1 //===----- X86WinAllocaExpander.cpp - Expand WinAlloca pseudo instruction -===//
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 // This file defines a pass that expands WinAlloca pseudo-instructions.
11 //
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.
15 //
16 //===----------------------------------------------------------------------===//
17 
18 #include "X86.h"
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/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 X86WinAllocaExpander : public MachineFunctionPass {
37 public:
38   X86WinAllocaExpander() : MachineFunctionPass(ID) {}
39 
40   bool runOnMachineFunction(MachineFunction &MF) override;
41 
42 private:
43   /// Strategies for lowering a WinAlloca.
44   enum Lowering { TouchAndSub, Sub, Probe };
45 
46   /// Deterministic-order map from WinAlloca instruction to desired lowering.
47   typedef MapVector<MachineInstr*, Lowering> LoweringMap;
48 
49   /// Compute which lowering to use for each WinAlloca 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 WinAlloca instruction.
56   void lower(MachineInstr* MI, Lowering L);
57 
58   MachineRegisterInfo *MRI;
59   const X86Subtarget *STI;
60   const TargetInstrInfo *TII;
61   const X86RegisterInfo *TRI;
62   unsigned StackPtr;
63   unsigned SlotSize;
64   int64_t StackProbeSize;
65   bool NoStackArgProbe;
66 
67   StringRef getPassName() const override { return "X86 WinAlloca Expander"; }
68   static char ID;
69 };
70 
71 char X86WinAllocaExpander::ID = 0;
72 
73 } // end anonymous namespace
74 
75 FunctionPass *llvm::createX86WinAllocaExpander() {
76   return new X86WinAllocaExpander();
77 }
78 
79 /// Return the allocation amount for a WinAlloca instruction, or -1 if unknown.
80 static int64_t getWinAllocaAmount(MachineInstr *MI, MachineRegisterInfo *MRI) {
81   assert(MI->getOpcode() == X86::WIN_ALLOCA_32 ||
82          MI->getOpcode() == X86::WIN_ALLOCA_64);
83   assert(MI->getOperand(0).isReg());
84 
85   unsigned AmountReg = MI->getOperand(0).getReg();
86   MachineInstr *Def = MRI->getUniqueVRegDef(AmountReg);
87 
88   // Look through copies.
89   while (Def && Def->isCopy() && Def->getOperand(1).isReg())
90     Def = MRI->getUniqueVRegDef(Def->getOperand(1).getReg());
91 
92   if (!Def ||
93       (Def->getOpcode() != X86::MOV32ri && Def->getOpcode() != X86::MOV64ri) ||
94       !Def->getOperand(1).isImm())
95     return -1;
96 
97   return Def->getOperand(1).getImm();
98 }
99 
100 X86WinAllocaExpander::Lowering
101 X86WinAllocaExpander::getLowering(int64_t CurrentOffset,
102                                   int64_t AllocaAmount) {
103   // For a non-constant amount or a large amount, we have to probe.
104   if (AllocaAmount < 0 || AllocaAmount > StackProbeSize)
105     return Probe;
106 
107   // If it fits within the safe region of the stack, just subtract.
108   if (CurrentOffset + AllocaAmount <= StackProbeSize)
109     return Sub;
110 
111   // Otherwise, touch the current tip of the stack, then subtract.
112   return TouchAndSub;
113 }
114 
115 static bool isPushPop(const MachineInstr &MI) {
116   switch (MI.getOpcode()) {
117   case X86::PUSH32i8:
118   case X86::PUSH32r:
119   case X86::PUSH32rmm:
120   case X86::PUSH32rmr:
121   case X86::PUSHi32:
122   case X86::PUSH64i8:
123   case X86::PUSH64r:
124   case X86::PUSH64rmm:
125   case X86::PUSH64rmr:
126   case X86::PUSH64i32:
127   case X86::POP32r:
128   case X86::POP64r:
129     return true;
130   default:
131     return false;
132   }
133 }
134 
135 void X86WinAllocaExpander::computeLowerings(MachineFunction &MF,
136                                             LoweringMap &Lowerings) {
137   // Do a one-pass reverse post-order walk of the CFG to conservatively estimate
138   // the offset between the stack pointer and the lowest touched part of the
139   // stack, and use that to decide how to lower each WinAlloca instruction.
140 
141   // Initialize OutOffset[B], the stack offset at exit from B, to something big.
142   DenseMap<MachineBasicBlock *, int64_t> OutOffset;
143   for (MachineBasicBlock &MBB : MF)
144     OutOffset[&MBB] = INT32_MAX;
145 
146   // Note: we don't know the offset at the start of the entry block since the
147   // prologue hasn't been inserted yet, and how much that will adjust the stack
148   // pointer depends on register spills, which have not been computed yet.
149 
150   // Compute the reverse post-order.
151   ReversePostOrderTraversal<MachineFunction*> RPO(&MF);
152 
153   for (MachineBasicBlock *MBB : RPO) {
154     int64_t Offset = -1;
155     for (MachineBasicBlock *Pred : MBB->predecessors())
156       Offset = std::max(Offset, OutOffset[Pred]);
157     if (Offset == -1) Offset = INT32_MAX;
158 
159     for (MachineInstr &MI : *MBB) {
160       if (MI.getOpcode() == X86::WIN_ALLOCA_32 ||
161           MI.getOpcode() == X86::WIN_ALLOCA_64) {
162         // A WinAlloca moves StackPtr, and potentially touches it.
