1//===-- WebAssemblyFrameLowering.cpp - WebAssembly Frame Lowering ----------==//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8///
9/// \file
10/// This file contains the WebAssembly implementation of
11/// TargetFrameLowering class.
12///
13/// On WebAssembly, there aren't a lot of things to do here. There are no
14/// callee-saved registers to save, and no spill slots.
15///
16/// The stack grows downward.
17///
18//===----------------------------------------------------------------------===//
19
20#include "WebAssemblyFrameLowering.h"
21#include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
22#include "Utils/WebAssemblyUtilities.h"
23#include "WebAssembly.h"
24#include "WebAssemblyInstrInfo.h"
25#include "WebAssemblyMachineFunctionInfo.h"
26#include "WebAssemblySubtarget.h"
27#include "WebAssemblyTargetMachine.h"
28#include "llvm/CodeGen/MachineFrameInfo.h"
29#include "llvm/CodeGen/MachineFunction.h"
30#include "llvm/CodeGen/MachineInstrBuilder.h"
31#include "llvm/CodeGen/MachineModuleInfoImpls.h"
32#include "llvm/CodeGen/MachineRegisterInfo.h"
33#include "llvm/MC/MCAsmInfo.h"
34#include "llvm/Support/Debug.h"
35using namespace llvm;
36
37#define DEBUG_TYPE "wasm-frame-info"
38
39// TODO: wasm64
40// TODO: Emit TargetOpcode::CFI_INSTRUCTION instructions
41
42/// We need a base pointer in the case of having items on the stack that
43/// require stricter alignment than the stack pointer itself.  Because we need
44/// to shift the stack pointer by some unknown amount to force the alignment,
45/// we need to record the value of the stack pointer on entry to the function.
46bool WebAssemblyFrameLowering::hasBP(const MachineFunction &MF) const {
47  const auto *RegInfo =
48      MF.getSubtarget<WebAssemblySubtarget>().getRegisterInfo();
49  return RegInfo->hasStackRealignment(MF);
50}
51
52/// Return true if the specified function should have a dedicated frame pointer
53/// register.
54bool WebAssemblyFrameLowering::hasFP(const MachineFunction &MF) const {
55  const MachineFrameInfo &MFI = MF.getFrameInfo();
56
57  // When we have var-sized objects, we move the stack pointer by an unknown
58  // amount, and need to emit a frame pointer to restore the stack to where we
59  // were on function entry.
60  // If we already need a base pointer, we use that to fix up the stack pointer.
61  // If there are no fixed-size objects, we would have no use of a frame
62  // pointer, and thus should not emit one.
63  bool HasFixedSizedObjects = MFI.getStackSize() > 0;
64  bool NeedsFixedReference = !hasBP(MF) || HasFixedSizedObjects;
65
66  return MFI.isFrameAddressTaken() ||
67         (MFI.hasVarSizedObjects() && NeedsFixedReference) ||
68         MFI.hasStackMap() || MFI.hasPatchPoint();
69}
70
71/// Under normal circumstances, when a frame pointer is not required, we reserve
72/// argument space for call sites in the function immediately on entry to the
73/// current function. This eliminates the need for add/sub sp brackets around
74/// call sites. Returns true if the call frame is included as part of the stack
75/// frame.
76bool WebAssemblyFrameLowering::hasReservedCallFrame(
77    const MachineFunction &MF) const {
78  return !MF.getFrameInfo().hasVarSizedObjects();
79}
80
81// Returns true if this function needs a local user-space stack pointer for its
82// local frame (not for exception handling).
83bool WebAssemblyFrameLowering::needsSPForLocalFrame(
84    const MachineFunction &MF) const {
85  auto &MFI = MF.getFrameInfo();
86  return MFI.getStackSize() || MFI.adjustsStack() || hasFP(MF);
87}
88
89// In function with EH pads, we need to make a copy of the value of
90// __stack_pointer global in SP32/64 register, in order to use it when
91// restoring __stack_pointer after an exception is caught.
92bool WebAssemblyFrameLowering::needsPrologForEH(
93    const MachineFunction &MF) const {
94  auto EHType = MF.getTarget().getMCAsmInfo()->getExceptionHandlingType();
95  return EHType == ExceptionHandling::Wasm &&
96         MF.getFunction().hasPersonalityFn() && MF.getFrameInfo().hasCalls();
97}
98
99/// Returns true if this function needs a local user-space stack pointer.
100/// Unlike a machine stack pointer, the wasm user stack pointer is a global
101/// variable, so it is loaded into a register in the prolog.
102bool WebAssemblyFrameLowering::needsSP(const MachineFunction &MF) const {
103  return needsSPForLocalFrame(MF) || needsPrologForEH(MF);
104}
105
106/// Returns true if the local user-space stack pointer needs to be written back
107/// to __stack_pointer global by this function (this is not meaningful if
108/// needsSP is false). If false, the stack red zone can be used and only a local
109/// SP is needed.
