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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 "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 =
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();
// 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;
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))
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))
uint64_t StackSize = MFI.getStackSize();
const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
auto &MRI = MF.getRegInfo();
auto InsertPt = MBB.begin();
while (InsertPt != MBB.end() &&
DebugLoc DL;
const TargetRegisterClass *PtrRC =
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)
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(WebAssembly::CONST_I32), OffsetReg)
BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::SUB_I32),
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),
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)
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))
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 =
Register OffsetReg = MRI.createVirtualRegister(PtrRC);
BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), OffsetReg)
// 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)
} else {
SPReg = hasFP(MF) ? WebAssembly::FP32 : WebAssembly::SP32;
writeSPToGlobal(SPReg, MF, MBB, InsertPt, DL);