SjLjEHPrepare.cpp

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//===- SjLjEHPrepare.cpp - Eliminate Invoke & Unwind instructions ---------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This transformation is designed for use by code generators which use SjLj
// based exception handling.
//
//===----------------------------------------------------------------------===//

#include "llvm/CodeGen/Passes.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Intrinsics.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Module.h"
#include "llvm/Pass.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetLowering.h"
#include "llvm/Target/TargetSubtargetInfo.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Local.h"
#include <set>
using namespace llvm;

#define DEBUG_TYPE "sjljehprepare"

STATISTIC(NumInvokes, "Number of invokes replaced");
STATISTIC(NumSpilled, "Number of registers live across unwind edges");

namespace {
class SjLjEHPrepare : public FunctionPass {
  const TargetMachine *TM;
  Type *FunctionContextTy;
  Constant *RegisterFn;
  Constant *UnregisterFn;
  Constant *BuiltinSetjmpFn;
  Constant *FrameAddrFn;
  Constant *StackAddrFn;
  Constant *StackRestoreFn;
  Constant *LSDAAddrFn;
  Value *PersonalityFn;
  Constant *CallSiteFn;
  Constant *FuncCtxFn;
  AllocaInst *FuncCtx;

public:
  static char ID; // Pass identification, replacement for typeid
  explicit SjLjEHPrepare(const TargetMachine *TM) : FunctionPass(ID), TM(TM) {}
  bool doInitialization(Module &M) override;
  bool runOnFunction(Function &F) override;

  void getAnalysisUsage(AnalysisUsage &AU) const override {}
  const char *getPassName() const override {
    return "SJLJ Exception Handling preparation";
  }

private:
  bool setupEntryBlockAndCallSites(Function &F);
  void substituteLPadValues(LandingPadInst *LPI, Value *ExnVal, Value *SelVal);
  Value *setupFunctionContext(Function &F, ArrayRef<LandingPadInst *> LPads);
  void lowerIncomingArguments(Function &F);
  void lowerAcrossUnwindEdges(Function &F, ArrayRef<InvokeInst *> Invokes);
  void insertCallSiteStore(Instruction *I, int Number);
};
} // end anonymous namespace

char SjLjEHPrepare::ID = 0;

// Public Interface To the SjLjEHPrepare pass.
FunctionPass *llvm::createSjLjEHPreparePass(const TargetMachine *TM) {
  return new SjLjEHPrepare(TM);
}
// doInitialization - Set up decalarations and types needed to process
// exceptions.
bool SjLjEHPrepare::doInitialization(Module &M) {
  // Build the function context structure.
  // builtin_setjmp uses a five word jbuf
  Type *VoidPtrTy = Type::getInt8PtrTy(M.getContext());
  Type *Int32Ty = Type::getInt32Ty(M.getContext());
  FunctionContextTy = StructType::get(VoidPtrTy,                  // __prev
                                      Int32Ty,                    // call_site
                                      ArrayType::get(Int32Ty, 4), // __data
                                      VoidPtrTy, // __personality
                                      VoidPtrTy, // __lsda
                                      ArrayType::get(VoidPtrTy, 5), // __jbuf
                                      NULL);
  RegisterFn = M.getOrInsertFunction(
      "_Unwind_SjLj_Register", Type::getVoidTy(M.getContext()),
      PointerType::getUnqual(FunctionContextTy), (Type *)nullptr);
  UnregisterFn = M.getOrInsertFunction(
      "_Unwind_SjLj_Unregister", Type::getVoidTy(M.getContext()),
      PointerType::getUnqual(FunctionContextTy), (Type *)nullptr);
  FrameAddrFn = Intrinsic::getDeclaration(&M, Intrinsic::frameaddress);
  StackAddrFn = Intrinsic::getDeclaration(&M, Intrinsic::stacksave);
  StackRestoreFn = Intrinsic::getDeclaration(&M, Intrinsic::stackrestore);
  BuiltinSetjmpFn = Intrinsic::getDeclaration(&M, Intrinsic::eh_sjlj_setjmp);
  LSDAAddrFn = Intrinsic::getDeclaration(&M, Intrinsic::eh_sjlj_lsda);
  CallSiteFn = Intrinsic::getDeclaration(&M, Intrinsic::eh_sjlj_callsite);
  FuncCtxFn = Intrinsic::getDeclaration(&M, Intrinsic::eh_sjlj_functioncontext);
  PersonalityFn = nullptr;

  return true;
}

/// insertCallSiteStore - Insert a store of the call-site value to the
/// function context
void SjLjEHPrepare::insertCallSiteStore(Instruction *I, int Number) {
  IRBuilder<> Builder(I);