110bool WebAssemblyFrameLowering::needsSPWriteback(
111    const MachineFunction &MF) const {
112  auto &MFI = MF.getFrameInfo();
113  assert(needsSP(MF));
114  // When we don't need a local stack pointer for its local frame but only to
115  // support EH, we don't need to write SP back in the epilog, because we don't
116  // bump down the stack pointer in the prolog. We need to write SP back in the
117  // epilog only if
118  // 1. We need SP not only for EH support but also because we actually use
119  // stack or we have a frame address taken.
120  // 2. We cannot use the red zone.
121  bool CanUseRedZone = MFI.getStackSize() <= RedZoneSize && !MFI.hasCalls() &&
122                       !MF.getFunction().hasFnAttribute(Attribute::NoRedZone);
123  return needsSPForLocalFrame(MF) && !CanUseRedZone;
124}
125
126unsigned WebAssemblyFrameLowering::getSPReg(const MachineFunction &MF) {
127  return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
128             ? WebAssembly::SP64
129             : WebAssembly::SP32;
130}
131
132unsigned WebAssemblyFrameLowering::getFPReg(const MachineFunction &MF) {
133  return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
134             ? WebAssembly::FP64
135             : WebAssembly::FP32;
136}
137
138unsigned
139WebAssemblyFrameLowering::getOpcConst(const MachineFunction &MF) {
140  return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
141             ? WebAssembly::CONST_I64
142             : WebAssembly::CONST_I32;
143}
144
145unsigned WebAssemblyFrameLowering::getOpcAdd(const MachineFunction &MF) {
146  return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
147             ? WebAssembly::ADD_I64
148             : WebAssembly::ADD_I32;
149}
150
151unsigned WebAssemblyFrameLowering::getOpcSub(const MachineFunction &MF) {
152  return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
153             ? WebAssembly::SUB_I64
154             : WebAssembly::SUB_I32;
155}
156
157unsigned WebAssemblyFrameLowering::getOpcAnd(const MachineFunction &MF) {
158  return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
159             ? WebAssembly::AND_I64
160             : WebAssembly::AND_I32;
161}
162
163unsigned
164WebAssemblyFrameLowering::getOpcGlobGet(const MachineFunction &MF) {
165  return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
166             ? WebAssembly::GLOBAL_GET_I64
167             : WebAssembly::GLOBAL_GET_I32;
168}
169
170unsigned
171WebAssemblyFrameLowering::getOpcGlobSet(const MachineFunction &MF) {
172  return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
173             ? WebAssembly::GLOBAL_SET_I64
174             : WebAssembly::GLOBAL_SET_I32;
175}
176
177void WebAssemblyFrameLowering::writeSPToGlobal(
178    unsigned SrcReg, MachineFunction &MF, MachineBasicBlock &MBB,
179    MachineBasicBlock::iterator &InsertStore, const DebugLoc &DL) const {
180  const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
181
182  const char *ES = "__stack_pointer";
183  auto *SPSymbol = MF.createExternalSymbolName(ES);
184
185  BuildMI(MBB, InsertStore, DL, TII->get(getOpcGlobSet(MF)))
186      .addExternalSymbol(SPSymbol)
187      .addReg(SrcReg);
188}
189
190MachineBasicBlock::iterator
191WebAssemblyFrameLowering::eliminateCallFramePseudoInstr(
192    MachineFunction &MF, MachineBasicBlock &MBB,
193    MachineBasicBlock::iterator I) const {
194  assert(!I->getOperand(0).getImm() && (hasFP(MF) || hasBP(MF)) &&
195         "Call frame pseudos should only be used for dynamic stack adjustment");
196  auto &ST = MF.getSubtarget<WebAssemblySubtarget>();
197  const auto *TII = ST.getInstrInfo();
198  if (I->getOpcode() == TII->getCallFrameDestroyOpcode() &&
199      needsSPWriteback(MF)) {
200    DebugLoc DL = I->getDebugLoc();
201    writeSPToGlobal(getSPReg(MF), MF, MBB, I, DL);
202  }
203  return MBB.erase(I);
204}
205
206void WebAssemblyFrameLowering::emitPrologue(MachineFunction &MF,
207                                            MachineBasicBlock &MBB) const {
208  // TODO: Do ".setMIFlag(MachineInstr::FrameSetup)" on emitted instructions
209  auto &MFI = MF.getFrameInfo();
210  assert(MFI.getCalleeSavedInfo().empty() &&
211         "WebAssembly should not have callee-saved registers");
212
213  if (!needsSP(MF))
214    return;
215  uint64_t StackSize = MFI.getStackSize();
216
217  auto &ST = MF.getSubtarget<WebAssemblySubtarget>();
218  const auto *TII = ST.getInstrInfo();
219  auto &MRI = MF.getRegInfo();
