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   unsigned AmountReg = MI->getOperand(0).getReg();
  85   MachineInstr *Def = MRI->getUniqueVRegDef(AmountReg);
  86
  87   // Look through copies.
  88   while (Def && Def->isCopy() && Def->getOperand(1).isReg())
  89     Def = MRI->getUniqueVRegDef(Def->getOperand(1).getReg());
  90
  91   if (!Def ||
  92       (Def->getOpcode() != X86::MOV32ri && Def->getOpcode() != X86::MOV64ri) ||
  93       !Def->getOperand(1).isImm())
  94     return -1;
  95
  96   return Def->getOperand(1).getImm();
  97 }
  98
  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)
 104     return Probe;
 105
 106   // If it fits within the safe region of the stack, just subtract.
 107   if (CurrentOffset + AllocaAmount <= StackProbeSize)
 108     return Sub;
 109
 110   // Otherwise, touch the current tip of the stack, then subtract.
 111   return TouchAndSub;
 112 }
 113
 114 static bool isPushPop(const MachineInstr &MI) {
 115   switch (MI.getOpcode()) {
 116   case X86::PUSH32i8:
 117   case X86::PUSH32r:
 118   case X86::PUSH32rmm:
 119   case X86::PUSH32rmr:
 120   case X86::PUSHi32:
 121   case X86::PUSH64i8:
 122   case X86::PUSH64r:
 123   case X86::PUSH64rmm:
 124   case X86::PUSH64rmr:
 125   case X86::PUSH64i32:
 126   case X86::POP32r:
 127   case X86::POP64r:
 128     return true;
 129   default:
 130     return false;
 131   }
 132 }
 133
 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.
 139
 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;
 144
 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.
 148
 149   // Compute the reverse post-order.
 150   ReversePostOrderTraversal<MachineFunction*> RPO(&MF);
 151
 152   for (MachineBasicBlock *MBB : RPO) {
 153     int64_t Offset = -1;
 154     for (MachineBasicBlock *Pred : MBB->predecessors())
 155       Offset = std::max(Offset, OutOffset[Pred]);
 156     if (Offset == -1) Offset = INT32_MAX;
 157
 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);
 164         Lowerings[&MI] = L;
 165         switch (L) {
 166         case Sub:
 167           Offset += Amount;
 168           break;
 169         case TouchAndSub:
 170           Offset = Amount;
 171           break;
 172         case Probe:
 173           Offset = 0;
 174           break;
 175         }
 176       } else if (MI.isCall() || isPushPop(MI)) {
 177         // Calls, pushes and pops touch the tip of the stack.
 178         Offset = 0;
 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.
 187         Offset = INT32_MAX;
 188       }
 189     }
 190
 191     OutOffset[MBB] = Offset;
 192   }
 193 }
 194
 195 static unsigned getSubOpcode(bool Is64Bit, int64_t Amount) {
 196   if (Is64Bit)
 197     return isInt<8>(Amount) ? X86::SUB64ri8 : X86::SUB64ri32;
 198   return isInt<8>(Amount) ? X86::SUB32ri8 : X86::SUB32ri;
 199 }
 200
 201 void X86WinAllocaExpander::lower(MachineInstr* MI, Lowering L) {
 202   DebugLoc DL = MI->getDebugLoc();
 203   MachineBasicBlock *MBB = MI->getParent();
 204   MachineBasicBlock::iterator I = *MI;
 205
 206   int64_t Amount = getWinAllocaAmount(MI, MRI);
 207   if (Amount == 0) {
 208     MI->eraseFromParent();
 209     return;
 210   }
 211
 212   bool Is64Bit = STI->is64Bit();
 213   assert(SlotSize == 4 || SlotSize == 8);
 214   unsigned RegA = (SlotSize == 8) ? X86::RAX : X86::EAX;
 215
 216   switch (L) {
 217   case TouchAndSub:
 218     assert(Amount >= SlotSize);
 219
 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);
 223     Amount -= SlotSize;
 224     if (!Amount)
 225       break;
 226
 227     // Fall through to make any remaining adjustment.
 228     LLVM_FALLTHROUGH;
 229   case Sub:
 230     assert(Amount > 0);
 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);
 235     } else {
 236       // Sub.
 237       BuildMI(*MBB, I, DL, TII->get(getSubOpcode(Is64Bit, Amount)), StackPtr)
 238           .addReg(StackPtr)
 239           .addImm(Amount);
 240     }
 241     break;
 242   case Probe:
 243     if (!NoStackArgProbe) {
 244       // The probe lowering expects the amount in RAX/EAX.
 245       BuildMI(*MBB, MI, DL, TII->get(TargetOpcode::COPY), RegA)
 246           .addReg(MI->getOperand(0).getReg());
 247
 248       // Do the probe.
 249       STI->getFrameLowering()->emitStackProbe(*MBB->getParent(), *MBB, MI, DL,
 250                                               /*InPrologue=*/false);
 251     } else {
 252       // Sub
 253       BuildMI(*MBB, I, DL, TII->get(Is64Bit ? X86::SUB64rr : X86::SUB32rr),
 254               StackPtr)
 255           .addReg(StackPtr)
 256           .addReg(MI->getOperand(0).getReg());
 257     }
 258     break;
 259   }
 260
 261   unsigned AmountReg = MI->getOperand(0).getReg();
 262   MI->eraseFromParent();
 263
 264   // Delete the definition of AmountReg, possibly walking a chain of copies.
 265   for (;;) {
 266     if (!MRI->use_empty(AmountReg))
 267       break;
 268     MachineInstr *AmountDef = MRI->getUniqueVRegDef(AmountReg);
 269     if (!AmountDef)
 270       break;
 271     if (AmountDef->isCopy() && AmountDef->getOperand(1).isReg())
 272       AmountReg = AmountDef->getOperand(1).isReg();
 273     AmountDef->eraseFromParent();
 274     break;
 275   }
 276 }
 277
 278 bool X86WinAllocaExpander::runOnMachineFunction(MachineFunction &MF) {
 279   if (!MF.getInfo<X86MachineFunctionInfo>()->hasWinAlloca())
 280     return false;
 281
 282   MRI = &MF.getRegInfo();
 283   STI = &MF.getSubtarget<X86Subtarget>();
 284   TII = STI->getInstrInfo();
 285   TRI = STI->getRegisterInfo();
 286   StackPtr = TRI->getStackRegister();
 287   SlotSize = TRI->getSlotSize();
 288
 289   StackProbeSize = 4096;
 290   if (MF.getFunction().hasFnAttribute("stack-probe-size")) {
 291     MF.getFunction()
 292         .getFnAttribute("stack-probe-size")
 293         .getValueAsString()
 294         .getAsInteger(0, StackProbeSize);
 295   }
 296   NoStackArgProbe = MF.getFunction().hasFnAttribute("no-stack-arg-probe");
 297   if (NoStackArgProbe)
 298     StackProbeSize = INT64_MAX;
 299
 300   LoweringMap Lowerings;
 301   computeLowerings(MF, Lowerings);
 302   for (auto &P : Lowerings)
 303     lower(P.first, P.second);
 304
 305   return true;
 306 }