RegScavenging: Add scavengeRegisterBackwards()

This is a variant of scavengeRegister() that works for
enterBasicBlockEnd()/backward(). The benefit of the backward mode is
that it is not affected by incomplete kill flags.

This patch also changes
PrologEpilogInserter::doScavengeFrameVirtualRegs() to use the register
scavenger in backwards mode.

Differential Revision: http://reviews.llvm.org/D21885

llvm-svn: 276044

2 files changed, 240 insertions, 108 deletions

diff --git a/llvm/lib/CodeGen/PrologEpilogInserter.cpp b/llvm/lib/CodeGen/PrologEpilogInserter.cpp
index 20a9a39..0d90fd3 100644
--- a/llvm/lib/CodeGen/PrologEpilogInserter.cpp
+++ b/llvm/lib/CodeGen/PrologEpilogInserter.cpp
@@ -41,6 +41,7 @@
 #include "llvm/Target/TargetMachine.h"
 #include "llvm/Target/TargetRegisterInfo.h"
 #include "llvm/Target/TargetSubtargetInfo.h"
+#include <algorithm>
 #include <climits>
 
 using namespace llvm;
@@ -1146,6 +1147,55 @@ void PEI::replaceFrameIndices(MachineBasicBlock *BB, MachineFunction &Fn,
   }
 }
 
+/// Allocate a register for the virtual register \p VReg. The last use of
+/// \p VReg is around the current position of the register scavenger \p RS.
+/// \p ReserveAfter controls whether the scavenged register needs to be reserved
+/// after the current instruction, otherwise it will only be reserved before the
+/// current instruction.
+static unsigned scavengeVReg(MachineRegisterInfo &MRI, RegScavenger &RS,
+                             unsigned VReg, bool ReserveAfter) {
+#ifndef NDEBUG
+  // Verify that all definitions and uses are in the same basic block.
+  const MachineBasicBlock *CommonMBB = nullptr;
+  bool HadDef = false;
+  for (MachineOperand &MO : MRI.reg_nodbg_operands(VReg)) {
+    MachineBasicBlock *MBB = MO.getParent()->getParent();
+    if (CommonMBB == nullptr)
+      CommonMBB = MBB;
+    assert(MBB == CommonMBB && "All defs+uses must be in the same basic block");
+    if (MO.isDef())
+      HadDef = true;
+  }
+  assert(HadDef && "Must have at least 1 Def");
+#endif
+
+  // We should only have one definition of the register. However to accomodate
+  // the requirements of two address code we also allow definitions in
+  // subsequent instructions provided they also read the register. That way
+  // we get a single contiguous lifetime.
+  //
+  // Definitions in MRI.def_begin() are unordered, search for the first.
+  const TargetRegisterInfo *TRI = MRI.getTargetRegisterInfo();
+  MachineRegisterInfo::def_iterator FirstDef =
+    std::find_if(MRI.def_begin(VReg), MRI.def_end(),
+                 [VReg, TRI](const MachineOperand &MO) {
+      return !MO.getParent()->readsRegister(VReg, TRI);
+    });
+  assert(FirstDef != MRI.def_end() &&
+         "Must have one definition that does not redefine vreg");
+  MachineInstr &DefMI = *FirstDef->getParent();
+
+  // The register scavenger will report a free register inserting an emergency
+  // spill/reload if necessary.
+  int SPAdj = 0;
+  const TargetRegisterClass &RC = *MRI.getRegClass(VReg);
+  unsigned SReg = RS.scavengeRegisterBackwards(RC, DefMI.getIterator(),
+                                               ReserveAfter, SPAdj);
+  MRI.replaceRegWith(VReg, SReg);
+  ++NumScavengedRegs;
+  return SReg;
+}
+
 /// doScavengeFrameVirtualRegs - Replace all frame index virtual registers
 /// with physical registers. Use the register scavenger to find an
 /// appropriate register to use.
@@ -1158,77 +1208,46 @@ doScavengeFrameVirtualRegs(MachineFunction &MF, RegScavenger *RS) {
   // Run through the instructions and find any virtual registers.
   MachineRegisterInfo &MRI = MF.getRegInfo();
   for (MachineBasicBlock &MBB : MF) {
-    RS->enterBasicBlock(MBB);
-
-    int SPAdj = 0;
-
-    // The instruction stream may change in the loop, so check MBB.end()
-    // directly.
-    for (MachineBasicBlock::iterator I = MBB.begin(); I != MBB.end(); ) {
-      // We might end up here again with a NULL iterator if we scavenged a
-      // register for which we inserted spill code for definition by what was
-      // originally the first instruction in MBB.
-      if (I == MachineBasicBlock::iterator(nullptr))
-        I = MBB.begin();
+    RS->enterBasicBlockEnd(MBB);
+
+    bool LastIterationHadVRegUses = false;
+    for (MachineBasicBlock::iterator I = MBB.end(); I != MBB.begin(); ) {
+      --I;
+      // Move RegScavenger to the position between *I and *std::next(I).
+      RS->backward(I);
+
+      // Look for unassigned vregs in the uses of *std::next(I).
+      MachineBasicBlock::iterator N = std::next(I);
+      if (LastIterationHadVRegUses) {
+        const MachineInstr &NMI = *N;
+        for (const MachineOperand &MO : NMI.operands()) {
+          if (!MO.isReg() || !MO.readsReg())
+            continue;
+          unsigned Reg = MO.getReg();
+          if (TargetRegisterInfo::isVirtualRegister(Reg)) {
+            unsigned SReg = scavengeVReg(MRI, *RS, Reg, true);
+            RS->setRegUsed(SReg);
+          }
+        }
+      }
 
