[PHIElimination] Compile time optimization for huge functions.

This is a compile-time optimization for PHIElimination (splitting of critical
edges), which was reported at https://bugs.llvm.org/show_bug.cgi?id=44249. As
discussed there, the way to remedy the slowdowns with huge functions is to
pre-compute the live-in registers for each MBB in an efficient way in
PHIElimination.cpp and then pass that information along to
LiveVariabless::addNewBlock().

In all the huge test programs where this slowdown has been noticable, it has
dissapeared entirely with this patch.

Review: Björn Pettersson, Quentin Colombet.

Differential Revision: https://reviews.llvm.org/D73152

include/llvm/CodeGen/LiveVariables.h

@@ -297,6 +297,11 @@ public:
                    MachineBasicBlock *DomBB,
                    MachineBasicBlock *SuccBB);
 
+  void addNewBlock(MachineBasicBlock *BB,
+                   MachineBasicBlock *DomBB,
+                   MachineBasicBlock *SuccBB,
+                   std::vector<SparseBitVector<>> &LiveInSets);
+
   /// isPHIJoin - Return true if Reg is a phi join register.
   bool isPHIJoin(unsigned Reg) { return PHIJoins.test(Reg); }
 

include/llvm/CodeGen/MachineBasicBlock.h

@@ -18,6 +18,7 @@
 #include "llvm/ADT/ilist_node.h"
 #include "llvm/ADT/iterator_range.h"
 #include "llvm/ADT/simple_ilist.h"
+#include "llvm/ADT/SparseBitVector.h"
 #include "llvm/CodeGen/MachineInstr.h"
 #include "llvm/CodeGen/MachineInstrBundleIterator.h"
 #include "llvm/IR/DebugLoc.h"
@@ -588,7 +589,9 @@ public:
   ///
   /// This function updates LiveVariables, MachineDominatorTree, and
   /// MachineLoopInfo, as applicable.
-  MachineBasicBlock *SplitCriticalEdge(MachineBasicBlock *Succ, Pass &P);
+  MachineBasicBlock *
+  SplitCriticalEdge(MachineBasicBlock *Succ, Pass &P,
+                    std::vector<SparseBitVector<>> *LiveInSets = nullptr);
 
   /// Check if the edge between this block and the given successor \p
   /// Succ, can be split. If this returns true a subsequent call to

lib/CodeGen/LiveVariables.cpp

@@ -806,3 +806,30 @@ void LiveVariables::addNewBlock(MachineBasicBlock *BB,
       VI.AliveBlocks.set(NumNew);
   }
 }
+
+/// addNewBlock - Add a new basic block BB as an empty succcessor to DomBB. All
+/// variables that are live out of DomBB will be marked as passing live through
+/// BB. LiveInSets[BB] is *not* updated (because it is not needed during
+/// PHIElimination).
+void LiveVariables::addNewBlock(MachineBasicBlock *BB,
+                                MachineBasicBlock *DomBB,
+                                MachineBasicBlock *SuccBB,
+                                std::vector<SparseBitVector<>> &LiveInSets) {
+  const unsigned NumNew = BB->getNumber();
+
+  SparseBitVector<> &BV = LiveInSets[SuccBB->getNumber()];
+  for (auto R = BV.begin(), E = BV.end(); R != E; R++) {
+    unsigned VirtReg = Register::index2VirtReg(*R);
+    LiveVariables::VarInfo &VI = getVarInfo(VirtReg);
+    VI.AliveBlocks.set(NumNew);
+  }
+  // All registers used by PHI nodes in SuccBB must be live through BB.
+  for (MachineBasicBlock::iterator BBI = SuccBB->begin(),
+         BBE = SuccBB->end();
+       BBI != BBE && BBI->isPHI(); ++BBI) {
+    for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2)
+      if (BBI->getOperand(i + 1).getMBB() == BB)
+        getVarInfo(BBI->getOperand(i).getReg())
+          .AliveBlocks.set(NumNew);
+  }
+}

lib/CodeGen/MachineBasicBlock.cpp

@@ -871,8 +871,9 @@ bool MachineBasicBlock::canFallThrough() {
   return getFallThrough() != nullptr;
 }
 
-MachineBasicBlock *MachineBasicBlock::SplitCriticalEdge(MachineBasicBlock *Succ,
-                                                        Pass &P) {
+MachineBasicBlock *MachineBasicBlock::SplitCriticalEdge(
+    MachineBasicBlock *Succ, Pass &P,
+    std::vector<SparseBitVector<>> *LiveInSets) {
   if (!canSplitCriticalEdge(Succ))
     return nullptr;
 