163         int64_t Amount = getWinAllocaAmount(&MI, MRI);
164         Lowering L = getLowering(Offset, Amount);
165         Lowerings[&MI] = L;
166         switch (L) {
167         case Sub:
168           Offset += Amount;
169           break;
170         case TouchAndSub:
171           Offset = Amount;
172           break;
173         case Probe:
174           Offset = 0;
175           break;
176         }
177       } else if (MI.isCall() || isPushPop(MI)) {
178         // Calls, pushes and pops touch the tip of the stack.
179         Offset = 0;
180       } else if (MI.getOpcode() == X86::ADJCALLSTACKUP32 ||
181                  MI.getOpcode() == X86::ADJCALLSTACKUP64) {
182         Offset -= MI.getOperand(0).getImm();
183       } else if (MI.getOpcode() == X86::ADJCALLSTACKDOWN32 ||
184                  MI.getOpcode() == X86::ADJCALLSTACKDOWN64) {
185         Offset += MI.getOperand(0).getImm();
186       } else if (MI.modifiesRegister(StackPtr, TRI)) {
187         // Any other modification of SP means we've lost track of it.
188         Offset = INT32_MAX;
189       }
190     }
191 
192     OutOffset[MBB] = Offset;
193   }
194 }
195 
196 static unsigned getSubOpcode(bool Is64Bit, int64_t Amount) {
197   if (Is64Bit)
198     return isInt<8>(Amount) ? X86::SUB64ri8 : X86::SUB64ri32;
199   return isInt<8>(Amount) ? X86::SUB32ri8 : X86::SUB32ri;
200 }
201 
202 void X86WinAllocaExpander::lower(MachineInstr* MI, Lowering L) {
203   DebugLoc DL = MI->getDebugLoc();
204   MachineBasicBlock *MBB = MI->getParent();
205   MachineBasicBlock::iterator I = *MI;
206 
207   int64_t Amount = getWinAllocaAmount(MI, MRI);
208   if (Amount == 0) {
209     MI->eraseFromParent();
210     return;
211   }
212 
213   bool Is64Bit = STI->is64Bit();
214   assert(SlotSize == 4 || SlotSize == 8);
215   unsigned RegA = (SlotSize == 8) ? X86::RAX : X86::EAX;
216 
217   switch (L) {
218   case TouchAndSub:
219     assert(Amount >= SlotSize);
220 
221     // Use a push to touch the top of the stack.
222     BuildMI(*MBB, I, DL, TII->get(Is64Bit ? X86::PUSH64r : X86::PUSH32r))
223         .addReg(RegA, RegState::Undef);
224     Amount -= SlotSize;
225     if (!Amount)
226       break;
227 
228     // Fall through to make any remaining adjustment.
229     LLVM_FALLTHROUGH;
230   case Sub:
231     assert(Amount > 0);
232     if (Amount == SlotSize) {
233       // Use push to save size.
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, TII->get(getSubOpcode(Is64Bit, Amount)), StackPtr)
239           .addReg(StackPtr)
240           .addImm(Amount);
241     }
242     break;
243   case Probe:
244     if (!NoStackArgProbe) {
245       // The probe lowering expects the amount in RAX/EAX.
246       BuildMI(*MBB, MI, DL, TII->get(TargetOpcode::COPY), RegA)
247           .addReg(MI->getOperand(0).getReg());
248 
249       // Do the probe.
250       STI->getFrameLowering()->emitStackProbe(*MBB->getParent(), *MBB, MI, DL,
251                                               /*InPrologue=*/false);
252     } else {
253       // Sub
254       BuildMI(*MBB, I, DL, TII->get(Is64Bit ? X86::SUB64rr : X86::SUB32rr),
255               StackPtr)
256           .addReg(StackPtr)
257           .addReg(MI->getOperand(0).getReg());
258     }
259     break;
260   }
261 
262   unsigned AmountReg = MI->getOperand(0).getReg();
263   MI->eraseFromParent();
264 
265   // Delete the definition of AmountReg, possibly walking a chain of copies.
266   for (;;) {
267     if (!MRI->use_empty(AmountReg))
268       break;
269     MachineInstr *AmountDef = MRI->getUniqueVRegDef(AmountReg);
270     if (!AmountDef)
271       break;
272     if (AmountDef->isCopy() && AmountDef->getOperand(1).isReg())
273       AmountReg = AmountDef->getOperand(1).isReg();
274     AmountDef->eraseFromParent();
275     break;
276   }
277 }
278 
279 bool X86WinAllocaExpander::runOnMachineFunction(MachineFunction &MF) {
280   if (!MF.getInfo<X86MachineFunctionInfo>()->hasWinAlloca())
281     return false;
282 
283   MRI = &MF.getRegInfo();
284   STI = &MF.getSubtarget<X86Subtarget>();
285   TII = STI->getInstrInfo();
286   TRI = STI->getRegisterInfo();
287   StackPtr = TRI->getStackRegister();
288   SlotSize = TRI->getSlotSize();
289 
290   StackProbeSize = 4096;
291   if (MF.getFunction().hasFnAttribute("stack-probe-size")) {
292     MF.getFunction()
293         .getFnAttribute("stack-probe-size")
294         .getValueAsString()
295         .getAsInteger(0, StackProbeSize);
296   }
297   NoStackArgProbe = MF.getFunction().hasFnAttribute("no-stack-arg-probe");
298   if (NoStackArgProbe)
299     StackProbeSize = INT64_MAX;
300 
301   LoweringMap Lowerings;
302   computeLowerings(MF, Lowerings);
303   for (auto &P : Lowerings)
304     lower(P.first, P.second);
305 
306   return true;
307 }
308