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//===-- WebAssemblyFrameLowering.cpp - WebAssembly Frame Lowering ----------==//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See 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/WebAssemblyUtilities.h"
#include "WebAssembly.h"
#include "WebAssemblyInstrInfo.h"
#include "WebAssemblyMachineFunctionInfo.h"
#include "WebAssemblySubtarget.h"
#include "WebAssemblyTargetMachine.h"
#include "llvm/CodeGen/Analysis.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/IR/Instructions.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
// 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.
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 ||
return None;
// Otherwise, allocate this object in the named value stack, outside of linear
// memory.
SmallVector<EVT, 4> ValueVTs;
const WebAssemblyTargetLowering &TLI =
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)
// Abuse object size to record number of WebAssembly locals allocated to
// this object.
MFI.setObjectSize(FrameIndex, ValueVTs.size());
return static_cast<unsigned>(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 =
return RegInfo->hasStackRealignment(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 SP32/64 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();
// 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() &&
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;
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;
WebAssemblyFrameLowering::getOpcGlobGet(const MachineFunction &MF) {
return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
? WebAssembly::GLOBAL_GET_I64
: WebAssembly::GLOBAL_GET_I32;
WebAssemblyFrameLowering::getOpcGlobSet(const MachineFunction &MF) {
return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
? WebAssembly::GLOBAL_SET_I64
: WebAssembly::GLOBAL_SET_I32;
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(getOpcGlobSet(MF)))
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();
writeSPToGlobal(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))
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() &&
DebugLoc DL;
const TargetRegisterClass *PtrRC =
unsigned SPReg = getSPReg(MF);
if (StackSize)
SPReg = MRI.createVirtualRegister(PtrRC);
const char *ES = "__stack_pointer";
auto *SPSymbol = MF.createExternalSymbolName(ES);
BuildMI(MBB, InsertPt, DL, TII->get(getOpcGlobGet(MF)), SPReg)
bool HasBP = hasBP(MF);
if (HasBP) {
auto FI = MF.getInfo<WebAssemblyFunctionInfo>();
Register BasePtr = MRI.createVirtualRegister(PtrRC);
BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::COPY), BasePtr)
if (StackSize) {
// Subtract the frame size
Register OffsetReg = MRI.createVirtualRegister(PtrRC);
BuildMI(MBB, InsertPt, DL, TII->get(getOpcConst(MF)), OffsetReg)
BuildMI(MBB, InsertPt, DL, TII->get(getOpcSub(MF)), getSPReg(MF))
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))
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))
if (StackSize && needsSPWriteback(MF)) {
writeSPToGlobal(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))
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 =
Register OffsetReg = MRI.createVirtualRegister(PtrRC);
BuildMI(MBB, InsertPt, DL, TII->get(getOpcConst(MF)), OffsetReg)
// 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)
} else {
SPReg = SPFPReg;
writeSPToGlobal(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);
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 {
// TODO: This should work on a breakpoint at a function with no frame,
// but probably won't work for traversing up the stack.
Loc.Location.WasmLoc = {WebAssembly::TI_GLOBAL_RELOC, 0};
return Loc;