@@ -1003,7 +1004,10 @@ MachineBasicBlock *MachineBasicBlock::SplitCriticalEdge(MachineBasicBlock *Succ,
       }
     }
     // Update relevant live-through information.
-    LV->addNewBlock(NMBB, this, Succ);
+    if (LiveInSets != nullptr)
+      LV->addNewBlock(NMBB, this, Succ, *LiveInSets);
+    else
+      LV->addNewBlock(NMBB, this, Succ);
   }
 
   if (LIS) {

lib/CodeGen/PHIElimination.cpp

@@ -96,7 +96,8 @@ namespace {
 
     /// Split critical edges where necessary for good coalescer performance.
     bool SplitPHIEdges(MachineFunction &MF, MachineBasicBlock &MBB,
-                       MachineLoopInfo *MLI);
+                       MachineLoopInfo *MLI,
+                       std::vector<SparseBitVector<>> *LiveInSets);
 
     // These functions are temporary abstractions around LiveVariables and
     // LiveIntervals, so they can go away when LiveVariables does.
@@ -151,16 +152,45 @@ bool PHIElimination::runOnMachineFunction(MachineFunction &MF) {
 
   bool Changed = false;
 
-  // This pass takes the function out of SSA form.
-  MRI->leaveSSA();
-
   // Split critical edges to help the coalescer.
   if (!DisableEdgeSplitting && (LV || LIS)) {
+    // A set of live-in regs for each MBB which is used to update LV
+    // efficiently also with large functions.
+    std::vector<SparseBitVector<>> LiveInSets;
+    if (LV) {
+      LiveInSets.resize(MF.size());
+      for (unsigned Index = 0, e = MRI->getNumVirtRegs(); Index != e; ++Index) {
+        // Set the bit for this register for each MBB where it is
+        // live-through or live-in (killed).
+        unsigned VirtReg = Register::index2VirtReg(Index);
+        MachineInstr *DefMI = MRI->getVRegDef(VirtReg);
+        if (!DefMI)
+          continue;
+        LiveVariables::VarInfo &VI = LV->getVarInfo(VirtReg);
+        SparseBitVector<>::iterator AliveBlockItr = VI.AliveBlocks.begin();
+        SparseBitVector<>::iterator EndItr = VI.AliveBlocks.end();
+        while (AliveBlockItr != EndItr) {
+          unsigned BlockNum = *(AliveBlockItr++);
+          LiveInSets[BlockNum].set(Index);
+        }
+        // The register is live into an MBB in which it is killed but not
+        // defined. See comment for VarInfo in LiveVariables.h.
+        MachineBasicBlock *DefMBB = DefMI->getParent();
+        if (VI.Kills.size() > 1 ||
+            (!VI.Kills.empty() && VI.Kills.front()->getParent() != DefMBB))
+          for (auto *MI : VI.Kills)
+            LiveInSets[MI->getParent()->getNumber()].set(Index);
+      }
+    }
+
     MachineLoopInfo *MLI = getAnalysisIfAvailable<MachineLoopInfo>();
     for (auto &MBB : MF)
-      Changed |= SplitPHIEdges(MF, MBB, MLI);
+      Changed |= SplitPHIEdges(MF, MBB, MLI, (LV ? &LiveInSets : nullptr));
   }
 
+  // This pass takes the function out of SSA form.
+  MRI->leaveSSA();
+
   // Populate VRegPHIUseCount
   analyzePHINodes(MF);
 
@@ -561,7 +591,8 @@ void PHIElimination::analyzePHINodes(const MachineFunction& MF) {
 
 bool PHIElimination::SplitPHIEdges(MachineFunction &MF,
                                    MachineBasicBlock &MBB,
-                                   MachineLoopInfo *MLI) {
+                                   MachineLoopInfo *MLI,
+                                   std::vector<SparseBitVector<>> *LiveInSets) {
   if (MBB.empty() || !MBB.front().isPHI() || MBB.isEHPad())
     return false;   // Quick exit for basic blocks without PHIs.
 
@@ -628,7 +659,7 @@ bool PHIElimination::SplitPHIEdges(MachineFunction &MF,
       }
       if (!ShouldSplit && !SplitAllCriticalEdges)
         continue;
-      if (!PreMBB->SplitCriticalEdge(&MBB, *this)) {
+      if (!PreMBB->SplitCriticalEdge(&MBB, *this, LiveInSets)) {
         LLVM_DEBUG(dbgs() << "Failed to split critical edge.\n");
         continue;
       }