WebAssemblyFrameLowering.cpp

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//===-- 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 "Utils/WebAssemblyTypeUtilities.h"
#include "WebAssembly.h"
#include "WebAssemblyInstrInfo.h"
#include "WebAssemblyMachineFunctionInfo.h"
#include "WebAssemblySubtarget.h"
#include "llvm/CodeGen/Analysis.h"
#include "llvm/CodeGen/MachineFrameInfo.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Module.h"
using namespace llvm;
 
#define DEBUG_TYPE "wasm-frame-info"
 
// TODO: wasm64
// TODO: Emit TargetOpcode::CFI_INSTRUCTION instructions
 
// In an ideal world, when objects are added to the MachineFrameInfo by
// FunctionLoweringInfo::set, we could somehow hook into target-specific code to
// ensure they are assigned the right stack ID.  However there isn't a hook that
// runs between then and DAG building time, though, so instead we hoist stack
// objects lazily when they are first used, and comprehensively after the DAG is
// built via the PreprocessISelDAG hook, called by the
// SelectionDAGISel::runOnMachineFunction.  We have to do it in two places
// because we want to do it while building the selection DAG for uses of alloca,
// but not all alloca instructions are used so we have to follow up afterwards.
std::optional<unsigned>
WebAssemblyFrameLowering::getLocalForStackObject(MachineFunction &MF,
                                                 int FrameIndex) {
  MachineFrameInfo &MFI = MF.getFrameInfo();
 
  // If already hoisted to a local, done.
  if (MFI.getStackID(FrameIndex) == TargetStackID::WasmLocal)
    return static_cast<unsigned>(MFI.getObjectOffset(FrameIndex));
 
  // If not allocated in the object address space, this object will be in
  // linear memory.
  const AllocaInst *AI = MFI.getObjectAllocation(FrameIndex);
  if (!AI || !WebAssembly::isWasmVarAddressSpace(AI->getAddressSpace()))
    return std::nullopt;
 
  // Otherwise, allocate this object in the named value stack, outside of linear
  // memory.
  SmallVector<EVT, 4> ValueVTs;
  const WebAssemblyTargetLowering &TLI =
      *MF.getSubtarget<WebAssemblySubtarget>().getTargetLowering();
  WebAssemblyFunctionInfo *FuncInfo = MF.getInfo<WebAssemblyFunctionInfo>();
  ComputeValueVTs(TLI, MF.getDataLayout(), AI->getAllocatedType(), ValueVTs);
  MFI.setStackID(FrameIndex, TargetStackID::WasmLocal);
  // Abuse SP offset to record the index of the first local in the object.
  unsigned Local = FuncInfo->getParams().size() + FuncInfo->getLocals().size();
  MFI.setObjectOffset(FrameIndex, Local);
  // Allocate WebAssembly locals for each non-aggregate component of the
  // allocation.
  for (EVT ValueVT : ValueVTs)
    FuncInfo->addLocal(ValueVT.getSimpleVT());
  // Abuse object size to record number of WebAssembly locals allocated to
  // this object.
  MFI.setObjectSize(FrameIndex, ValueVTs.size());
  return Local;
}
 
/// 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->hasStackRealignment(MF);
}
 
/// Return true if the specified function should have a dedicated frame pointer
/// register.
bool WebAssemblyFrameLowering::hasFPImpl(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();
  auto &MRI = MF.getRegInfo();
  // llvm.stacksave can explicitly read SP register and it can appear without
  // dynamic alloca.
  bool HasExplicitSPUse =
      any_of(MRI.use_operands(getSPReg(MF)),
             [](MachineOperand &MO) { return !MO.isImplicit(); });
 
  // With libcall thread context, we need SP in the prolog when debug
  // info is present so we can allocate a local for DWARF to reference.
  bool NeedsSPForDebug =
      MF.getFunction().getSubprogram() &&
      MF.getSubtarget<WebAssemblySubtarget>().hasLibcallThreadContext();
 
  return MFI.getStackSize() || MFI.adjustsStack() || hasFP(MF) ||
         HasExplicitSPUse || NeedsSPForDebug;
}
 
