1//===----- X86WinAllocaExpander.cpp - Expand WinAlloca pseudo instruction -===//
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// This file defines a pass that expands WinAlloca pseudo-instructions.
10//
11// It performs a conservative analysis to determine whether each allocation
12// falls within a region of the stack that is safe to use, or whether stack
13// probes must be emitted.
14//
15//===----------------------------------------------------------------------===//
16
17#include "X86.h"
18#include "X86InstrBuilder.h"
19#include "X86InstrInfo.h"
20#include "X86MachineFunctionInfo.h"
21#include "X86Subtarget.h"
22#include "llvm/ADT/PostOrderIterator.h"
23#include "llvm/CodeGen/MachineFunctionPass.h"
24#include "llvm/CodeGen/MachineInstrBuilder.h"
25#include "llvm/CodeGen/MachineRegisterInfo.h"
26#include "llvm/CodeGen/Passes.h"
27#include "llvm/CodeGen/TargetInstrInfo.h"
28#include "llvm/IR/Function.h"
29#include "llvm/Support/raw_ostream.h"
30
31using namespace llvm;
32
33namespace {
34
35class X86WinAllocaExpander : public MachineFunctionPass {
36public:
37  X86WinAllocaExpander() : MachineFunctionPass(ID) {}
38
39  bool runOnMachineFunction(MachineFunction &MF) override;
40
41private:
42  /// Strategies for lowering a WinAlloca.
43  enum Lowering { TouchAndSub, Sub, Probe };
44
45  /// Deterministic-order map from WinAlloca instruction to desired lowering.
46  typedef MapVector<MachineInstr*, Lowering> LoweringMap;
47
48  /// Compute which lowering to use for each WinAlloca instruction.
49  void computeLowerings(MachineFunction &MF, LoweringMap& Lowerings);
50
51  /// Get the appropriate lowering based on current offset and amount.
52  Lowering getLowering(int64_t CurrentOffset, int64_t AllocaAmount);
53
54  /// Lower a WinAlloca instruction.
55  void lower(MachineInstr* MI, Lowering L);
56
57  MachineRegisterInfo *MRI = nullptr;
58  const X86Subtarget *STI = nullptr;
59  const TargetInstrInfo *TII = nullptr;
60  const X86RegisterInfo *TRI = nullptr;
61  unsigned StackPtr = 0;
62  unsigned SlotSize = 0;
63  int64_t StackProbeSize = 0;
64  bool NoStackArgProbe = false;
65
66  StringRef getPassName() const override { return "X86 WinAlloca Expander"; }
67  static char ID;
68};
69
70char X86WinAllocaExpander::ID = 0;
71
72} // end anonymous namespace
73
74FunctionPass *llvm::createX86WinAllocaExpander() {
75  return new X86WinAllocaExpander();
76}
77
78/// Return the allocation amount for a WinAlloca instruction, or -1 if unknown.
79static int64_t getWinAllocaAmount(MachineInstr *MI, MachineRegisterInfo *MRI) {
80  assert(MI->getOpcode() == X86::WIN_ALLOCA_32 ||
81         MI->getOpcode() == X86::WIN_ALLOCA_64);
82  assert(MI->getOperand(0).isReg());
83
84  Register AmountReg = MI->getOperand(0).getReg();
85  MachineInstr *Def = MRI->getUniqueVRegDef(AmountReg);
86
87  if (!Def ||
88      (Def->getOpcode() != X86::MOV32ri && Def->getOpcode() != X86::MOV64ri) ||
89      !Def->getOperand(1).isImm())
90    return -1;
91
92  return Def->getOperand(1).getImm();
93}
94
95X86WinAllocaExpander::Lowering
96X86WinAllocaExpander::getLowering(int64_t CurrentOffset,
97                                  int64_t AllocaAmount) {
98  // For a non-constant amount or a large amount, we have to probe.
99  if (AllocaAmount < 0 || AllocaAmount > StackProbeSize)
100    return Probe;
101
102  // If it fits within the safe region of the stack, just subtract.
103  if (CurrentOffset + AllocaAmount <= StackProbeSize)
104    return Sub;
105
106  // Otherwise, touch the current tip of the stack, then subtract.
