInstr*> Unsched;
   Unsched.reserve(NumRegionInstrs);
@@ -344,6 +380,15 @@ void GCNScheduleDAGMILive::schedule() {
   DEBUG(dbgs() << "Occupancy before scheduling: " << WavesBefore <<
                   ", after " << WavesAfter << ".\n");
 
+  // We could not keep current target occupancy because of the just scheduled
+  // region. Record new occupancy for next scheduling cycle.
+  unsigned NewOccupancy = std::max(WavesAfter, WavesBefore);
+  if (NewOccupancy < MinOccupancy) {
+    MinOccupancy = NewOccupancy;
+    DEBUG(dbgs() << "Occupancy lowered for the function to "
+                 << MinOccupancy << ".\n");
+  }
+
   if (WavesAfter >= WavesBefore)
     return;
 
@@ -485,5 +530,52 @@ GCNScheduleDAGMILive::getRealRegPressure() const {
 }
 
 void GCNScheduleDAGMILive::finalizeSchedule() {
+  // Retry function scheduling if we found resulting occupancy and it is
+  // lower than used for first pass scheduling. This will give more freedom
+  // to schedule low register pressure blocks.
+  // Code is partially copied from MachineSchedulerBase::scheduleRegions().
+
+  if (!LIS || StartingOccupancy <= MinOccupancy)
+    return;
+
+  DEBUG(dbgs() << "Retrying function scheduling with lowest recorded occupancy "
+               << MinOccupancy << ".\n");
+
+  Stage++;
+  GCNMaxOccupancySchedStrategy &S = (GCNMaxOccupancySchedStrategy&)*SchedImpl;
+  S.TargetOccupancy = MinOccupancy;
+
+  MachineBasicBlock *MBB = nullptr;
+  for (auto Region : Regions) {
+    RegionBegin = Region.first;
+    RegionEnd = Region.second;
+
+    if (RegionBegin->getParent() != MBB) {
+      if (MBB) finishBlock();
+      MBB = RegionBegin->getParent();
+      startBlock(MBB);
+    }
+
+    unsigned NumRegionInstrs = std::distance(begin(), end());
+    enterRegion(MBB, begin(), end(), NumRegionInstrs);
+
+    // Skip empty scheduling regions (0 or 1 schedulable instructions).
+    if (begin() == end() || begin() == std::prev(end())) {
+      exitRegion();
+      continue;
+    }
+    DEBUG(dbgs() << "********** MI Scheduling **********\n");
+    DEBUG(dbgs() << MF.getName()
+          << ":BB#" << MBB->getNumber() << " " << MBB->getName()
+          << "\n  From: " << *begin() << "    To: ";
+          if (RegionEnd != MBB->end()) dbgs() << *RegionEnd;
+          else dbgs() << "End";
+          dbgs() << " RegionInstrs: " << NumRegionInstrs << '\n');
+
+    schedule();
+
+    exitRegion();
+  }
+  finishBlock();
   LiveIns.shrink_and_clear();
 }

llvm/lib/Target/AMDGPU/GCNSchedStrategy.h

@@ -18,7 +18,9 @@
 
 namespace llvm {
 
+class SIMachineFunctionInfo;
 class SIRegisterInfo;
+class SISubtarget;
 
 /// This is a minimal scheduler strategy.  The main difference between this
 /// and the GenericScheduler is that GCNSchedStrategy uses different
@@ -43,6 +45,10 @@ class GCNMaxOccupancySchedStrategy : public GenericScheduler {
   unsigned SGPRCriticalLimit;
   unsigned VGPRCriticalLimit;
 
+  unsigned TargetOccupancy;
+
+  MachineFunction *MF;
+
 public:
   GCNMaxOccupancySchedStrategy(const MachineSchedContext *C);
 
@@ -53,6 +59,23 @@ public:
 
 class GCNScheduleDAGMILive : public ScheduleDAGMILive {
 
+  const SISubtarget &ST;
+
+  const SIMachineFunctionInfo &MFI;
+
+  // Occupancy target at the begining of function scheduling cycle.
+  unsigned StartingOccupancy;
+
+  // Minimal real occupancy recorder for the function.
+  unsigned MinOccupancy;
+
+  // Scheduling stage number.
+  unsigned Stage;
+
+  // Vecor of regions recorder for later rescheduling
+  SmallVector<std::pair<const MachineBasicBlock::iterator,
+                        const MachineBasicBlock::iterator>, 32> Regions;
+
   // Region live-ins.
   DenseMap<unsigned, LaneBitmask> LiveIns;
 
@@ -67,8 +90,12 @@ class GCNScheduleDAGMILive : public ScheduleDAGMILive {
 
 public:
   GCNScheduleDAGMILive(MachineSchedContext *C,
-                       std::unique_ptr<MachineSchedStrategy> S) :
-    ScheduleDAGMILive(C, std::move(S)) {}
+                       std::unique_ptr<MachineSchedStrategy> S);
+
+  void enterRegion(MachineBasicBlock *bb,
+                   MachineBasicBlock::iterator begin,
+                   MachineBasicBlock::iterator end,
+                   unsigned regioninstrs) override;
 
   void schedule() override;