WebAssemblyFrameLowering.cpp revision 327952 
1//===-- WebAssemblyFrameLowering.cpp - WebAssembly Frame Lowering ----------==//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9///
10/// \file
11/// \brief This file contains the WebAssembly implementation of
12/// TargetFrameLowering class.
13///
14/// On WebAssembly, there aren't a lot of things to do here. There are no
15/// callee-saved registers to save, and no spill slots.
16///
17/// The stack grows downward.
18///
19//===----------------------------------------------------------------------===//
20
21#include "WebAssemblyFrameLowering.h"
22#include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
23#include "WebAssemblyInstrInfo.h"
24#include "WebAssemblyMachineFunctionInfo.h"
25#include "WebAssemblySubtarget.h"
26#include "WebAssemblyTargetMachine.h"
27#include "WebAssemblyUtilities.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/Support/Debug.h"
34using namespace llvm;
35
36#define DEBUG_TYPE "wasm-frame-info"
37
38// TODO: wasm64
39// TODO: Emit TargetOpcode::CFI_INSTRUCTION instructions
40
41/// We need a base pointer in the case of having items on the stack that
42/// require stricter alignment than the stack pointer itself.  Because we need
43/// to shift the stack pointer by some unknown amount to force the alignment,
44/// we need to record the value of the stack pointer on entry to the function.
45bool WebAssemblyFrameLowering::hasBP(
46    const MachineFunction &MF) const {
47  const auto *RegInfo =
48      MF.getSubtarget<WebAssemblySubtarget>().getRegisterInfo();
49  return RegInfo->needsStackRealignment(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
82/// Returns true if this function needs a local user-space stack pointer.
83/// Unlike a machine stack pointer, the wasm user stack pointer is a global
84/// variable, so it is loaded into a register in the prolog.
85bool WebAssemblyFrameLowering::needsSP(const MachineFunction &MF,
86                                       const MachineFrameInfo &MFI) const {
87  return MFI.getStackSize() || MFI.adjustsStack() || hasFP(MF);
88}
89
90/// Returns true if the local user-space stack pointer needs to be written back
91/// to memory by this function (this is not meaningful if needsSP is false). If
92/// false, the stack red zone can be used and only a local SP is needed.
93bool WebAssemblyFrameLowering::needsSPWriteback(
94    const MachineFunction &MF, const MachineFrameInfo &MFI) const {
95  assert(needsSP(MF, MFI));
96  return MFI.getStackSize() > RedZoneSize || MFI.hasCalls() ||
97         MF.getFunction().hasFnAttribute(Attribute::NoRedZone);
98}
99
100static void writeSPToMemory(unsigned SrcReg, MachineFunction &MF,
101                            MachineBasicBlock &MBB,
102                            MachineBasicBlock::iterator &InsertAddr,
103                            MachineBasicBlock::iterator &InsertStore,
104                            const DebugLoc &DL) {
105  const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
106
107  const char *ES = "__stack_pointer";
108  auto *SPSymbol = MF.createExternalSymbolName(ES);
109  if (MF.getSubtarget<WebAssemblySubtarget>()
110        .getTargetTriple().isOSBinFormatELF()) {
111    MachineRegisterInfo &MRI = MF.getRegInfo();
112    const TargetRegisterClass *PtrRC =
113        MRI.getTargetRegisterInfo()->getPointerRegClass(MF);
114    unsigned Zero = MRI.createVirtualRegister(PtrRC);
115
116    BuildMI(MBB, InsertAddr, DL, TII->get(WebAssembly::CONST_I32), Zero)
117        .addImm(0);
118    MachineMemOperand *MMO = MF.getMachineMemOperand(
119        MachinePointerInfo(MF.getPSVManager().getExternalSymbolCallEntry(ES)),
120        MachineMemOperand::MOStore, 4, 4);
121    BuildMI(MBB, InsertStore, DL, TII->get(WebAssembly::STORE_I32))
122        .addImm(2)  // p2align
123        .addExternalSymbol(SPSymbol)
124        .addReg(Zero)
125        .addReg(SrcReg)
126        .addMemOperand(MMO);
127  } else {
128    BuildMI(MBB, InsertStore, DL, TII->get(WebAssembly::SET_GLOBAL_I32))
129        .addExternalSymbol(SPSymbol)
130        .addReg(SrcReg);
131  }
132}
133
134MachineBasicBlock::iterator
135WebAssemblyFrameLowering::eliminateCallFramePseudoInstr(
136    MachineFunction &MF, MachineBasicBlock &MBB,
137    MachineBasicBlock::iterator I) const {
138  assert(!I->getOperand(0).getImm() && (hasFP(MF) || hasBP(MF)) &&
139         "Call frame pseudos should only be used for dynamic stack adjustment");
140  const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
141  if (I->getOpcode() == TII->getCallFrameDestroyOpcode() &&
142      needsSPWriteback(MF, MF.getFrameInfo())) {
143    DebugLoc DL = I->getDebugLoc();
144    writeSPToMemory(WebAssembly::SP32, MF, MBB, I, I, DL);
145  }
146  return MBB.erase(I);
147}
148
149void WebAssemblyFrameLowering::emitPrologue(MachineFunction &MF,
150                                            MachineBasicBlock &MBB) const {
151  // TODO: Do ".setMIFlag(MachineInstr::FrameSetup)" on emitted instructions
152  auto &MFI = MF.getFrameInfo();
153  assert(MFI.getCalleeSavedInfo().empty() &&
154         "WebAssembly should not have callee-saved registers");
155
156  if (!needsSP(MF, MFI)) return;
157  uint64_t StackSize = MFI.getStackSize();
158
159  const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
160  auto &MRI = MF.getRegInfo();
161
162  auto InsertPt = MBB.begin();
163  while (InsertPt != MBB.end() && WebAssembly::isArgument(*InsertPt))
164    ++InsertPt;
165  DebugLoc DL;
166
167  const TargetRegisterClass *PtrRC =
168      MRI.getTargetRegisterInfo()->getPointerRegClass(MF);
169  unsigned SPReg = WebAssembly::SP32;
170  if (StackSize)
171    SPReg = MRI.createVirtualRegister(PtrRC);
172
173  const char *ES = "__stack_pointer";
174  auto *SPSymbol = MF.createExternalSymbolName(ES);
175  if (MF.getSubtarget<WebAssemblySubtarget>()
176        .getTargetTriple().isOSBinFormatELF()) {
177    unsigned Zero = MRI.createVirtualRegister(PtrRC);
178
179    BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), Zero)
180        .addImm(0);
181    MachineMemOperand *LoadMMO = MF.getMachineMemOperand(
182        MachinePointerInfo(MF.getPSVManager().getExternalSymbolCallEntry(ES)),
183        MachineMemOperand::MOLoad, 4, 4);
184    // Load the SP value.
