lib/Target/X86/X86WinAllocaExpander.cpp

   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
  31 using namespace llvm;
  32
  33 namespace {
  34
  35 class X86WinAllocaExpander : public MachineFunctionPass {
  36 public:
  37   X86WinAllocaExpander() : MachineFunctionPass(ID) {}
  38
  39   bool runOnMachineFunction(MachineFunction &MF) override;
  40
  41 private:
  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;
  58   const X86Subtarget *STI;
  59   const TargetInstrInfo *TII;
  60   const X86RegisterInfo *TRI;
  61   unsigned StackPtr;
  62   unsigned SlotSize;
  63   int64_t StackProbeSize;
  64   bool NoStackArgProbe;
  65
  66   StringRef getPassName() const override { return "X86 WinAlloca Expander"; }
  67   static char ID;
  68 };
  69
  70 char X86WinAllocaExpander::ID = 0;
  71
  72 } // end anonymous namespace
  73
  74 FunctionPass *llvm::createX86WinAllocaExpander() {
  75   return new X86WinAllocaExpander();
  76 }
  77
  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());
  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
  95 X86WinAllocaExpander::Lowering
  96 X86WinAllocaExpander::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
 110 static 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
 130 void 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
 191 static 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
 197 void 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
 273 bool 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 }