1 //===----- X86DynAllocaExpander.cpp - Expand DynAlloca 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 DynAlloca 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/MapVector.h"
23 #include "llvm/ADT/PostOrderIterator.h"
24 #include "llvm/CodeGen/MachineFunctionPass.h"
25 #include "llvm/CodeGen/MachineInstrBuilder.h"
26 #include "llvm/CodeGen/MachineRegisterInfo.h"
27 #include "llvm/CodeGen/Passes.h"
28 #include "llvm/CodeGen/TargetInstrInfo.h"
29 #include "llvm/IR/Function.h"
30 #include "llvm/Support/raw_ostream.h"
31
32 using namespace llvm;
33
34 namespace {
35
36 class X86DynAllocaExpander : public MachineFunctionPass {
37 public:
X86DynAllocaExpander()38 X86DynAllocaExpander() : MachineFunctionPass(ID) {}
39
40 bool runOnMachineFunction(MachineFunction &MF) override;
41
42 private:
43 /// Strategies for lowering a DynAlloca.
44 enum Lowering { TouchAndSub, Sub, Probe };
45
46 /// Deterministic-order map from DynAlloca instruction to desired lowering.
47 typedef MapVector<MachineInstr*, Lowering> LoweringMap;
48
49 /// Compute which lowering to use for each DynAlloca instruction.
50 void computeLowerings(MachineFunction &MF, LoweringMap& Lowerings);
51
52 /// Get the appropriate lowering based on current offset and amount.
53 Lowering getLowering(int64_t CurrentOffset, int64_t AllocaAmount);
54
55 /// Lower a DynAlloca instruction.
56 void lower(MachineInstr* MI, Lowering L);
57
58 MachineRegisterInfo *MRI = nullptr;
59 const X86Subtarget *STI = nullptr;
60 const TargetInstrInfo *TII = nullptr;
61 const X86RegisterInfo *TRI = nullptr;
62 unsigned StackPtr = 0;
63 unsigned SlotSize = 0;
64 int64_t StackProbeSize = 0;
65 bool NoStackArgProbe = false;
66
getPassName() const67 StringRef getPassName() const override { return "X86 DynAlloca Expander"; }
68 static char ID;
69 };
70
71 char X86DynAllocaExpander::ID = 0;
72
73 } // end anonymous namespace
74
createX86DynAllocaExpander()75 FunctionPass *llvm::createX86DynAllocaExpander() {
76 return new X86DynAllocaExpander();
77 }
78
79 /// Return the allocation amount for a DynAlloca instruction, or -1 if unknown.
getDynAllocaAmount(MachineInstr * MI,MachineRegisterInfo * MRI)80 static int64_t getDynAllocaAmount(MachineInstr *MI, MachineRegisterInfo *MRI) {
81 assert(MI->getOpcode() == X86::DYN_ALLOCA_32 ||
82 MI->getOpcode() == X86::DYN_ALLOCA_64);
83 assert(MI->getOperand(0).isReg());
84
85 Register AmountReg = MI->getOperand(0).getReg();
86 MachineInstr *Def = MRI->getUniqueVRegDef(AmountReg);
87
88 if (!Def ||
89 (Def->getOpcode() != X86::MOV32ri && Def->getOpcode() != X86::MOV64ri) ||
90 !Def->getOperand(1).isImm())
91 return -1;
92
93 return Def->getOperand(1).getImm();
94 }
95
96 X86DynAllocaExpander::Lowering
getLowering(int64_t CurrentOffset,int64_t AllocaAmount)97 X86DynAllocaExpander::getLowering(int64_t CurrentOffset,
98 int64_t AllocaAmount) {
99 // For a non-constant amount or a large amount, we have to probe.
100 if (AllocaAmount < 0 || AllocaAmount > StackProbeSize)
101 return Probe;
102
103 // If it fits within the safe region of the stack, just subtract.
104 if (CurrentOffset + AllocaAmount <= StackProbeSize)
105 return Sub;
106
107 // Otherwise, touch the current tip of the stack, then subtract.
