Use virtual base registers on PPC

On PowerPC, non-vector loads and stores have r+i forms; however, in functions
with large stack frames these were not being used to access slots far from the
stack pointer because such slots were out of range for the signed 16-bit
immediate offset field. This increases register pressure because we need a
separate register for each offset (when the r+r form is used). By enabling
virtual base registers, we can deal with large stack frames without unduly
increasing register pressure.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179105 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 151 insertions, 15 deletions

diff --git a/lib/Target/PowerPC/PPCRegisterInfo.cpp b/lib/Target/PowerPC/PPCRegisterInfo.cpp
index 3f5f6c26ff..2be6324fd7 100644
--- a/lib/Target/PowerPC/PPCRegisterInfo.cpp
+++ b/lib/Target/PowerPC/PPCRegisterInfo.cpp
@@ -454,6 +454,33 @@ PPCRegisterInfo::hasReservedSpillSlot(const MachineFunction &MF,
   return false;
 }
 
+// Figure out if the offset in the instruction is shifted right two bits. This
+// is true for instructions like "STD", which the machine implicitly adds two
+// low zeros to.
+static bool usesIXAddr(const MachineInstr &MI) {
+  unsigned OpC = MI.getOpcode();
+
+  switch (OpC) {
+  default:
+    return false;
+  case PPC::LWA:
+  case PPC::LD:
+  case PPC::STD:
+    return true;
+  }
+}
+
+// Return the OffsetOperandNo given the FIOperandNum (and the instruction).
+static unsigned getOffsetONFromFION(const MachineInstr &MI,
+                                    unsigned FIOperandNum) {
+  // Take into account whether it's an add or mem instruction
+  unsigned OffsetOperandNo = (FIOperandNum == 2) ? 1 : 2;
+  if (MI.isInlineAsm())
+    OffsetOperandNo = FIOperandNum-1;
+
+  return OffsetOperandNo;
+}
+
 void
 PPCRegisterInfo::eliminateFrameIndex(MachineBasicBlock::iterator II,
                                      int SPAdj, unsigned FIOperandNum,
@@ -471,10 +498,7 @@ PPCRegisterInfo::eliminateFrameIndex(MachineBasicBlock::iterator II,
   const TargetFrameLowering *TFI = MF.getTarget().getFrameLowering();
   DebugLoc dl = MI.getDebugLoc();
 
-  // Take into account whether it's an add or mem instruction
-  unsigned OffsetOperandNo = (FIOperandNum == 2) ? 1 : 2;
-  if (MI.isInlineAsm())
-    OffsetOperandNo = FIOperandNum-1;
+  unsigned OffsetOperandNo = getOffsetONFromFION(MI, FIOperandNum);
 
   // Get the frame index.
   int FrameIndex = MI.getOperand(FIOperandNum).getIndex();
@@ -516,17 +540,8 @@ PPCRegisterInfo::eliminateFrameIndex(MachineBasicBlock::iterator II,
                                                 (is64Bit ? PPC::X1 : PPC::R1),
                                               false);
 
-  // Figure out if the offset in the instruction is shifted right two bits. This
-  // is true for instructions like "STD", which the machine implicitly adds two
-  // low zeros to.
-  bool isIXAddr = false;
-  switch (OpC) {
-  case PPC::LWA:
-  case PPC::LD:
-  case PPC::STD:
-    isIXAddr = true;
-    break;
-  }
+  // Figure out if the offset in the instruction is shifted right two bits.
+  bool isIXAddr = usesIXAddr(MI);
 
