1 //===----- X86WinAllocaExpander.cpp - Expand WinAlloca pseudo instruction -===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file defines a pass that expands WinAlloca pseudo-instructions.
12 // It performs a conservative analysis to determine whether each allocation
13 // falls within a region of the stack that is safe to use, or whether stack
14 // probes must be emitted.
16 //===----------------------------------------------------------------------===//
19 #include "X86InstrBuilder.h"
20 #include "X86InstrInfo.h"
21 #include "X86MachineFunctionInfo.h"
22 #include "X86Subtarget.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/IR/Function.h"
29 #include "llvm/Support/raw_ostream.h"
30 #include "llvm/Target/TargetInstrInfo.h"
36 class X86WinAllocaExpander : public MachineFunctionPass {
38 X86WinAllocaExpander() : MachineFunctionPass(ID) {}
40 bool runOnMachineFunction(MachineFunction &MF) override;
43 /// Strategies for lowering a WinAlloca.
44 enum Lowering { TouchAndSub, Sub, Probe };
46 /// Deterministic-order map from WinAlloca instruction to desired lowering.
47 typedef MapVector<MachineInstr*, Lowering> LoweringMap;
49 /// Compute which lowering to use for each WinAlloca instruction.
50 void computeLowerings(MachineFunction &MF, LoweringMap& Lowerings);
52 /// Get the appropriate lowering based on current offset and amount.
53 Lowering getLowering(int64_t CurrentOffset, int64_t AllocaAmount);
55 /// Lower a WinAlloca instruction.
56 void lower(MachineInstr* MI, Lowering L);
58 MachineRegisterInfo *MRI;
59 const X86Subtarget *STI;
60 const TargetInstrInfo *TII;
61 const X86RegisterInfo *TRI;
64 int64_t StackProbeSize;
66 StringRef getPassName() const override { return "X86 WinAlloca Expander"; }
70 char X86WinAllocaExpander::ID = 0;
72 } // end anonymous namespace
74 FunctionPass *llvm::createX86WinAllocaExpander() {
75 return new X86WinAllocaExpander();
78 /// Return the allocation amount for a WinAlloca instruction, or -1 if unknown.
79 static 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());
84 unsigned AmountReg = MI->getOperand(0).getReg();
85 MachineInstr *Def = MRI->getUniqueVRegDef(AmountReg);
87 // Look through copies.
88 while (Def && Def->isCopy() && Def->getOperand(1).isReg())
89 Def = MRI->getUniqueVRegDef(Def->getOperand(1).getReg());
92 (Def->getOpcode() != X86::MOV32ri && Def->getOpcode() != X86::MOV64ri) ||
93 !Def->getOperand(1).isImm())
96 return Def->getOperand(1).getImm();
99 X86WinAllocaExpander::Lowering
100 X86WinAllocaExpander::getLowering(int64_t CurrentOffset,
101 int64_t AllocaAmount) {
102 // For a non-constant amount or a large amount, we have to probe.
103 if (AllocaAmount < 0 || AllocaAmount > StackProbeSize)
106 // If it fits within the safe region of the stack, just subtract.
107 if (CurrentOffset + AllocaAmount <= StackProbeSize)
110 // Otherwise, touch the current tip of the stack, then subtract.
114 static bool isPushPop(const MachineInstr &MI) {
115 switch (MI.getOpcode()) {
134 void X86WinAllocaExpander::computeLowerings(MachineFunction &MF,
135 LoweringMap &Lowerings) {
136 // Do a one-pass reverse post-order walk of the CFG to conservatively estimate
137 // the offset between the stack pointer and the lowest touched part of the
138 // stack, and use that to decide how to lower each WinAlloca instruction.
140 // Initialize OutOffset[B], the stack offset at exit from B, to something big.
141 DenseMap<MachineBasicBlock *, int64_t> OutOffset;
142 for (MachineBasicBlock &MBB : MF)
143 OutOffset[&MBB] = INT32_MAX;
145 // Note: we don't know the offset at the start of the entry block since the
146 // prologue hasn't been inserted yet, and how much that will adjust the stack
147 // pointer depends on register spills, which have not been computed yet.
149 // Compute the reverse post-order.
150 ReversePostOrderTraversal<MachineFunction*> RPO(&MF);
152 for (MachineBasicBlock *MBB : RPO) {
154 for (MachineBasicBlock *Pred : MBB->predecessors())
155 Offset = std::max(Offset, OutOffset[Pred]);
156 if (Offset == -1) Offset = INT32_MAX;
158 for (MachineInstr &MI : *MBB) {
159 if (MI.getOpcode() == X86::WIN_ALLOCA_32 ||
160 MI.getOpcode() == X86::WIN_ALLOCA_64) {
161 // A WinAlloca moves StackPtr, and potentially touches it.
