lib/Target/Mips/MipsFrameLowering.cpp

//=======- MipsFrameLowering.cpp - Mips Frame Information ------*- C++ -*-====//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file contains the Mips implementation of TargetFrameLowering class.
//
//===----------------------------------------------------------------------===//

#include "MipsFrameLowering.h"
#include "MipsInstrInfo.h"
#include "MipsMachineFunction.h"
#include "llvm/Function.h"
#include "llvm/CodeGen/MachineFrameInfo.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineModuleInfo.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Target/TargetOptions.h"
#include "llvm/Support/CommandLine.h"

using namespace llvm;


//===----------------------------------------------------------------------===//
//
// Stack Frame Processing methods
// +----------------------------+
//
// The stack is allocated decrementing the stack pointer on
// the first instruction of a function prologue. Once decremented,
// all stack references are done thought a positive offset
// from the stack/frame pointer, so the stack is considering
// to grow up! Otherwise terrible hacks would have to be made
// to get this stack ABI compliant :)
//
//  The stack frame required by the ABI (after call):
//  Offset
//
//  0                 ----------
//  4                 Args to pass
//  .                 saved $GP  (used in PIC)
//  .                 Alloca allocations
//  .                 Local Area
//  .                 CPU "Callee Saved" Registers
//  .                 saved FP
//  .                 saved RA
//  .                 FPU "Callee Saved" Registers
//  StackSize         -----------
//
// Offset - offset from sp after stack allocation on function prologue
//
// The sp is the stack pointer subtracted/added from the stack size
// at the Prologue/Epilogue
//
// References to the previous stack (to obtain arguments) are done
// with offsets that exceeds the stack size: (stacksize+(4*(num_arg-1))
//
// Examples:
// - reference to the actual stack frame
//   for any local area var there is smt like : FI >= 0, StackOffset: 4
//     sw REGX, 4(SP)
//
// - reference to previous stack frame
//   suppose there's a load to the 5th arguments : FI < 0, StackOffset: 16.
//   The emitted instruction will be something like:
//     lw REGX, 16+StackSize(SP)
//
// Since the total stack size is unknown on LowerFormalArguments, all
// stack references (ObjectOffset) created to reference the function
// arguments, are negative numbers. This way, on eliminateFrameIndex it's
// possible to detect those references and the offsets are adjusted to
// their real location.
//
//===----------------------------------------------------------------------===//

// hasFP - Return true if the specified function should have a dedicated frame
// pointer register.  This is true if the function has variable sized allocas or
// if frame pointer elimination is disabled.
bool MipsFrameLowering::hasFP(const MachineFunction &MF) const {
  const MachineFrameInfo *MFI = MF.getFrameInfo();
  return DisableFramePointerElim(MF) || MFI->hasVarSizedObjects();
}

bool MipsFrameLowering::targetHandlesStackFrameRounding() const {
  return true;
}

static unsigned AlignOffset(unsigned Offset, unsigned Align) {
  return (Offset + Align - 1) / Align * Align; 
} 

// expand pair of register and immediate if the immediate doesn't fit in the
// 16-bit offset field.
// e.g.
//  if OrigImm = 0x10000, OrigReg = $sp:
//    generate the following sequence of instrs:
//      lui  $at, hi(0x10000)
//      addu $at, $sp, $at
//
//    (NewReg, NewImm) = ($at, lo(Ox10000))
//    return true
static bool expandRegLargeImmPair(unsigned OrigReg, int OrigImm,
                                  unsigned& NewReg, int& NewImm,
                                  MachineBasicBlock& MBB,
                                  MachineBasicBlock::iterator I) {
  // OrigImm fits in the 16-bit field
  if (OrigImm < 0x8000 && OrigImm >= -0x8000) {
    NewReg = OrigReg;
    NewImm = OrigImm;
    return false;
  }

  MachineFunction* MF = MBB.getParent();
  const TargetInstrInfo *TII = MF->getTarget().getInstrInfo();
  DebugLoc DL = I->getDebugLoc();
  int ImmLo = (short)(OrigImm & 0xffff);
  int ImmHi = (((unsigned)OrigImm & 0xffff0000) >> 16) +
              ((OrigImm & 0x8000) != 0);

  // FIXME: change this when mips goes MC".
  BuildMI(MBB, I, DL, TII->get(Mips::NOAT));
  BuildMI(MBB, I, DL, TII->get(Mips::LUi), Mips::AT).addImm(ImmHi);
  BuildMI(MBB, I, DL, TII->get(Mips::ADDu), Mips::AT).addReg(OrigReg)
                                                     .addReg(Mips::AT);
  NewReg = Mips::AT;
  NewImm = ImmLo;

  return true;
}

void MipsFrameLowering::emitPrologue(MachineFunction &MF) const {
  MachineBasicBlock &MBB   = MF.front();
  MachineFrameInfo *MFI    = MF.getFrameInfo();
  MipsFunctionInfo *MipsFI = MF.getInfo<MipsFunctionInfo>();
  const MipsRegisterInfo *RegInfo =
    static_cast<const MipsRegisterInfo*>(MF.getTarget().getRegisterInfo());
  const MipsInstrInfo &TII =
    *static_cast<const MipsInstrInfo*>(MF.getTarget().getInstrInfo());
  MachineBasicBlock::iterator MBBI = MBB.begin();
  DebugLoc dl = MBBI != MBB.end() ? MBBI->getDebugLoc() : DebugLoc();
  bool isPIC = (MF.getTarget().getRelocationModel() == Reloc::PIC_);
  unsigned NewReg = 0;
  int NewImm = 0;
  bool ATUsed;

