MipsFrameLowering.cpp

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//===-- MipsFrameLowering.cpp - Mips Frame Information --------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file contains the Mips implementation of TargetFrameLowering class.
//
//===----------------------------------------------------------------------===//
 
#include "MipsFrameLowering.h"
#include "MipsInstrInfo.h"
#include "MipsTargetMachine.h"
#include "llvm/CodeGen/MachineFrameInfo.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineModuleInfo.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.
//
//===----------------------------------------------------------------------===//
 
const MipsFrameLowering *MipsFrameLowering::create(const MipsSubtarget &ST) {
  if (ST.inMips16Mode())
    return llvm::createMips16FrameLowering(ST);
 
  return llvm::createMipsSEFrameLowering(ST);
}
 
// hasFPImpl - Return true if the specified function should have a dedicated
// frame pointer register.  This is true if the function has variable sized
// allocas, if it needs dynamic stack realignment, if frame pointer elimination
// is disabled, or if the frame address is taken.
bool MipsFrameLowering::hasFPImpl(const MachineFunction &MF) const {
  const MachineFrameInfo &MFI = MF.getFrameInfo();
  const TargetRegisterInfo *TRI = STI.getRegisterInfo();
 
  return MF.disableFramePointerElim() || MFI.hasVarSizedObjects() ||
         MFI.isFrameAddressTaken() || TRI->hasStackRealignment(MF);
}
 
bool MipsFrameLowering::hasBP(const MachineFunction &MF) const {
  const MachineFrameInfo &MFI = MF.getFrameInfo();
  const TargetRegisterInfo *TRI = STI.getRegisterInfo();
 
  return MFI.hasVarSizedObjects() && TRI->hasStackRealignment(MF);
}
 
// Estimate the size of the stack, including the incoming arguments. We need to
// account for register spills, local objects, reserved call frame and incoming
// arguments. This is required to determine the largest possible positive offset
// from $sp so that it can be determined if an emergency spill slot for stack
// addresses is required.
uint64_t MipsFrameLowering::estimateStackSize(const MachineFunction &MF) const {
  const MachineFrameInfo &MFI = MF.getFrameInfo();
  const TargetRegisterInfo &TRI = *STI.getRegisterInfo();
 
  int64_t Size = 0;
 
  // Iterate over fixed sized objects which are incoming arguments.
  for (int I = MFI.getObjectIndexBegin(); I != 0; ++I)
    if (MFI.getObjectOffset(I) > 0)
      Size += MFI.getObjectSize(I);
 
  // Conservatively assume all callee-saved registers will be saved.
  for (const MCPhysReg *R = TRI.getCalleeSavedRegs(&MF); *R; ++R) {
    unsigned RegSize = TRI.getSpillSize(*TRI.getMinimalPhysRegClass(*R));
    Size = alignTo(Size + RegSize, RegSize);
  }
 
  // Get the size of the rest of the frame objects and any possible reserved
  // call frame, accounting for alignment.
  return Size + MFI.estimateStackSize(MF);
}
 
// Eliminate ADJCALLSTACKDOWN, ADJCALLSTACKUP pseudo instructions
MachineBasicBlock::iterator MipsFrameLowering::
eliminateCallFramePseudoInstr(MachineFunction &MF, MachineBasicBlock &MBB,
                              MachineBasicBlock::iterator I) const {
  unsigned SP = STI.getABI().IsN64() ? Mips::SP_64 : Mips::SP;
 
  if (!hasReservedCallFrame(MF)) {
    int64_t Amount = I->getOperand(0).getImm();
    if (I->getOpcode() == Mips::ADJCALLSTACKDOWN)
      Amount = -Amount;
 
    STI.getInstrInfo()->adjustStackPtr(SP, Amount, MBB, I);
  }
 
  return MBB.erase(I);
}