220
221  auto InsertPt = MBB.begin();
222  while (InsertPt != MBB.end() &&
223         WebAssembly::isArgument(InsertPt->getOpcode()))
224    ++InsertPt;
225  DebugLoc DL;
226
227  const TargetRegisterClass *PtrRC =
228      MRI.getTargetRegisterInfo()->getPointerRegClass(MF);
229  unsigned SPReg = getSPReg(MF);
230  if (StackSize)
231    SPReg = MRI.createVirtualRegister(PtrRC);
232
233  const char *ES = "__stack_pointer";
234  auto *SPSymbol = MF.createExternalSymbolName(ES);
235  BuildMI(MBB, InsertPt, DL, TII->get(getOpcGlobGet(MF)), SPReg)
236      .addExternalSymbol(SPSymbol);
237
238  bool HasBP = hasBP(MF);
239  if (HasBP) {
240    auto FI = MF.getInfo<WebAssemblyFunctionInfo>();
241    Register BasePtr = MRI.createVirtualRegister(PtrRC);
242    FI->setBasePointerVreg(BasePtr);
243    BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::COPY), BasePtr)
244        .addReg(SPReg);
245  }
246  if (StackSize) {
247    // Subtract the frame size
248    Register OffsetReg = MRI.createVirtualRegister(PtrRC);
249    BuildMI(MBB, InsertPt, DL, TII->get(getOpcConst(MF)), OffsetReg)
250        .addImm(StackSize);
251    BuildMI(MBB, InsertPt, DL, TII->get(getOpcSub(MF)), getSPReg(MF))
252        .addReg(SPReg)
253        .addReg(OffsetReg);
254  }
255  if (HasBP) {
256    Register BitmaskReg = MRI.createVirtualRegister(PtrRC);
257    Align Alignment = MFI.getMaxAlign();
258    BuildMI(MBB, InsertPt, DL, TII->get(getOpcConst(MF)), BitmaskReg)
259        .addImm((int64_t) ~(Alignment.value() - 1));
260    BuildMI(MBB, InsertPt, DL, TII->get(getOpcAnd(MF)), getSPReg(MF))
261        .addReg(getSPReg(MF))
262        .addReg(BitmaskReg);
263  }
264  if (hasFP(MF)) {
265    // Unlike most conventional targets (where FP points to the saved FP),
266    // FP points to the bottom of the fixed-size locals, so we can use positive
267    // offsets in load/store instructions.
268    BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::COPY), getFPReg(MF))
269        .addReg(getSPReg(MF));
270  }
271  if (StackSize && needsSPWriteback(MF)) {
272    writeSPToGlobal(getSPReg(MF), MF, MBB, InsertPt, DL);
273  }
274}
275
276void WebAssemblyFrameLowering::emitEpilogue(MachineFunction &MF,
277                                            MachineBasicBlock &MBB) const {
278  uint64_t StackSize = MF.getFrameInfo().getStackSize();
279  if (!needsSP(MF) || !needsSPWriteback(MF))
280    return;
281  auto &ST = MF.getSubtarget<WebAssemblySubtarget>();
282  const auto *TII = ST.getInstrInfo();
283  auto &MRI = MF.getRegInfo();
284  auto InsertPt = MBB.getFirstTerminator();
285  DebugLoc DL;
286
287  if (InsertPt != MBB.end())
288    DL = InsertPt->getDebugLoc();
289
290  // Restore the stack pointer. If we had fixed-size locals, add the offset
291  // subtracted in the prolog.
292  unsigned SPReg = 0;
293  unsigned SPFPReg = hasFP(MF) ? getFPReg(MF) : getSPReg(MF);
294  if (hasBP(MF)) {
295    auto FI = MF.getInfo<WebAssemblyFunctionInfo>();
296    SPReg = FI->getBasePointerVreg();
297  } else if (StackSize) {
298    const TargetRegisterClass *PtrRC =
299        MRI.getTargetRegisterInfo()->getPointerRegClass(MF);
300    Register OffsetReg = MRI.createVirtualRegister(PtrRC);
301    BuildMI(MBB, InsertPt, DL, TII->get(getOpcConst(MF)), OffsetReg)
302        .addImm(StackSize);
303    // In the epilog we don't need to write the result back to the SP32/64
304    // physreg because it won't be used again. We can use a stackified register
305    // instead.
306    SPReg = MRI.createVirtualRegister(PtrRC);
307    BuildMI(MBB, InsertPt, DL, TII->get(getOpcAdd(MF)), SPReg)
308        .addReg(SPFPReg)
309        .addReg(OffsetReg);
310  } else {
311    SPReg = SPFPReg;
312  }
313
314  writeSPToGlobal(SPReg, MF, MBB, InsertPt, DL);
315}
316
317TargetFrameLowering::DwarfFrameBase
318WebAssemblyFrameLowering::getDwarfFrameBase(const MachineFunction &MF) const {
319  DwarfFrameBase Loc;
320  Loc.Kind = DwarfFrameBase::WasmFrameBase;
321  const WebAssemblyFunctionInfo &MFI = *MF.getInfo<WebAssemblyFunctionInfo>();
322  if (needsSP(MF) && MFI.isFrameBaseVirtual()) {
323    unsigned LocalNum = MFI.getFrameBaseLocal();
324    Loc.Location.WasmLoc = {WebAssembly::TI_LOCAL, LocalNum};
325  } else {
326    // TODO: This should work on a breakpoint at a function with no frame,
327    // but probably won't work for traversing up the stack.
328    Loc.Location.WasmLoc = {WebAssembly::TI_GLOBAL_RELOC, 0};
329  }
330  return Loc;
331}
332