   // If the instruction is not present in ImmToIdxMap, then it has no immediate
   // form (and must be r+r).
@@ -618,3 +633,124 @@ unsigned PPCRegisterInfo::getEHExceptionRegister() const {
 unsigned PPCRegisterInfo::getEHHandlerRegister() const {
   return !Subtarget.isPPC64() ? PPC::R4 : PPC::X4;
 }
+
+/// Returns true if the instruction's frame index
+/// reference would be better served by a base register other than FP
+/// or SP. Used by LocalStackFrameAllocation to determine which frame index
+/// references it should create new base registers for.
+bool PPCRegisterInfo::
+needsFrameBaseReg(MachineInstr *MI, int64_t Offset) const {
+  assert(Offset < 0 && "Local offset must be negative");
+
+  unsigned FIOperandNum = 0;
+  while (!MI->getOperand(FIOperandNum).isFI()) {
+    ++FIOperandNum;
+    assert(FIOperandNum < MI->getNumOperands() &&
+           "Instr doesn't have FrameIndex operand!");
+  }
+
+  unsigned OffsetOperandNo = getOffsetONFromFION(*MI, FIOperandNum);
+
+  if (!usesIXAddr(*MI))
+    Offset += MI->getOperand(OffsetOperandNo).getImm();
+  else
+    Offset += MI->getOperand(OffsetOperandNo).getImm() << 2;
+
+  // It's the load/store FI references that cause issues, as it can be difficult
+  // to materialize the offset if it won't fit in the literal field. Estimate
+  // based on the size of the local frame and some conservative assumptions
+  // about the rest of the stack frame (note, this is pre-regalloc, so
+  // we don't know everything for certain yet) whether this offset is likely
+  // to be out of range of the immediate. Return true if so.
+
+  // We only generate virtual base registers for loads and stores that have
+  // an r+i form. Return false for everything else.
+  unsigned OpC = MI->getOpcode();
+  if (!ImmToIdxMap.count(OpC))
+    return false;
+
+  // Don't generate a new virtual base register just to add zero to it.
+  if ((OpC == PPC::ADDI || OpC == PPC::ADDI8) &&
+      MI->getOperand(2).getImm() == 0)
+    return false;
+
+  MachineBasicBlock &MBB = *MI->getParent();
+  MachineFunction &MF = *MBB.getParent();
+
+  const PPCFrameLowering *PPCFI =
+    static_cast<const PPCFrameLowering*>(MF.getTarget().getFrameLowering());
+  unsigned StackEst =
+    PPCFI->determineFrameLayout(MF, false, true);
+
+  // If we likely don't need a stack frame, then we probably don't need a
+  // virtual base register either.
+  if (!StackEst)
+    return false;
+
+  // Estimate an offset from the stack pointer.
+  // The incoming offset is relating to the SP at the start of the function,
+  // but when we access the local it'll be relative to the SP after local
+  // allocation, so adjust our SP-relative offset by that allocation size.
+  Offset += StackEst;
+
+  // The frame pointer will point to the end of the stack, so estimate the
+  // offset as the difference between the object offset and the FP location.
+  return !isFrameOffsetLegal(MI, Offset);
+}
+
+/// Insert defining instruction(s) for BaseReg to
+/// be a pointer to FrameIdx at the beginning of the basic block.
+void PPCRegisterInfo::
+materializeFrameBaseRegister(MachineBasicBlock *MBB,
+                             unsigned BaseReg, int FrameIdx,
+                             int64_t Offset) const {
+  unsigned ADDriOpc = Subtarget.isPPC64() ? PPC::ADDI8 : PPC::ADDI;
+
+  MachineBasicBlock::iterator Ins = MBB->begin();
+  DebugLoc DL;                  // Defaults to "unknown"
+  if (Ins != MBB->end())
+    DL = Ins->getDebugLoc();
+
+  const MCInstrDesc &MCID = TII.get(ADDriOpc);
+  MachineRegisterInfo &MRI = MBB->getParent()->getRegInfo();
+  const MachineFunction &MF = *MBB->getParent();
+  MRI.constrainRegClass(BaseReg, TII.getRegClass(MCID, 0, this, MF));
+
+  BuildMI(*MBB, Ins, DL, MCID, BaseReg)
+    .addFrameIndex(FrameIdx).addImm(Offset);
+}
+
+void
+PPCRegisterInfo::resolveFrameIndex(MachineBasicBlock::iterator I,
+                                   unsigned BaseReg, int64_t Offset) const {
+  MachineInstr &MI = *I;
+
+  unsigned FIOperandNum = 0;
+  while (!MI.getOperand(FIOperandNum).isFI()) {
+    ++FIOperandNum;
+    assert(FIOperandNum < MI.getNumOperands() &&
+           "Instr doesn't have FrameIndex operand!");
+  }
+
+  MI.getOperand(FIOperandNum).ChangeToRegister(BaseReg, false);
+  unsigned OffsetOperandNo = getOffsetONFromFION(MI, FIOperandNum);
+
+  bool isIXAddr = usesIXAddr(MI);
+  if (!isIXAddr)
+    Offset += MI.getOperand(OffsetOperandNo).getImm();
+  else
+    Offset += MI.getOperand(OffsetOperandNo).getImm() << 2;
+
+  // Figure out if the offset in the instruction is shifted right two bits.
+  if (isIXAddr)
+    Offset >>= 2;    // The actual encoded value has the low two bits zero.
+
+  MI.getOperand(OffsetOperandNo).ChangeToImmediate(Offset);
+}
+
+bool PPCRegisterInfo::isFrameOffsetLegal(const MachineInstr *MI,
+                                         int64_t Offset) const {
+  return MI->getOpcode() == PPC::DBG_VALUE || // DBG_VALUE is always Reg+Imm
+         (isInt<16>(Offset) && (!usesIXAddr(*MI) || (Offset & 3) == 0));
+}
+