108 return TouchAndSub;
109 }
110
isPushPop(const MachineInstr & MI)111 static bool isPushPop(const MachineInstr &MI) {
112 switch (MI.getOpcode()) {
113 case X86::PUSH32r:
114 case X86::PUSH32rmm:
115 case X86::PUSH32rmr:
116 case X86::PUSH32i:
117 case X86::PUSH64r:
118 case X86::PUSH64rmm:
119 case X86::PUSH64rmr:
120 case X86::PUSH64i32:
121 case X86::POP32r:
122 case X86::POP64r:
123 return true;
124 default:
125 return false;
126 }
127 }
128
computeLowerings(MachineFunction & MF,LoweringMap & Lowerings)129 void X86DynAllocaExpander::computeLowerings(MachineFunction &MF,
130 LoweringMap &Lowerings) {
131 // Do a one-pass reverse post-order walk of the CFG to conservatively estimate
132 // the offset between the stack pointer and the lowest touched part of the
133 // stack, and use that to decide how to lower each DynAlloca instruction.
134
135 // Initialize OutOffset[B], the stack offset at exit from B, to something big.
136 DenseMap<MachineBasicBlock *, int64_t> OutOffset;
137 for (MachineBasicBlock &MBB : MF)
138 OutOffset[&MBB] = INT32_MAX;
139
140 // Note: we don't know the offset at the start of the entry block since the
141 // prologue hasn't been inserted yet, and how much that will adjust the stack
142 // pointer depends on register spills, which have not been computed yet.
143
144 // Compute the reverse post-order.
145 ReversePostOrderTraversal<MachineFunction*> RPO(&MF);
146
147 for (MachineBasicBlock *MBB : RPO) {
148 int64_t Offset = -1;
149 for (MachineBasicBlock *Pred : MBB->predecessors())
150 Offset = std::max(Offset, OutOffset[Pred]);
151 if (Offset == -1) Offset = INT32_MAX;
152
153 for (MachineInstr &MI : *MBB) {
154 if (MI.getOpcode() == X86::DYN_ALLOCA_32 ||
155 MI.getOpcode() == X86::DYN_ALLOCA_64) {
156 // A DynAlloca moves StackPtr, and potentially touches it.
157 int64_t Amount = getDynAllocaAmount(&MI, MRI);
158 Lowering L = getLowering(Offset, Amount);
159 Lowerings[&MI] = L;
160 switch (L) {
161 case Sub:
162 Offset += Amount;
163 break;
164 case TouchAndSub:
165 Offset = Amount;
166 break;
167 case Probe:
168 Offset = 0;
169 break;
170 }
171 } else if (MI.isCall() || isPushPop(MI)) {
172 // Calls, pushes and pops touch the tip of the stack.
173 Offset = 0;
174 } else if (MI.getOpcode() == X86::ADJCALLSTACKUP32 ||
175 MI.getOpcode() == X86::ADJCALLSTACKUP64) {
176 Offset -= MI.getOperand(0).getImm();
177 } else if (MI.getOpcode() == X86::ADJCALLSTACKDOWN32 ||
178 MI.getOpcode() == X86::ADJCALLSTACKDOWN64) {
179 Offset += MI.getOperand(0).getImm();
180 } else if (MI.modifiesRegister(StackPtr, TRI)) {
181 // Any other modification of SP means we've lost track of it.
182 Offset = INT32_MAX;
183 }
184 }
185
186 OutOffset[MBB] = Offset;
187 }
188 }
189
getSubOpcode(bool Is64Bit)190 static unsigned getSubOpcode(bool Is64Bit) {
191 if (Is64Bit)
192 return X86::SUB64ri32;
193 return X86::SUB32ri;
194 }
195
lower(MachineInstr * MI,Lowering L)196 void X86DynAllocaExpander::lower(MachineInstr *MI, Lowering L) {
197 const DebugLoc &DL = MI->getDebugLoc();
198 MachineBasicBlock *MBB = MI->getParent();
199 MachineBasicBlock::iterator I = *MI;
200
201 int64_t Amount = getDynAllocaAmount(MI, MRI);
202 if (Amount == 0) {
203 MI->eraseFromParent();
204 return;
205 }
206
207 // These two variables differ on x32, which is a 64-bit target with a
208 // 32-bit alloca.