+      // Look for unassigned vregs in the defs of *I.
+      LastIterationHadVRegUses = false;
       const MachineInstr &MI = *I;
-      MachineBasicBlock::iterator J = std::next(I);
-      MachineBasicBlock::iterator P =
-                         I == MBB.begin() ? MachineBasicBlock::iterator(nullptr)
-                                          : std::prev(I);
-
-      // RS should process this instruction before we might scavenge at this
-      // location. This is because we might be replacing a virtual register
-      // defined by this instruction, and if so, registers killed by this
-      // instruction are available, and defined registers are not.
-      RS->forward(I);
-
       for (const MachineOperand &MO : MI.operands()) {
         if (!MO.isReg())
           continue;
         unsigned Reg = MO.getReg();
         if (!TargetRegisterInfo::isVirtualRegister(Reg))
           continue;
-
-        // When we first encounter a new virtual register, it
-        // must be a definition.
-        assert(MO.isDef() && "frame index virtual missing def!");
-        // Scavenge a new scratch register
-        const TargetRegisterClass *RC = MRI.getRegClass(Reg);
-        unsigned ScratchReg = RS->scavengeRegister(RC, J, SPAdj);
-
-        ++NumScavengedRegs;
-
-        // Replace this reference to the virtual register with the
-        // scratch register.
-        assert(ScratchReg && "Missing scratch register!");
-        MRI.replaceRegWith(Reg, ScratchReg);
-
-        // Because this instruction was processed by the RS before this
-        // register was allocated, make sure that the RS now records the
-        // register as being used.
-        RS->setRegUsed(ScratchReg);
+        // We have to look at all operands anyway so we can precalculate here
+        // whether there is a reading operand. This allows use to skip the use
+        // step in the next iteration if there was none.
+        if (MO.readsReg())
+          LastIterationHadVRegUses = true;
+        if (MO.isDef())
+          scavengeVReg(MRI, *RS, Reg, false);
       }
-
-      // If the scavenger needed to use one of its spill slots, the
-      // spill code will have been inserted in between I and J. This is a
-      // problem because we need the spill code before I: Move I to just
-      // prior to J.
-      if (I != std::prev(J)) {
-        MBB.splice(J, &MBB, I);
-
-        // Before we move I, we need to prepare the RS to visit I again.
-        // Specifically, RS will assert if it sees uses of registers that
-        // it believes are undefined. Because we have already processed
-        // register kills in I, when it visits I again, it will believe that
-        // those registers are undefined. To avoid this situation, unprocess
-        // the instruction I.
-        assert(RS->getCurrentPosition() == I &&
-          "The register scavenger has an unexpected position");
-        I = P;
-        RS->unprocess(P);
-      } else
-        ++I;
     }
   }
 }
diff --git a/llvm/lib/CodeGen/RegisterScavenging.cpp b/llvm/lib/CodeGen/RegisterScavenging.cpp
index 79a698d..c062bd0 100644
--- a/llvm/lib/CodeGen/RegisterScavenging.cpp
+++ b/llvm/lib/CodeGen/RegisterScavenging.cpp
@@ -299,6 +299,14 @@ void RegScavenger::backward() {
     }
   }
 