  // Get a reference to the call_site field.
  Type *Int32Ty = Type::getInt32Ty(I->getContext());
  Value *Zero = ConstantInt::get(Int32Ty, 0);
  Value *One = ConstantInt::get(Int32Ty, 1);
  Value *Idxs[2] = { Zero, One };
  Value *CallSite = Builder.CreateGEP(FuncCtx, Idxs, "call_site");

  // Insert a store of the call-site number
  ConstantInt *CallSiteNoC =
      ConstantInt::get(Type::getInt32Ty(I->getContext()), Number);
  Builder.CreateStore(CallSiteNoC, CallSite, true /*volatile*/);
}

/// MarkBlocksLiveIn - Insert BB and all of its predescessors into LiveBBs until
/// we reach blocks we've already seen.
static void MarkBlocksLiveIn(BasicBlock *BB,
                             SmallPtrSetImpl<BasicBlock *> &LiveBBs) {
  if (!LiveBBs.insert(BB))
    return; // already been here.

  for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
    MarkBlocksLiveIn(*PI, LiveBBs);
}

/// substituteLPadValues - Substitute the values returned by the landingpad
/// instruction with those returned by the personality function.
void SjLjEHPrepare::substituteLPadValues(LandingPadInst *LPI, Value *ExnVal,
                                         Value *SelVal) {
  SmallVector<Value *, 8> UseWorkList(LPI->user_begin(), LPI->user_end());
  while (!UseWorkList.empty()) {
    Value *Val = UseWorkList.pop_back_val();
    ExtractValueInst *EVI = dyn_cast<ExtractValueInst>(Val);
    if (!EVI)
      continue;
    if (EVI->getNumIndices() != 1)
      continue;
    if (*EVI->idx_begin() == 0)
      EVI->replaceAllUsesWith(ExnVal);
    else if (*EVI->idx_begin() == 1)
      EVI->replaceAllUsesWith(SelVal);
    if (EVI->getNumUses() == 0)
      EVI->eraseFromParent();
  }

  if (LPI->getNumUses() == 0)
    return;

  // There are still some uses of LPI. Construct an aggregate with the exception
  // values and replace the LPI with that aggregate.
  Type *LPadType = LPI->getType();
  Value *LPadVal = UndefValue::get(LPadType);
  IRBuilder<> Builder(
      std::next(BasicBlock::iterator(cast<Instruction>(SelVal))));
  LPadVal = Builder.CreateInsertValue(LPadVal, ExnVal, 0, "lpad.val");
  LPadVal = Builder.CreateInsertValue(LPadVal, SelVal, 1, "lpad.val");

  LPI->replaceAllUsesWith(LPadVal);
}

/// setupFunctionContext - Allocate the function context on the stack and fill
/// it with all of the data that we know at this point.
Value *SjLjEHPrepare::setupFunctionContext(Function &F,
                                           ArrayRef<LandingPadInst *> LPads) {
  BasicBlock *EntryBB = F.begin();

  // Create an alloca for the incoming jump buffer ptr and the new jump buffer
  // that needs to be restored on all exits from the function. This is an alloca
  // because the value needs to be added to the global context list.
  const TargetLowering *TLI = TM->getSubtargetImpl()->getTargetLowering();
  unsigned Align =
      TLI->getDataLayout()->getPrefTypeAlignment(FunctionContextTy);
  FuncCtx = new AllocaInst(FunctionContextTy, nullptr, Align, "fn_context",
                           EntryBB->begin());

  // Fill in the function context structure.
  for (unsigned I = 0, E = LPads.size(); I != E; ++I) {
    LandingPadInst *LPI = LPads[I];
    IRBuilder<> Builder(LPI->getParent()->getFirstInsertionPt());