209 bool Is64Bit = STI->is64Bit();
210 bool Is64BitAlloca = MI->getOpcode() == X86::DYN_ALLOCA_64;
211 assert(SlotSize == 4 || SlotSize == 8);
212
213 std::optional<MachineFunction::DebugInstrOperandPair> InstrNum;
214 if (unsigned Num = MI->peekDebugInstrNum()) {
215 // Operand 2 of DYN_ALLOCAs contains the stack def.
216 InstrNum = {Num, 2};
217 }
218
219 switch (L) {
220 case TouchAndSub: {
221 assert(Amount >= SlotSize);
222
223 // Use a push to touch the top of the stack.
224 unsigned RegA = Is64Bit ? X86::RAX : X86::EAX;
225 BuildMI(*MBB, I, DL, TII->get(Is64Bit ? X86::PUSH64r : X86::PUSH32r))
226 .addReg(RegA, RegState::Undef);
227 Amount -= SlotSize;
228 if (!Amount)
229 break;
230
231 // Fall through to make any remaining adjustment.
232 [[fallthrough]];
233 }
234 case Sub:
235 assert(Amount > 0);
236 if (Amount == SlotSize) {
237 // Use push to save size.
238 unsigned RegA = Is64Bit ? X86::RAX : X86::EAX;
239 BuildMI(*MBB, I, DL, TII->get(Is64Bit ? X86::PUSH64r : X86::PUSH32r))
240 .addReg(RegA, RegState::Undef);
241 } else {
242 // Sub.
243 BuildMI(*MBB, I, DL, TII->get(getSubOpcode(Is64BitAlloca)), StackPtr)
244 .addReg(StackPtr)
245 .addImm(Amount);
246 }
247 break;
248 case Probe:
249 if (!NoStackArgProbe) {
250 // The probe lowering expects the amount in RAX/EAX.
251 unsigned RegA = Is64BitAlloca ? X86::RAX : X86::EAX;
252 BuildMI(*MBB, MI, DL, TII->get(TargetOpcode::COPY), RegA)
253 .addReg(MI->getOperand(0).getReg());
254
255 // Do the probe.
256 STI->getFrameLowering()->emitStackProbe(*MBB->getParent(), *MBB, MI, DL,
257 /*InProlog=*/false, InstrNum);
258 } else {
259 // Sub
260 BuildMI(*MBB, I, DL,
261 TII->get(Is64BitAlloca ? X86::SUB64rr : X86::SUB32rr), StackPtr)
262 .addReg(StackPtr)
263 .addReg(MI->getOperand(0).getReg());
264 }
265 break;
266 }
267
268 Register AmountReg = MI->getOperand(0).getReg();
269 MI->eraseFromParent();
270
271 // Delete the definition of AmountReg.
272 if (MRI->use_empty(AmountReg))
273 if (MachineInstr *AmountDef = MRI->getUniqueVRegDef(AmountReg))
274 AmountDef->eraseFromParent();
275 }
276
runOnMachineFunction(MachineFunction & MF)277 bool X86DynAllocaExpander::runOnMachineFunction(MachineFunction &MF) {
278 if (!MF.getInfo<X86MachineFunctionInfo>()->hasDynAlloca())
279 return false;
280
281 MRI = &MF.getRegInfo();
282 STI = &MF.getSubtarget<X86Subtarget>();
283 TII = STI->getInstrInfo();
284 TRI = STI->getRegisterInfo();
285 StackPtr = TRI->getStackRegister();
286 SlotSize = TRI->getSlotSize();
287 StackProbeSize = STI->getTargetLowering()->getStackProbeSize(MF);
288 NoStackArgProbe = MF.getFunction().hasFnAttribute("no-stack-arg-probe");
289 if (NoStackArgProbe)
290 StackProbeSize = INT64_MAX;
291
292 LoweringMap Lowerings;
293 computeLowerings(MF, Lowerings);
294 for (auto &P : Lowerings)
295 lower(P.first, P.second);
296
297 return true;
298 }
299