// In function with EH pads, we need to make a copy of the value of
// the stack pointer in the SP32/64 register, in order to use it when
// restoring the stack pointer after an exception is caught.
bool WebAssemblyFrameLowering::needsPrologForEH(
    const MachineFunction &MF) const {
  return MF.getFunction().hasPersonalityFn() && MF.getFrameInfo().hasCalls() &&
         MF.getFunction().getParent()->getExceptionModel() ==
             ExceptionHandling::Wasm;
}
 
/// 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 or managed by library calls, 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 the stack pointer global/thread context 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;
}
 
unsigned WebAssemblyFrameLowering::getSPReg(const MachineFunction &MF) {
  return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
             ? WebAssembly::SP64
             : WebAssembly::SP32;
}
 
unsigned WebAssemblyFrameLowering::getFPReg(const MachineFunction &MF) {
  return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
             ? WebAssembly::FP64
             : WebAssembly::FP32;
}
 
unsigned
WebAssemblyFrameLowering::getOpcConst(const MachineFunction &MF) {
  return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
             ? WebAssembly::CONST_I64
             : WebAssembly::CONST_I32;
}
 
unsigned WebAssemblyFrameLowering::getOpcAdd(const MachineFunction &MF) {
  return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
             ? WebAssembly::ADD_I64
             : WebAssembly::ADD_I32;
}
 
unsigned WebAssemblyFrameLowering::getOpcSub(const MachineFunction &MF) {
  return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
             ? WebAssembly::SUB_I64
             : WebAssembly::SUB_I32;
}
 
unsigned WebAssemblyFrameLowering::getOpcAnd(const MachineFunction &MF) {
  return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
             ? WebAssembly::AND_I64
             : WebAssembly::AND_I32;
}
 
unsigned
WebAssemblyFrameLowering::getOpcGlobGet(const MachineFunction &MF) {
  return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
             ? WebAssembly::GLOBAL_GET_I64
             : WebAssembly::GLOBAL_GET_I32;
}
 
unsigned
WebAssemblyFrameLowering::getOpcGlobSet(const MachineFunction &MF) {
  return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
             ? WebAssembly::GLOBAL_SET_I64
             : WebAssembly::GLOBAL_SET_I32;
}
 
void WebAssemblyFrameLowering::writeBackSP(
    unsigned SrcReg, MachineFunction &MF, MachineBasicBlock &MBB,
    MachineBasicBlock::iterator &InsertStore, const DebugLoc &DL) const {
  const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
 
  if (MF.getSubtarget<WebAssemblySubtarget>().hasLibcallThreadContext()) {
    const char *ES = "__wasm_set_stack_pointer";
    auto *SPSymbol = MF.createExternalSymbolName(ES);
    BuildMI(MBB, InsertStore, DL, TII->get(WebAssembly::CALL))
        .addExternalSymbol(SPSymbol)
        .addReg(SrcReg);
  } else {
    const char *ES = "__stack_pointer";
    auto *SPSymbol = MF.createExternalSymbolName(ES);
 
    BuildMI(MBB, InsertStore, DL, TII->get(getOpcGlobSet(MF)))
        .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");
  auto &ST = MF.getSubtarget<WebAssemblySubtarget>();
  const auto *TII = ST.getInstrInfo();
  if (I->getOpcode() == TII->getCallFrameDestroyOpcode() &&
      needsSPWriteback(MF)) {
    DebugLoc DL = I->getDebugLoc();
    writeBackSP(getSPReg(MF), 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();
 
  auto &ST = MF.getSubtarget<WebAssemblySubtarget>();
  const auto *TII = ST.getInstrInfo();
  auto &MRI = MF.getRegInfo();
 
  auto InsertPt = MBB.begin();
  while (InsertPt != MBB.end() &&
         WebAssembly::isArgument(InsertPt->getOpcode()))
    ++InsertPt;
  DebugLoc DL;
 
  const TargetRegisterClass *PtrRC =
      TII->getRegClass(TII->get(getOpcConst(MF)), 0);
  unsigned SPReg = getSPReg(MF);
  if (StackSize)
    SPReg = MRI.createVirtualRegister(PtrRC);
 