    // Reference the __data field.
    Value *FCData = Builder.CreateConstGEP2_32(FuncCtx, 0, 2, "__data");

    // The exception values come back in context->__data[0].
    Value *ExceptionAddr =
        Builder.CreateConstGEP2_32(FCData, 0, 0, "exception_gep");
    Value *ExnVal = Builder.CreateLoad(ExceptionAddr, true, "exn_val");
    ExnVal = Builder.CreateIntToPtr(ExnVal, Builder.getInt8PtrTy());

    Value *SelectorAddr =
        Builder.CreateConstGEP2_32(FCData, 0, 1, "exn_selector_gep");
    Value *SelVal = Builder.CreateLoad(SelectorAddr, true, "exn_selector_val");

    substituteLPadValues(LPI, ExnVal, SelVal);
  }

  // Personality function
  IRBuilder<> Builder(EntryBB->getTerminator());
  if (!PersonalityFn)
    PersonalityFn = LPads[0]->getPersonalityFn();
  Value *PersonalityFieldPtr =
      Builder.CreateConstGEP2_32(FuncCtx, 0, 3, "pers_fn_gep");
  Builder.CreateStore(
      Builder.CreateBitCast(PersonalityFn, Builder.getInt8PtrTy()),
      PersonalityFieldPtr, /*isVolatile=*/true);

  // LSDA address
  Value *LSDA = Builder.CreateCall(LSDAAddrFn, "lsda_addr");
  Value *LSDAFieldPtr = Builder.CreateConstGEP2_32(FuncCtx, 0, 4, "lsda_gep");
  Builder.CreateStore(LSDA, LSDAFieldPtr, /*isVolatile=*/true);

  return FuncCtx;
}

/// lowerIncomingArguments - To avoid having to handle incoming arguments
/// specially, we lower each arg to a copy instruction in the entry block. This
/// ensures that the argument value itself cannot be live out of the entry
/// block.
void SjLjEHPrepare::lowerIncomingArguments(Function &F) {
  BasicBlock::iterator AfterAllocaInsPt = F.begin()->begin();
  while (isa<AllocaInst>(AfterAllocaInsPt) &&
         isa<ConstantInt>(cast<AllocaInst>(AfterAllocaInsPt)->getArraySize()))
    ++AfterAllocaInsPt;

  for (Function::arg_iterator AI = F.arg_begin(), AE = F.arg_end(); AI != AE;
       ++AI) {
    Type *Ty = AI->getType();

    // Use 'select i8 true, %arg, undef' to simulate a 'no-op' instruction.
    Value *TrueValue = ConstantInt::getTrue(F.getContext());
    Value *UndefValue = UndefValue::get(Ty);
    Instruction *SI = SelectInst::Create(TrueValue, AI, UndefValue,
                                         AI->getName() + ".tmp",
                                         AfterAllocaInsPt);
    AI->replaceAllUsesWith(SI);

    // Reset the operand, because it  was clobbered by the RAUW above.
    SI->setOperand(1, AI);
  }
}

/// lowerAcrossUnwindEdges - Find all variables which are alive across an unwind
/// edge and spill them.
void SjLjEHPrepare::lowerAcrossUnwindEdges(Function &F,
                                           ArrayRef<InvokeInst *> Invokes) {
  // Finally, scan the code looking for instructions with bad live ranges.
  for (Function::iterator BB = F.begin(), BBE = F.end(); BB != BBE; ++BB) {
    for (BasicBlock::iterator II = BB->begin(), IIE = BB->end(); II != IIE;
         ++II) {
      // Ignore obvious cases we don't have to handle. In particular, most
      // instructions either have no uses or only have a single use inside the
      // current block. Ignore them quickly.
      Instruction *Inst = II;
      if (Inst->use_empty())
        continue;
      if (Inst->hasOneUse() &&
          cast<Instruction>(Inst->user_back())->getParent() == BB &&
          !isa<PHINode>(Inst->user_back()))
        continue;

      // If this is an alloca in the entry block, it's not a real register
      // value.
      if (AllocaInst *AI = dyn_cast<AllocaInst>(Inst))
        if (isa<ConstantInt>(AI->getArraySize()) && BB == F.begin())
          continue;