107  return TouchAndSub;
108}
109
110static bool isPushPop(const MachineInstr &MI) {
111  switch (MI.getOpcode()) {
112  case X86::PUSH32i8:
113  case X86::PUSH32r:
114  case X86::PUSH32rmm:
115  case X86::PUSH32rmr:
116  case X86::PUSHi32:
117  case X86::PUSH64i8:
118  case X86::PUSH64r:
119  case X86::PUSH64rmm:
120  case X86::PUSH64rmr:
121  case X86::PUSH64i32:
122  case X86::POP32r:
123  case X86::POP64r:
124    return true;
125  default:
126    return false;
127  }
128}
129
130void X86WinAllocaExpander::computeLowerings(MachineFunction &MF,
131                                            LoweringMap &Lowerings) {
132  // Do a one-pass reverse post-order walk of the CFG to conservatively estimate
133  // the offset between the stack pointer and the lowest touched part of the
134  // stack, and use that to decide how to lower each WinAlloca instruction.
135
136  // Initialize OutOffset[B], the stack offset at exit from B, to something big.
137  DenseMap<MachineBasicBlock *, int64_t> OutOffset;
138  for (MachineBasicBlock &MBB : MF)
139    OutOffset[&MBB] = INT32_MAX;
140
141  // Note: we don't know the offset at the start of the entry block since the
142  // prologue hasn't been inserted yet, and how much that will adjust the stack
143  // pointer depends on register spills, which have not been computed yet.
144
145  // Compute the reverse post-order.
146  ReversePostOrderTraversal<MachineFunction*> RPO(&MF);
147
148  for (MachineBasicBlock *MBB : RPO) {
149    int64_t Offset = -1;
150    for (MachineBasicBlock *Pred : MBB->predecessors())
151      Offset = std::max(Offset, OutOffset[Pred]);
152    if (Offset == -1) Offset = INT32_MAX;
153
154    for (MachineInstr &MI : *MBB) {
155      if (MI.getOpcode() == X86::WIN_ALLOCA_32 ||
156          MI.getOpcode() == X86::WIN_ALLOCA_64) {
157        // A WinAlloca moves StackPtr, and potentially touches it.
158        int64_t Amount = getWinAllocaAmount(&MI, MRI);
159        Lowering L = getLowering(Offset, Amount);
160        Lowerings[&MI] = L;
161        switch (L) {
162        case Sub:
163          Offset += Amount;
164          break;
165        case TouchAndSub:
166          Offset = Amount;
167          break;
168        case Probe:
169          Offset = 0;
170          break;
171        }
172      } else if (MI.isCall() || isPushPop(MI)) {
173        // Calls, pushes and pops touch the tip of the stack.
174        Offset = 0;
175      } else if (MI.getOpcode() == X86::ADJCALLSTACKUP32 ||
176                 MI.getOpcode() == X86::ADJCALLSTACKUP64) {
177        Offset -= MI.getOperand(0).getImm();
178      } else if (MI.getOpcode() == X86::ADJCALLSTACKDOWN32 ||
179                 MI.getOpcode() == X86::ADJCALLSTACKDOWN64) {
180        Offset += MI.getOperand(0).getImm();
181      } else if (MI.modifiesRegister(StackPtr, TRI)) {
182        // Any other modification of SP means we've lost track of it.
183        Offset = INT32_MAX;
184      }
185    }
186
187    OutOffset[MBB] = Offset;
188  }
189}
190
191static unsigned getSubOpcode(bool Is64Bit, int64_t Amount) {
192  if (Is64Bit)
193    return isInt<8>(Amount) ? X86::SUB64ri8 : X86::SUB64ri32;
194  return isInt<8>(Amount) ? X86::SUB32ri8 : X86::SUB32ri;
195}
196
197void X86WinAllocaExpander::lower(MachineInstr* MI, Lowering L) {
198  DebugLoc DL = MI->getDebugLoc();
199  MachineBasicBlock *MBB = MI->getParent();
200  MachineBasicBlock::iterator I = *MI;
201
202  int64_t Amount = getWinAllocaAmount(MI, MRI);
203  if (Amount == 0) {
204    MI->eraseFromParent();
205    return;
206  }
207
208  // These two variables differ on x32, which is a 64-bit target with a
209  // 32-bit alloca.