  if (ST.hasLibcallThreadContext()) {
    const char *ES = "__wasm_get_stack_pointer";
    auto *SPSymbol = MF.createExternalSymbolName(ES);
    BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CALL), SPReg)
        .addExternalSymbol(SPSymbol);
  } else {
    const char *ES = "__stack_pointer";
    auto *SPSymbol = MF.createExternalSymbolName(ES);
    BuildMI(MBB, InsertPt, DL, TII->get(getOpcGlobGet(MF)), 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(getOpcConst(MF)), OffsetReg)
        .addImm(StackSize);
    BuildMI(MBB, InsertPt, DL, TII->get(getOpcSub(MF)), getSPReg(MF))
        .addReg(SPReg)
        .addReg(OffsetReg);
  }
  if (HasBP) {
    Register BitmaskReg = MRI.createVirtualRegister(PtrRC);
    Align Alignment = MFI.getMaxAlign();
    BuildMI(MBB, InsertPt, DL, TII->get(getOpcConst(MF)), BitmaskReg)
        .addImm((int64_t) ~(Alignment.value() - 1));
    BuildMI(MBB, InsertPt, DL, TII->get(getOpcAnd(MF)), getSPReg(MF))
        .addReg(getSPReg(MF))
        .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), getFPReg(MF))
        .addReg(getSPReg(MF));
  }
  if (StackSize && needsSPWriteback(MF)) {
    writeBackSP(getSPReg(MF), MF, MBB, InsertPt, DL);
  }
}
 
void WebAssemblyFrameLowering::emitEpilogue(MachineFunction &MF,
                                            MachineBasicBlock &MBB) const {
  uint64_t StackSize = MF.getFrameInfo().getStackSize();
  if (!needsSP(MF) || !needsSPWriteback(MF))
    return;
  auto &ST = MF.getSubtarget<WebAssemblySubtarget>();
  const auto *TII = ST.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;
  unsigned SPFPReg = hasFP(MF) ? getFPReg(MF) : getSPReg(MF);
  if (hasBP(MF)) {
    auto FI = MF.getInfo<WebAssemblyFunctionInfo>();
    SPReg = FI->getBasePointerVreg();
  } else if (StackSize) {
    const TargetRegisterClass *PtrRC =
        TII->getRegClass(TII->get(getOpcConst(MF)), 0);
    Register OffsetReg = MRI.createVirtualRegister(PtrRC);
    BuildMI(MBB, InsertPt, DL, TII->get(getOpcConst(MF)), OffsetReg)
        .addImm(StackSize);
    // In the epilog we don't need to write the result back to the SP32/64
    // 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(getOpcAdd(MF)), SPReg)
        .addReg(SPFPReg)
        .addReg(OffsetReg);
  } else {
    SPReg = SPFPReg;
  }
 
  writeBackSP(SPReg, MF, MBB, InsertPt, DL);
}
 
bool WebAssemblyFrameLowering::isSupportedStackID(
    TargetStackID::Value ID) const {
  // Use the Object stack for WebAssembly locals which can only be accessed
  // by name, not via an address in linear memory.
  if (ID == TargetStackID::WasmLocal)
    return true;
 
  return TargetFrameLowering::isSupportedStackID(ID);
}
 
TargetFrameLowering::DwarfFrameBase
WebAssemblyFrameLowering::getDwarfFrameBase(const MachineFunction &MF) const {
  DwarfFrameBase Loc;
  Loc.Kind = DwarfFrameBase::WasmFrameBase;
  const WebAssemblyFunctionInfo &MFI = *MF.getInfo<WebAssemblyFunctionInfo>();
  if (needsSP(MF) && MFI.isFrameBaseVirtual()) {
    unsigned LocalNum = MFI.getFrameBaseLocal();
    Loc.Location.WasmLoc = {WebAssembly::TI_LOCAL, LocalNum};
  } else {
    // There is no __stack_pointer global in libcall thread context mode, so
    // TI_GLOBAL_RELOC would produce a bogus relocation. We take care to ensure
    // that this code is not reached in that case, but assert here to be sure.
    assert(!MF.getSubtarget<WebAssemblySubtarget>().hasLibcallThreadContext());
 
    // The __stack_pointer global holds only the innermost frame's stack
    // pointer, so this frame base is correct only for the innermost frame.
    // TODO: Describe outer frames, which need a per-frame stack pointer value.
    Loc.Location.WasmLoc = {WebAssembly::TI_GLOBAL_RELOC, 0};
  }
  return Loc;
}