+  // Expire scavenge spill frameindex uses.
+  for (ScavengedInfo &I : Scavenged) {
+    if (I.Restore == &MI) {
+      I.Reg = 0;
+      I.Restore = nullptr;
+    }
+  }
+
   if (MBBI == MBB->begin()) {
     MBBI = MachineBasicBlock::iterator(nullptr);
     Tracking = false;
@@ -398,6 +406,72 @@ unsigned RegScavenger::findSurvivorReg(MachineBasicBlock::iterator StartMI,
   return Survivor;
 }
 
+static std::pair<unsigned, MachineBasicBlock::iterator>
+findSurvivorBackwards(const TargetRegisterInfo &TRI,
+    MachineBasicBlock::iterator From, MachineBasicBlock::iterator To,
+    BitVector &Available, BitVector &Candidates) {
+  bool FoundTo = false;
+  unsigned Survivor = 0;
+  MachineBasicBlock::iterator Pos;
+  MachineBasicBlock &MBB = *From->getParent();
+  MachineBasicBlock::iterator I = From;
+  unsigned InstrLimit = 25;
+  unsigned InstrCountDown = InstrLimit;
+  for (;;) {
+    const MachineInstr &MI = *I;
+    if (MI.isDebugValue())
+      continue;
+
+    // Remove any candidates touched by instruction.
+    bool FoundVReg = false;
+    for (const MachineOperand &MO : MI.operands()) {
+      if (MO.isRegMask()) {
+        Candidates.clearBitsNotInMask(MO.getRegMask());
+        continue;
+      }
+      if (!MO.isReg() || MO.isUndef())
+        continue;
+      unsigned Reg = MO.getReg();
+      if (TargetRegisterInfo::isVirtualRegister(Reg)) {
+        FoundVReg = true;
+      } else if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
+        for (MCRegAliasIterator AI(Reg, &TRI, true); AI.isValid(); ++AI)
+          Candidates.reset(*AI);
+      }
+    }
+
+    if (I == To) {
+      // If one of the available registers survived this long take it.
+      Available &= Candidates;
+      int Reg = Available.find_first();
+      if (Reg != -1)
+        return std::make_pair(Reg, MBB.end());
+      // Otherwise we will continue up to InstrLimit instructions to fine
+      // the register which is not defined/used for the longest time.
+      FoundTo = true;
+      Pos = To;
+    }
+    if (FoundTo) {
+      if (Survivor == 0 || !Candidates.test(Survivor)) {
+        int Reg = Candidates.find_first();
+        if (Reg == -1)
+          break;
+        Survivor = Reg;
+      }
+      if (--InstrCountDown == 0 || I == MBB.begin())
+        break;
+      if (FoundVReg) {
+        // We found a vreg, reset the InstrLimit counter.
+        InstrCountDown = InstrLimit;
+        Pos = I;
+      }
+    }
+    --I;
+  }
+
+  return std::make_pair(Survivor, Pos);
+}
+
 static unsigned getFrameIndexOperandNum(MachineInstr &MI) {
   unsigned i = 0;
   while (!MI.getOperand(i).isFI()) {
@@ -407,43 +481,16 @@ static unsigned getFrameIndexOperandNum(MachineInstr &MI) {
   return i;
 }
 
