llvm/lib/Target/WebAssembly/WebAssemblyFrameLowering.cpp

//===-- WebAssemblyFrameLowering.cpp - WebAssembly Frame Lowering ----------==//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
///
/// \file
/// \brief This file contains the WebAssembly implementation of
/// TargetFrameLowering class.
///
/// On WebAssembly, there aren't a lot of things to do here. There are no
/// callee-saved registers to save, and no spill slots.
///
/// The stack grows downward.
///
//===----------------------------------------------------------------------===//

#include "WebAssemblyFrameLowering.h"
#include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
#include "WebAssemblyInstrInfo.h"
#include "WebAssemblyMachineFunctionInfo.h"
#include "WebAssemblySubtarget.h"
#include "WebAssemblyTargetMachine.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/Support/Debug.h"
using namespace llvm;

#define DEBUG_TYPE "wasm-frame-info"

// TODO: Implement a red zone?
// TODO: wasm64
// TODO: Prolog/epilog should be stackified too. This pass runs after register
//       stackification, so we'll have to do it manually.
// TODO: Emit TargetOpcode::CFI_INSTRUCTION instructions

/// Return true if the specified function should have a dedicated frame pointer
/// register.
bool WebAssemblyFrameLowering::hasFP(const MachineFunction &MF) const {
  const MachineFrameInfo *MFI = MF.getFrameInfo();
  const auto *RegInfo =
      MF.getSubtarget<WebAssemblySubtarget>().getRegisterInfo();
  return MFI->hasVarSizedObjects() || MFI->hasStackMap() ||
         MFI->hasPatchPoint() || RegInfo->needsStackRealignment(MF);
}

/// Under normal circumstances, when a frame pointer is not required, we reserve
/// argument space for call sites in the function immediately on entry to the
/// current function. This eliminates the need for add/sub sp brackets around
/// call sites. Returns true if the call frame is included as part of the stack
/// frame.
bool WebAssemblyFrameLowering::hasReservedCallFrame(
    const MachineFunction &MF) const {
  return !MF.getFrameInfo()->hasVarSizedObjects();
}

void WebAssemblyFrameLowering::eliminateCallFramePseudoInstr(
    MachineFunction &MF, MachineBasicBlock &MBB,
    MachineBasicBlock::iterator I) const {
  // TODO: can we avoid using call frame pseudos altogether?
  assert(!I->getOperand(0).getImm() &&
         "Stack should not be adjusted around calls");
  MBB.erase(I);
}

void WebAssemblyFrameLowering::emitPrologue(MachineFunction &MF,
                                            MachineBasicBlock &MBB) const {
  // TODO: Do ".setMIFlag(MachineInstr::FrameSetup)" on emitted instructions
  auto *MFI = MF.getFrameInfo();
  assert(MFI->getCalleeSavedInfo().empty() &&
         "WebAssembly should not have callee-saved registers");
  assert(!MFI->isFrameAddressTaken());

  uint64_t StackSize = MFI->getStackSize();
  if (!StackSize && !MFI->adjustsStack() && !hasFP(MF)) return;

  const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
  auto &MRI = MF.getRegInfo();

  auto InsertPt = MBB.begin();
  DebugLoc DL;

  unsigned SPReg = MRI.createVirtualRegister(&WebAssembly::I32RegClass);
  auto *SPSymbol = MF.createExternalSymbolName("__stack_pointer");
  BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), SPReg)
      .addExternalSymbol(SPSymbol);
  // This MachinePointerInfo should reference __stack_pointer as well but
  // doesn't because MachinePointerInfo() takes a GV which we don't have for
  // __stack_pointer. TODO: check if PseudoSourceValue::ExternalSymbolCallEntry
  // is appropriate instead. (likewise for EmitEpologue below)
  auto *LoadMMO = new MachineMemOperand(MachinePointerInfo(),
                                        MachineMemOperand::MOLoad, 4, 4);
  // Load the SP value.
  BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::LOAD_I32),
          StackSize ? SPReg : (unsigned)WebAssembly::SP32)
      .addImm(0)     // offset
      .addReg(SPReg) // addr
      .addImm(2)     // p2align
      .addMemOperand(LoadMMO);

  unsigned OffsetReg = 0;
  if (StackSize) {
    // Subtract the frame size
    OffsetReg = MRI.createVirtualRegister(&WebAssembly::I32RegClass);
    BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), OffsetReg)
        .addImm(StackSize);
    BuildMI(MBB, InsertPt, DL,
            TII->get(WebAssembly::SUB_I32),
            WebAssembly::SP32)
        .addReg(SPReg)
        .addReg(OffsetReg);
  }
  if (hasFP(MF)) {
    // Unlike most conventional targets (where FP points to the saved FP),
    // FP points to the bottom of the fixed-size locals, so we can use positive
    // offsets in load/store instructions.
    BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::COPY_LOCAL_I32),
            WebAssembly::FP32)
        .addReg(WebAssembly::SP32);
  }
  if (StackSize) {
    assert(OffsetReg);
  // The SP32 register now has the new stacktop. Also write it back to memory.
  BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), OffsetReg)
      .addExternalSymbol(SPSymbol);
  auto *MMO = new MachineMemOperand(MachinePointerInfo(),
                                    MachineMemOperand::MOStore, 4, 4);
  BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::STORE_I32), WebAssembly::SP32)
      .addImm(0)
      .addReg(OffsetReg)
      .addImm(2) // p2align
      .addReg(WebAssembly::SP32)
      .addMemOperand(MMO);
  }
}

void WebAssemblyFrameLowering::emitEpilogue(MachineFunction &MF,
                                            MachineBasicBlock &MBB) const {
  auto *MFI = MF.getFrameInfo();
  uint64_t StackSize = MFI->getStackSize();
  if (!StackSize && !MFI->adjustsStack() && !hasFP(MF)) return;
  const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
  auto &MRI = MF.getRegInfo();
  unsigned OffsetReg = MRI.createVirtualRegister(&WebAssembly::I32RegClass);
  auto InsertPt = MBB.getFirstTerminator();
  DebugLoc DL;

  if (InsertPt != MBB.end()) {
    DL = InsertPt->getDebugLoc();
  }

  // Restore the stack pointer. If we had fixed-size locals, add the offset
  // subtracted in the prolog.
  if (StackSize) {
    BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), OffsetReg)
        .addImm(StackSize);
    BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::ADD_I32), WebAssembly::SP32)
        .addReg(hasFP(MF) ? WebAssembly::FP32 : WebAssembly::SP32)
        .addReg(OffsetReg);
  }

  auto *SPSymbol = MF.createExternalSymbolName("__stack_pointer");
  // Re-use OffsetReg to hold the address of the stacktop
  BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), OffsetReg)
      .addExternalSymbol(SPSymbol);
  auto *MMO = new MachineMemOperand(MachinePointerInfo(),
                                    MachineMemOperand::MOStore, 4, 4);
  BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::STORE_I32), WebAssembly::SP32)
      .addImm(0)
      .addReg(OffsetReg)
      .addImm(2) // p2align
      .addReg((!StackSize && hasFP(MF)) ? WebAssembly::FP32 : WebAssembly::SP32)
      .addMemOperand(MMO);
}