WebAssemblyFrameLowering.cpp

//===-- WebAssemblyFrameLowering.cpp - WebAssembly Frame Lowering ----------==//
//
// 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
//
//===----------------------------------------------------------------------===//
///
/// \file
/// 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 "WebAssemblyUtilities.h"
#include "llvm/CodeGen/MachineFrameInfo.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineModuleInfoImpls.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/MC/MCAsmInfo.h"
#include "llvm/Support/Debug.h"
using namespace llvm;

#define DEBUG_TYPE "wasm-frame-info"

// TODO: wasm64
// TODO: Emit TargetOpcode::CFI_INSTRUCTION instructions

/// We need a base pointer in the case of having items on the stack that
/// require stricter alignment than the stack pointer itself.  Because we need
/// to shift the stack pointer by some unknown amount to force the alignment,
/// we need to record the value of the stack pointer on entry to the function.
bool WebAssemblyFrameLowering::hasBP(const MachineFunction &MF) const {
  const auto *RegInfo =
      MF.getSubtarget<WebAssemblySubtarget>().getRegisterInfo();
  return RegInfo->needsStackRealignment(MF);
}

/// 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();

  // When we have var-sized objects, we move the stack pointer by an unknown
  // amount, and need to emit a frame pointer to restore the stack to where we
  // were on function entry.
  // If we already need a base pointer, we use that to fix up the stack pointer.
  // If there are no fixed-size objects, we would have no use of a frame
  // pointer, and thus should not emit one.
  bool HasFixedSizedObjects = MFI.getStackSize() > 0;
  bool NeedsFixedReference = !hasBP(MF) || HasFixedSizedObjects;

  return MFI.isFrameAddressTaken() ||
         (MFI.hasVarSizedObjects() && NeedsFixedReference) ||
         MFI.hasStackMap() || MFI.hasPatchPoint();
}

/// 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();
}

// Returns true if this function needs a local user-space stack pointer for its
// local frame (not for exception handling).
bool WebAssemblyFrameLowering::needsSPForLocalFrame(
    const MachineFunction &MF) const {
  auto &MFI = MF.getFrameInfo();
  return MFI.getStackSize() || MFI.adjustsStack() || hasFP(MF);
}

// In function with EH pads, we need to make a copy of the value of
// __stack_pointer global in SP32 register, in order to use it when restoring
// __stack_pointer after an exception is caught.
bool WebAssemblyFrameLowering::needsPrologForEH(
    const MachineFunction &MF) const {
  auto EHType = MF.getTarget().getMCAsmInfo()->getExceptionHandlingType();
  return EHType == ExceptionHandling::Wasm &&
         MF.getFunction().hasPersonalityFn() && MF.getFrameInfo().hasCalls();
}

/// Returns true if this function needs a local user-space stack pointer.
/// Unlike a machine stack pointer, the wasm user stack pointer is a global
/// variable, so it is loaded into a register in the prolog.
bool WebAssemblyFrameLowering::needsSP(const MachineFunction &MF) const {
  return needsSPForLocalFrame(MF) || needsPrologForEH(MF);
}

/// Returns true if the local user-space stack pointer needs to be written back
/// to __stack_pointer global by this function (this is not meaningful if
/// needsSP is false). If false, the stack red zone can be used and only a local
/// SP is needed.
bool WebAssemblyFrameLowering::needsSPWriteback(
    const MachineFunction &MF) const {
  auto &MFI = MF.getFrameInfo();
  assert(needsSP(MF));
  // When we don't need a local stack pointer for its local frame but only to
  // support EH, we don't need to write SP back in the epilog, because we don't
  // bump down the stack pointer in the prolog. We need to write SP back in the
  // epilog only if
  // 1. We need SP not only for EH support but also because we actually use
  // stack or we have a frame address taken.
  // 2. We cannot use the red zone.
  bool CanUseRedZone = MFI.getStackSize() <= RedZoneSize && !MFI.hasCalls() &&
                       !MF.getFunction().hasFnAttribute(Attribute::NoRedZone);
  return needsSPForLocalFrame(MF) && !CanUseRedZone;
}

void WebAssemblyFrameLowering::writeSPToGlobal(
    unsigned SrcReg, MachineFunction &MF, MachineBasicBlock &MBB,
    MachineBasicBlock::iterator &InsertStore, const DebugLoc &DL) const {
  const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();

  const char *ES = "__stack_pointer";
  auto *SPSymbol = MF.createExternalSymbolName(ES);
  BuildMI(MBB, InsertStore, DL, TII->get(WebAssembly::GLOBAL_SET_I32))
      .addExternalSymbol(SPSymbol)
      .addReg(SrcReg);
}