-unsigned RegScavenger::scavengeRegister(const TargetRegisterClass *RC,
-                                        MachineBasicBlock::iterator I,
-                                        int SPAdj) {
-  MachineInstr &MI = *I;
-  const MachineFunction &MF = *MI.getParent()->getParent();
-  // Consider all allocatable registers in the register class initially
-  BitVector Candidates = TRI->getAllocatableSet(MF, RC);
-
-  // Exclude all the registers being used by the instruction.
-  for (const MachineOperand &MO : MI.operands()) {
-    if (MO.isReg() && MO.getReg() != 0 && !(MO.isUse() && MO.isUndef()) &&
-        !TargetRegisterInfo::isVirtualRegister(MO.getReg()))
-      Candidates.reset(MO.getReg());
-  }
-
-  // Try to find a register that's unused if there is one, as then we won't
-  // have to spill.
-  BitVector Available = getRegsAvailable(RC);
-  Available &= Candidates;
-  if (Available.any())
-    Candidates = Available;
-
-  // Find the register whose use is furthest away.
-  MachineBasicBlock::iterator UseMI;
-  unsigned SReg = findSurvivorReg(I, Candidates, 25, UseMI);
-
-  // If we found an unused register there is no reason to spill it.
-  if (!isRegUsed(SReg)) {
-    DEBUG(dbgs() << "Scavenged register: " << TRI->getName(SReg) << "\n");
-    return SReg;
-  }
-
+RegScavenger::ScavengedInfo &
+RegScavenger::spill(unsigned Reg, const TargetRegisterClass &RC, int SPAdj,
+                    MachineBasicBlock::iterator Before,
+                    MachineBasicBlock::iterator &UseMI) {
   // Find an available scavenging slot with size and alignment matching
   // the requirements of the class RC.
+  const MachineFunction &MF = *Before->getParent()->getParent();
   const MachineFrameInfo &MFI = *MF.getFrameInfo();
-  unsigned NeedSize = RC->getSize();
-  unsigned NeedAlign = RC->getAlignment();
+  unsigned NeedSize = RC.getSize();
+  unsigned NeedAlign = RC.getAlignment();
 
   unsigned SI = Scavenged.size(), Diff = UINT_MAX;
   int FIB = MFI.getObjectIndexBegin(), FIE = MFI.getObjectIndexEnd();
@@ -478,42 +525,108 @@ unsigned RegScavenger::scavengeRegister(const TargetRegisterClass *RC,
   }
 
   // Avoid infinite regress
-  Scavenged[SI].Reg = SReg;
+  Scavenged[SI].Reg = Reg;
 
   // If the target knows how to save/restore the register, let it do so;
   // otherwise, use the emergency stack spill slot.
-  if (!TRI->saveScavengerRegister(*MBB, I, UseMI, RC, SReg)) {
-    // Spill the scavenged register before I.
+  if (!TRI->saveScavengerRegister(*MBB, Before, UseMI, &RC, Reg)) {
+    // Spill the scavenged register before \p Before.
     int FI = Scavenged[SI].FrameIndex;
     if (FI < FIB || FI >= FIE) {
       std::string Msg = std::string("Error while trying to spill ") +
-          TRI->getName(SReg) + " from class " + TRI->getRegClassName(RC) +
+          TRI->getName(Reg) + " from class " + TRI->getRegClassName(&RC) +
           ": Cannot scavenge register without an emergency spill slot!";
       report_fatal_error(Msg.c_str());
     }
-    TII->storeRegToStackSlot(*MBB, I, SReg, true, Scavenged[SI].FrameIndex,
-                             RC, TRI);
-    MachineBasicBlock::iterator II = std::prev(I);
+    TII->storeRegToStackSlot(*MBB, Before, Reg, true, Scavenged[SI].FrameIndex,
+                             &RC, TRI);
+    MachineBasicBlock::iterator II = std::prev(Before);
 
     unsigned FIOperandNum = getFrameIndexOperandNum(*II);
     TRI->eliminateFrameIndex(II, SPAdj, FIOperandNum, this);
 
     // Restore the scavenged register before its use (or first terminator).
-    TII->loadRegFromStackSlot(*MBB, UseMI, SReg, Scavenged[SI].FrameIndex,
-                              RC, TRI);
+    TII->loadRegFromStackSlot(*MBB, UseMI, Reg, Scavenged[SI].FrameIndex,
+                              &RC, TRI);
     II = std::prev(UseMI);
 