      // Avoid iterator invalidation by copying users to a temporary vector.
      SmallVector<Instruction *, 16> Users;
      for (User *U : Inst->users()) {
        Instruction *UI = cast<Instruction>(U);
        if (UI->getParent() != BB || isa<PHINode>(UI))
          Users.push_back(UI);
      }

      // Find all of the blocks that this value is live in.
      SmallPtrSet<BasicBlock *, 64> LiveBBs;
      LiveBBs.insert(Inst->getParent());
      while (!Users.empty()) {
        Instruction *U = Users.back();
        Users.pop_back();

        if (!isa<PHINode>(U)) {
          MarkBlocksLiveIn(U->getParent(), LiveBBs);
        } else {
          // Uses for a PHI node occur in their predecessor block.
          PHINode *PN = cast<PHINode>(U);
          for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
            if (PN->getIncomingValue(i) == Inst)
              MarkBlocksLiveIn(PN->getIncomingBlock(i), LiveBBs);
        }
      }

      // Now that we know all of the blocks that this thing is live in, see if
      // it includes any of the unwind locations.
      bool NeedsSpill = false;
      for (unsigned i = 0, e = Invokes.size(); i != e; ++i) {
        BasicBlock *UnwindBlock = Invokes[i]->getUnwindDest();
        if (UnwindBlock != BB && LiveBBs.count(UnwindBlock)) {
          DEBUG(dbgs() << "SJLJ Spill: " << *Inst << " around "
                       << UnwindBlock->getName() << "\n");
          NeedsSpill = true;
          break;
        }
      }

      // If we decided we need a spill, do it.
      // FIXME: Spilling this way is overkill, as it forces all uses of
      // the value to be reloaded from the stack slot, even those that aren't
      // in the unwind blocks. We should be more selective.
      if (NeedsSpill) {
        DemoteRegToStack(*Inst, true);
        ++NumSpilled;
      }
    }
  }

  // Go through the landing pads and remove any PHIs there.
  for (unsigned i = 0, e = Invokes.size(); i != e; ++i) {
    BasicBlock *UnwindBlock = Invokes[i]->getUnwindDest();
    LandingPadInst *LPI = UnwindBlock->getLandingPadInst();

    // Place PHIs into a set to avoid invalidating the iterator.
    SmallPtrSet<PHINode *, 8> PHIsToDemote;
    for (BasicBlock::iterator PN = UnwindBlock->begin(); isa<PHINode>(PN); ++PN)
      PHIsToDemote.insert(cast<PHINode>(PN));
    if (PHIsToDemote.empty())
      continue;

    // Demote the PHIs to the stack.
    for (PHINode *PN : PHIsToDemote)
      DemotePHIToStack(PN);

    // Move the landingpad instruction back to the top of the landing pad block.
    LPI->moveBefore(UnwindBlock->begin());
  }
}

/// setupEntryBlockAndCallSites - Setup the entry block by creating and filling
/// the function context and marking the call sites with the appropriate
/// values. These values are used by the DWARF EH emitter.
bool SjLjEHPrepare::setupEntryBlockAndCallSites(Function &F) {
  SmallVector<ReturnInst *, 16> Returns;
  SmallVector<InvokeInst *, 16> Invokes;
  SmallSetVector<LandingPadInst *, 16> LPads;

  // Look through the terminators of the basic blocks to find invokes.
  for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
    if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator())) {
      if (Function *Callee = II->getCalledFunction())
        if (Callee->isIntrinsic() &&
            Callee->getIntrinsicID() == Intrinsic::donothing) {
          // Remove the NOP invoke.
          BranchInst::Create(II->getNormalDest(), II);
          II->eraseFromParent();
          continue;
        }

      Invokes.push_back(II);
      LPads.insert(II->getUnwindDest()->getLandingPadInst());
    } else if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
      Returns.push_back(RI);
    }

  if (Invokes.empty())
    return false;

  NumInvokes += Invokes.size();

  lowerIncomingArguments(F);
  lowerAcrossUnwindEdges(F, Invokes);