185    BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::LOAD_I32), SPReg)
186        .addImm(2)       // p2align
187        .addExternalSymbol(SPSymbol)
188        .addReg(Zero)    // addr
189        .addMemOperand(LoadMMO);
190  } else {
191    BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::GET_GLOBAL_I32), SPReg)
192        .addExternalSymbol(SPSymbol);
193  }
194
195  bool HasBP = hasBP(MF);
196  if (HasBP) {
197    auto FI = MF.getInfo<WebAssemblyFunctionInfo>();
198    unsigned BasePtr = MRI.createVirtualRegister(PtrRC);
199    FI->setBasePointerVreg(BasePtr);
200    BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::COPY), BasePtr)
201        .addReg(SPReg);
202  }
203  if (StackSize) {
204    // Subtract the frame size
205    unsigned OffsetReg = MRI.createVirtualRegister(PtrRC);
206    BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), OffsetReg)
207        .addImm(StackSize);
208    BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::SUB_I32),
209            WebAssembly::SP32)
210        .addReg(SPReg)
211        .addReg(OffsetReg);
212  }
213  if (HasBP) {
214    unsigned BitmaskReg = MRI.createVirtualRegister(PtrRC);
215    unsigned Alignment = MFI.getMaxAlignment();
216    assert((1u << countTrailingZeros(Alignment)) == Alignment &&
217      "Alignment must be a power of 2");
218    BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), BitmaskReg)
219        .addImm((int)~(Alignment - 1));
220    BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::AND_I32),
221            WebAssembly::SP32)
222        .addReg(WebAssembly::SP32)
223        .addReg(BitmaskReg);
224  }
225  if (hasFP(MF)) {
226    // Unlike most conventional targets (where FP points to the saved FP),
227    // FP points to the bottom of the fixed-size locals, so we can use positive
228    // offsets in load/store instructions.
229    BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::COPY),
230            WebAssembly::FP32)
231        .addReg(WebAssembly::SP32);
232  }
233  if (StackSize && needsSPWriteback(MF, MFI)) {
234    writeSPToMemory(WebAssembly::SP32, MF, MBB, InsertPt, InsertPt, DL);
235  }
236}
237
238void WebAssemblyFrameLowering::emitEpilogue(MachineFunction &MF,
239                                            MachineBasicBlock &MBB) const {
240  auto &MFI = MF.getFrameInfo();
241  uint64_t StackSize = MFI.getStackSize();
242  if (!needsSP(MF, MFI) || !needsSPWriteback(MF, MFI)) return;
243  const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
244  auto &MRI = MF.getRegInfo();
245  auto InsertPt = MBB.getFirstTerminator();
246  DebugLoc DL;
247
248  if (InsertPt != MBB.end())
249    DL = InsertPt->getDebugLoc();
250
251  // Restore the stack pointer. If we had fixed-size locals, add the offset
252  // subtracted in the prolog.
253  unsigned SPReg = 0;
254  MachineBasicBlock::iterator InsertAddr = InsertPt;
255  if (hasBP(MF)) {
256    auto FI = MF.getInfo<WebAssemblyFunctionInfo>();
257    SPReg = FI->getBasePointerVreg();
258  } else if (StackSize) {
259    const TargetRegisterClass *PtrRC =
260        MRI.getTargetRegisterInfo()->getPointerRegClass(MF);
261    unsigned OffsetReg = MRI.createVirtualRegister(PtrRC);
262    InsertAddr =
263        BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), OffsetReg)
264            .addImm(StackSize);
265    // In the epilog we don't need to write the result back to the SP32 physreg
266    // because it won't be used again. We can use a stackified register instead.
267    SPReg = MRI.createVirtualRegister(PtrRC);
268    BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::ADD_I32), SPReg)
269        .addReg(hasFP(MF) ? WebAssembly::FP32 : WebAssembly::SP32)
270        .addReg(OffsetReg);
271  } else {
272    SPReg = hasFP(MF) ? WebAssembly::FP32 : WebAssembly::SP32;
273  }
274
275  writeSPToMemory(SPReg, MF, MBB, InsertAddr, InsertPt, DL);
276}
277