     FIOperandNum = getFrameIndexOperandNum(*II);
     TRI->eliminateFrameIndex(II, SPAdj, FIOperandNum, this);
   }
+  return Scavenged[SI];
+}
+
+unsigned RegScavenger::scavengeRegister(const TargetRegisterClass *RC,
+                                        MachineBasicBlock::iterator I,
+                                        int SPAdj) {
+  MachineInstr &MI = *I;
+  const MachineFunction &MF = *MI.getParent()->getParent();
+  // Consider all allocatable registers in the register class initially
+  BitVector Candidates = TRI->getAllocatableSet(MF, RC);
+
+  // Exclude all the registers being used by the instruction.
+  for (const MachineOperand &MO : MI.operands()) {
+    if (MO.isReg() && MO.getReg() != 0 && !(MO.isUse() && MO.isUndef()) &&
+        !TargetRegisterInfo::isVirtualRegister(MO.getReg()))
+      Candidates.reset(MO.getReg());
+  }
+
+  // Try to find a register that's unused if there is one, as then we won't
+  // have to spill.
+  BitVector Available = getRegsAvailable(RC);
+  Available &= Candidates;
+  if (Available.any())
+    Candidates = Available;
+
+  // Find the register whose use is furthest away.
+  MachineBasicBlock::iterator UseMI;
+  unsigned SReg = findSurvivorReg(I, Candidates, 25, UseMI);
 
-  Scavenged[SI].Restore = &*std::prev(UseMI);
+  // If we found an unused register there is no reason to spill it.
+  if (!isRegUsed(SReg)) {
+    DEBUG(dbgs() << "Scavenged register: " << TRI->getName(SReg) << "\n");
+    return SReg;
+  }
 
-  // Doing this here leads to infinite regress.
-  // Scavenged[SI].Reg = SReg;
+  ScavengedInfo &Scavenged = spill(SReg, *RC, SPAdj, I, UseMI);
+  Scavenged.Restore = std::prev(UseMI);
 
   DEBUG(dbgs() << "Scavenged register (with spill): " << TRI->getName(SReg) <<
         "\n");
 
   return SReg;
 }
+
+unsigned RegScavenger::scavengeRegisterBackwards(const TargetRegisterClass &RC,
+                                                 MachineBasicBlock::iterator To,
+                                                 bool RestoreAfter, int SPAdj) {
+  const MachineBasicBlock &MBB = *To->getParent();
+  const MachineFunction &MF = *MBB.getParent();
+  // Consider all allocatable registers in the register class initially
+  BitVector Candidates = TRI->getAllocatableSet(MF, &RC);
+
+  // Try to find a register that's unused if there is one, as then we won't
+  // have to spill.
+  BitVector Available = getRegsAvailable(&RC);
+
+  // Find the register whose use is furthest away.
+  MachineBasicBlock::iterator UseMI;
+  std::pair<unsigned, MachineBasicBlock::iterator> P =
+      findSurvivorBackwards(*TRI, MBBI, To, Available, Candidates);
+  unsigned Reg = P.first;
+  assert(Reg != 0 && "No register left to scavenge!");
+  // Found an available register?
+  if (!Available.test(Reg)) {
+    MachineBasicBlock::iterator ReloadBefore =
+      RestoreAfter ? std::next(MBBI) : MBBI;
+    DEBUG(dbgs() << "Reload before: " << *ReloadBefore << '\n');
+    ScavengedInfo &Scavenged = spill(Reg, RC, SPAdj, P.second, ReloadBefore);
+    Scavenged.Restore = std::prev(P.second);
+    addRegUnits(RegUnitsAvailable, Reg);
+    DEBUG(dbgs() << "Scavenged register with spill: " << PrintReg(Reg, TRI)
+          << " until " << *P.second);
+  } else {
+    DEBUG(dbgs() << "Scavenged free register: " << PrintReg(Reg, TRI) << '\n');
+  }
+  return Reg;
+}