  // First, compute final stack size.
  unsigned RegSize = STI.isGP32bit() ? 4 : 8;
  unsigned StackAlign = getStackAlignment();
  unsigned LocalVarAreaOffset = MipsFI->needGPSaveRestore() ? 
    (MFI->getObjectOffset(MipsFI->getGPFI()) + RegSize) :
    MFI->getMaxCallFrameSize();
  unsigned StackSize = AlignOffset(LocalVarAreaOffset, StackAlign) +
    AlignOffset(MFI->getStackSize(), StackAlign);

   // Update stack size
  MFI->setStackSize(StackSize); 
  
  BuildMI(MBB, MBBI, dl, TII.get(Mips::NOREORDER));

  // TODO: check need from GP here.
  if (isPIC && STI.isABI_O32())
    BuildMI(MBB, MBBI, dl, TII.get(Mips::CPLOAD))
      .addReg(RegInfo->getPICCallReg());
  BuildMI(MBB, MBBI, dl, TII.get(Mips::NOMACRO));

  // No need to allocate space on the stack.
  if (StackSize == 0 && !MFI->adjustsStack()) return;

  // Adjust stack : addi sp, sp, (-imm)
  ATUsed = expandRegLargeImmPair(Mips::SP, -StackSize, NewReg, NewImm, MBB,
                                 MBBI);
  BuildMI(MBB, MBBI, dl, TII.get(Mips::ADDiu), Mips::SP)
    .addReg(NewReg).addImm(NewImm);

  // FIXME: change this when mips goes MC".
  if (ATUsed)
    BuildMI(MBB, MBBI, dl, TII.get(Mips::ATMACRO));

  // if framepointer enabled, set it to point to the stack pointer.
  if (hasFP(MF)) {
    // Find the instruction past the last instruction that saves a callee-saved
    // register to the stack.
    const std::vector<CalleeSavedInfo> &CSI = MFI->getCalleeSavedInfo();
  
    for (unsigned i = 0; i < CSI.size(); ++i)
      ++MBBI;
 
    // Insert instruction "move $fp, $sp" at this location.    
    BuildMI(MBB, MBBI, dl, TII.get(Mips::ADDu), Mips::FP)
      .addReg(Mips::SP).addReg(Mips::ZERO);
  }

  // Restore GP from the saved stack location
  if (MipsFI->needGPSaveRestore())
    BuildMI(MBB, MBBI, dl, TII.get(Mips::CPRESTORE))
      .addImm(MFI->getObjectOffset(MipsFI->getGPFI()));
}

void MipsFrameLowering::emitEpilogue(MachineFunction &MF,
                                 MachineBasicBlock &MBB) const {
  MachineBasicBlock::iterator MBBI = MBB.getLastNonDebugInstr();
  MachineFrameInfo *MFI            = MF.getFrameInfo();
  const MipsInstrInfo &TII =
    *static_cast<const MipsInstrInfo*>(MF.getTarget().getInstrInfo());
  DebugLoc dl = MBBI->getDebugLoc();

  // Get the number of bytes from FrameInfo
  unsigned StackSize = MFI->getStackSize();

  unsigned NewReg = 0;
  int NewImm = 0;
  bool ATUsed = false;

  // if framepointer enabled, restore the stack pointer.
  if (hasFP(MF)) {
    // Find the first instruction that restores a callee-saved register.
    MachineBasicBlock::iterator I = MBBI;
    
    for (unsigned i = 0; i < MFI->getCalleeSavedInfo().size(); ++i)
      --I;

    // Insert instruction "move $sp, $fp" at this location.
    BuildMI(MBB, I, dl, TII.get(Mips::ADDu), Mips::SP)
      .addReg(Mips::FP).addReg(Mips::ZERO);
  }

  // adjust stack  : insert addi sp, sp, (imm)
  if (StackSize) {
    ATUsed = expandRegLargeImmPair(Mips::SP, StackSize, NewReg, NewImm, MBB,
                                   MBBI);
    BuildMI(MBB, MBBI, dl, TII.get(Mips::ADDiu), Mips::SP)
      .addReg(NewReg).addImm(NewImm);

    // FIXME: change this when mips goes MC".
    if (ATUsed)
      BuildMI(MBB, MBBI, dl, TII.get(Mips::ATMACRO));
  }
}

void MipsFrameLowering::
processFunctionBeforeCalleeSavedScan(MachineFunction &MF,
                                     RegScavenger *RS) const {
  MachineRegisterInfo& MRI = MF.getRegInfo();
  MipsFunctionInfo *MipsFI = MF.getInfo<MipsFunctionInfo>();

  // FIXME: remove this code if register allocator can correctly mark
  //        $fp and $ra used or unused.

  // Mark $fp and $ra as used or unused.
  if (hasFP(MF))
    MRI.setPhysRegUsed(Mips::FP);

  // The register allocator might determine $ra is used after seeing 
  // instruction "jr $ra", but we do not want PrologEpilogInserter to insert
  // instructions to save/restore $ra unless there is a function call.
  // To correct this, $ra is explicitly marked unused if there is no
  // function call.
  if (MipsFI->hasCall())
    MRI.setPhysRegUsed(Mips::RA);
  else
    MRI.setPhysRegUnused(Mips::RA);
}