  Value *FuncCtx =
      setupFunctionContext(F, makeArrayRef(LPads.begin(), LPads.end()));
  BasicBlock *EntryBB = F.begin();
  IRBuilder<> Builder(EntryBB->getTerminator());

  // Get a reference to the jump buffer.
  Value *JBufPtr = Builder.CreateConstGEP2_32(FuncCtx, 0, 5, "jbuf_gep");

  // Save the frame pointer.
  Value *FramePtr = Builder.CreateConstGEP2_32(JBufPtr, 0, 0, "jbuf_fp_gep");

  Value *Val = Builder.CreateCall(FrameAddrFn, Builder.getInt32(0), "fp");
  Builder.CreateStore(Val, FramePtr, /*isVolatile=*/true);

  // Save the stack pointer.
  Value *StackPtr = Builder.CreateConstGEP2_32(JBufPtr, 0, 2, "jbuf_sp_gep");

  Val = Builder.CreateCall(StackAddrFn, "sp");
  Builder.CreateStore(Val, StackPtr, /*isVolatile=*/true);

  // Call the setjmp instrinsic. It fills in the rest of the jmpbuf.
  Value *SetjmpArg = Builder.CreateBitCast(JBufPtr, Builder.getInt8PtrTy());
  Builder.CreateCall(BuiltinSetjmpFn, SetjmpArg);

  // Store a pointer to the function context so that the back-end will know
  // where to look for it.
  Value *FuncCtxArg = Builder.CreateBitCast(FuncCtx, Builder.getInt8PtrTy());
  Builder.CreateCall(FuncCtxFn, FuncCtxArg);

  // At this point, we are all set up, update the invoke instructions to mark
  // their call_site values.
  for (unsigned I = 0, E = Invokes.size(); I != E; ++I) {
    insertCallSiteStore(Invokes[I], I + 1);

    ConstantInt *CallSiteNum =
        ConstantInt::get(Type::getInt32Ty(F.getContext()), I + 1);

    // Record the call site value for the back end so it stays associated with
    // the invoke.
    CallInst::Create(CallSiteFn, CallSiteNum, "", Invokes[I]);
  }

  // Mark call instructions that aren't nounwind as no-action (call_site ==
  // -1). Skip the entry block, as prior to then, no function context has been
  // created for this function and any unexpected exceptions thrown will go
  // directly to the caller's context, which is what we want anyway, so no need
  // to do anything here.
  for (Function::iterator BB = F.begin(), E = F.end(); ++BB != E;)
    for (BasicBlock::iterator I = BB->begin(), end = BB->end(); I != end; ++I)
      if (CallInst *CI = dyn_cast<CallInst>(I)) {
        if (!CI->doesNotThrow())
          insertCallSiteStore(CI, -1);
      } else if (ResumeInst *RI = dyn_cast<ResumeInst>(I)) {
        insertCallSiteStore(RI, -1);
      }

  // Register the function context and make sure it's known to not throw
  CallInst *Register =
      CallInst::Create(RegisterFn, FuncCtx, "", EntryBB->getTerminator());
  Register->setDoesNotThrow();

  // Following any allocas not in the entry block, update the saved SP in the
  // jmpbuf to the new value.
  for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
    if (BB == F.begin())
      continue;
    for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
      if (CallInst *CI = dyn_cast<CallInst>(I)) {
        if (CI->getCalledFunction() != StackRestoreFn)
          continue;
      } else if (!isa<AllocaInst>(I)) {
        continue;
      }
      Instruction *StackAddr = CallInst::Create(StackAddrFn, "sp");
      StackAddr->insertAfter(I);
      Instruction *StoreStackAddr = new StoreInst(StackAddr, StackPtr, true);
      StoreStackAddr->insertAfter(StackAddr);
    }
  }

  // Finally, for any returns from this function, if this function contains an
  // invoke, add a call to unregister the function context.
  for (unsigned I = 0, E = Returns.size(); I != E; ++I)
    CallInst::Create(UnregisterFn, FuncCtx, "", Returns[I]);

  return true;
}

bool SjLjEHPrepare::runOnFunction(Function &F) {
  bool Res = setupEntryBlockAndCallSites(F);
  return Res;
}