MachineBasicBlock::iterator
WebAssemblyFrameLowering::eliminateCallFramePseudoInstr(
    MachineFunction &MF, MachineBasicBlock &MBB,
    MachineBasicBlock::iterator I) const {
  assert(!I->getOperand(0).getImm() && (hasFP(MF) || hasBP(MF)) &&
         "Call frame pseudos should only be used for dynamic stack adjustment");
  const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
  if (I->getOpcode() == TII->getCallFrameDestroyOpcode() &&
      needsSPWriteback(MF)) {
    DebugLoc DL = I->getDebugLoc();
    writeSPToGlobal(WebAssembly::SP32, MF, MBB, I, DL);
  }
  return 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");

  if (!needsSP(MF))
    return;
  uint64_t StackSize = MFI.getStackSize();

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

  auto InsertPt = MBB.begin();
  while (InsertPt != MBB.end() &&
         WebAssembly::isArgument(InsertPt->getOpcode()))
    ++InsertPt;
  DebugLoc DL;

  const TargetRegisterClass *PtrRC =
      MRI.getTargetRegisterInfo()->getPointerRegClass(MF);
  unsigned SPReg = WebAssembly::SP32;
  if (StackSize)
    SPReg = MRI.createVirtualRegister(PtrRC);

  const char *ES = "__stack_pointer";
  auto *SPSymbol = MF.createExternalSymbolName(ES);
  BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::GLOBAL_GET_I32), SPReg)
      .addExternalSymbol(SPSymbol);

  bool HasBP = hasBP(MF);
  if (HasBP) {
    auto FI = MF.getInfo<WebAssemblyFunctionInfo>();
    Register BasePtr = MRI.createVirtualRegister(PtrRC);
    FI->setBasePointerVreg(BasePtr);
    BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::COPY), BasePtr)
        .addReg(SPReg);
  }
  if (StackSize) {
    // Subtract the frame size
    Register OffsetReg = MRI.createVirtualRegister(PtrRC);
    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 (HasBP) {
    Register BitmaskReg = MRI.createVirtualRegister(PtrRC);
    unsigned Alignment = MFI.getMaxAlignment();
    assert((1u << countTrailingZeros(Alignment)) == Alignment &&
           "Alignment must be a power of 2");
    BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), BitmaskReg)
        .addImm((int)~(Alignment - 1));
    BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::AND_I32),
            WebAssembly::SP32)
        .addReg(WebAssembly::SP32)
        .addReg(BitmaskReg);
  }
  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), WebAssembly::FP32)
        .addReg(WebAssembly::SP32);
  }
  if (StackSize && needsSPWriteback(MF)) {
    writeSPToGlobal(WebAssembly::SP32, MF, MBB, InsertPt, DL);
  }
}

void WebAssemblyFrameLowering::emitEpilogue(MachineFunction &MF,
                                            MachineBasicBlock &MBB) const {
  uint64_t StackSize = MF.getFrameInfo().getStackSize();
  if (!needsSP(MF) || !needsSPWriteback(MF))
    return;
  const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
  auto &MRI = MF.getRegInfo();
  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.
  unsigned SPReg = 0;
  if (hasBP(MF)) {
    auto FI = MF.getInfo<WebAssemblyFunctionInfo>();
    SPReg = FI->getBasePointerVreg();
  } else if (StackSize) {
    const TargetRegisterClass *PtrRC =
        MRI.getTargetRegisterInfo()->getPointerRegClass(MF);
    Register OffsetReg = MRI.createVirtualRegister(PtrRC);
    BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), OffsetReg)
        .addImm(StackSize);
    // In the epilog we don't need to write the result back to the SP32 physreg
    // because it won't be used again. We can use a stackified register instead.
    SPReg = MRI.createVirtualRegister(PtrRC);
    BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::ADD_I32), SPReg)
        .addReg(hasFP(MF) ? WebAssembly::FP32 : WebAssembly::SP32)
        .addReg(OffsetReg);
  } else {
    SPReg = hasFP(MF) ? WebAssembly::FP32 : WebAssembly::SP32;
  }

  writeSPToGlobal(SPReg, MF, MBB, InsertPt, DL);
}