210  bool Is64Bit = STI->is64Bit();
211  bool Is64BitAlloca = MI->getOpcode() == X86::WIN_ALLOCA_64;
212  assert(SlotSize == 4 || SlotSize == 8);
213
214  switch (L) {
215  case TouchAndSub: {
216    assert(Amount >= SlotSize);
217
218    // Use a push to touch the top of the stack.
219    unsigned RegA = Is64Bit ? X86::RAX : X86::EAX;
220    BuildMI(*MBB, I, DL, TII->get(Is64Bit ? X86::PUSH64r : X86::PUSH32r))
221        .addReg(RegA, RegState::Undef);
222    Amount -= SlotSize;
223    if (!Amount)
224      break;
225
226    // Fall through to make any remaining adjustment.
227    LLVM_FALLTHROUGH;
228  }
229  case Sub:
230    assert(Amount > 0);
231    if (Amount == SlotSize) {
232      // Use push to save size.
233      unsigned RegA = Is64Bit ? X86::RAX : X86::EAX;
234      BuildMI(*MBB, I, DL, TII->get(Is64Bit ? X86::PUSH64r : X86::PUSH32r))
235          .addReg(RegA, RegState::Undef);
236    } else {
237      // Sub.
238      BuildMI(*MBB, I, DL,
239              TII->get(getSubOpcode(Is64BitAlloca, Amount)), StackPtr)
240          .addReg(StackPtr)
241          .addImm(Amount);
242    }
243    break;
244  case Probe:
245    if (!NoStackArgProbe) {
246      // The probe lowering expects the amount in RAX/EAX.
247      unsigned RegA = Is64BitAlloca ? X86::RAX : X86::EAX;
248      BuildMI(*MBB, MI, DL, TII->get(TargetOpcode::COPY), RegA)
249          .addReg(MI->getOperand(0).getReg());
250
251      // Do the probe.
252      STI->getFrameLowering()->emitStackProbe(*MBB->getParent(), *MBB, MI, DL,
253                                              /*InProlog=*/false);
254    } else {
255      // Sub
256      BuildMI(*MBB, I, DL,
257              TII->get(Is64BitAlloca ? X86::SUB64rr : X86::SUB32rr), StackPtr)
258          .addReg(StackPtr)
259          .addReg(MI->getOperand(0).getReg());
260    }
261    break;
262  }
263
264  Register AmountReg = MI->getOperand(0).getReg();
265  MI->eraseFromParent();
266
267  // Delete the definition of AmountReg.
268  if (MRI->use_empty(AmountReg))
269    if (MachineInstr *AmountDef = MRI->getUniqueVRegDef(AmountReg))
270      AmountDef->eraseFromParent();
271}
272
273bool X86WinAllocaExpander::runOnMachineFunction(MachineFunction &MF) {
274  if (!MF.getInfo<X86MachineFunctionInfo>()->hasWinAlloca())
275    return false;
276
277  MRI = &MF.getRegInfo();
278  STI = &MF.getSubtarget<X86Subtarget>();
279  TII = STI->getInstrInfo();
280  TRI = STI->getRegisterInfo();
281  StackPtr = TRI->getStackRegister();
282  SlotSize = TRI->getSlotSize();
283
284  StackProbeSize = 4096;
285  if (MF.getFunction().hasFnAttribute("stack-probe-size")) {
286    MF.getFunction()
287        .getFnAttribute("stack-probe-size")
288        .getValueAsString()
289        .getAsInteger(0, StackProbeSize);
290  }
291  NoStackArgProbe = MF.getFunction().hasFnAttribute("no-stack-arg-probe");
292  if (NoStackArgProbe)
293    StackProbeSize = INT64_MAX;
294
295  LoweringMap Lowerings;
296  computeLowerings(MF, Lowerings);
297  for (auto &P : Lowerings)
298    lower(P.first, P.second);
299
300  return true;
301}
302