162 int64_t Amount = getWinAllocaAmount(&MI, MRI);
163 Lowering L = getLowering(Offset, Amount);
176 } else if (MI.isCall() || isPushPop(MI)) {
177 // Calls, pushes and pops touch the tip of the stack.
179 } else if (MI.getOpcode() == X86::ADJCALLSTACKUP32 ||
180 MI.getOpcode() == X86::ADJCALLSTACKUP64) {
181 Offset -= MI.getOperand(0).getImm();
182 } else if (MI.getOpcode() == X86::ADJCALLSTACKDOWN32 ||
183 MI.getOpcode() == X86::ADJCALLSTACKDOWN64) {
184 Offset += MI.getOperand(0).getImm();
185 } else if (MI.modifiesRegister(StackPtr, TRI)) {
186 // Any other modification of SP means we've lost track of it.
191 OutOffset[MBB] = Offset;
195 static unsigned getSubOpcode(bool Is64Bit, int64_t Amount) {
197 return isInt<8>(Amount) ? X86::SUB64ri8 : X86::SUB64ri32;
198 return isInt<8>(Amount) ? X86::SUB32ri8 : X86::SUB32ri;
201 void X86WinAllocaExpander::lower(MachineInstr* MI, Lowering L) {
202 DebugLoc DL = MI->getDebugLoc();
203 MachineBasicBlock *MBB = MI->getParent();
204 MachineBasicBlock::iterator I = *MI;
206 int64_t Amount = getWinAllocaAmount(MI, MRI);
208 MI->eraseFromParent();
212 bool Is64Bit = STI->is64Bit();
213 assert(SlotSize == 4 || SlotSize == 8);
214 unsigned RegA = (SlotSize == 8) ? X86::RAX : X86::EAX;
218 assert(Amount >= SlotSize);
220 // Use a push to touch the top of the stack.
221 BuildMI(*MBB, I, DL, TII->get(Is64Bit ? X86::PUSH64r : X86::PUSH32r))
222 .addReg(RegA, RegState::Undef);
227 // Fall through to make any remaining adjustment.
231 if (Amount == SlotSize) {
232 // Use push to save size.
233 BuildMI(*MBB, I, DL, TII->get(Is64Bit ? X86::PUSH64r : X86::PUSH32r))
234 .addReg(RegA, RegState::Undef);
237 BuildMI(*MBB, I, DL, TII->get(getSubOpcode(Is64Bit, Amount)), StackPtr)
243 // The probe lowering expects the amount in RAX/EAX.
244 BuildMI(*MBB, MI, DL, TII->get(TargetOpcode::COPY), RegA)
245 .addReg(MI->getOperand(0).getReg());
248 STI->getFrameLowering()->emitStackProbe(*MBB->getParent(), *MBB, MI, DL,
249 /*InPrologue=*/false);
253 unsigned AmountReg = MI->getOperand(0).getReg();
254 MI->eraseFromParent();
256 // Delete the definition of AmountReg, possibly walking a chain of copies.
258 if (!MRI->use_empty(AmountReg))
260 MachineInstr *AmountDef = MRI->getUniqueVRegDef(AmountReg);
263 if (AmountDef->isCopy() && AmountDef->getOperand(1).isReg())
264 AmountReg = AmountDef->getOperand(1).isReg();
265 AmountDef->eraseFromParent();
270 bool X86WinAllocaExpander::runOnMachineFunction(MachineFunction &MF) {
271 if (!MF.getInfo<X86MachineFunctionInfo>()->hasWinAlloca())
274 MRI = &MF.getRegInfo();
275 STI = &MF.getSubtarget<X86Subtarget>();
276 TII = STI->getInstrInfo();
277 TRI = STI->getRegisterInfo();
278 StackPtr = TRI->getStackRegister();
279 SlotSize = TRI->getSlotSize();
281 StackProbeSize = 4096;
282 if (MF.getFunction()->hasFnAttribute("stack-probe-size")) {
284 ->getFnAttribute("stack-probe-size")
286 .getAsInteger(0, StackProbeSize);
289 LoweringMap Lowerings;
290 computeLowerings(MF, Lowerings);
291 for (auto &P : Lowerings)
292 lower(P.first, P.second);