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      1 //===- SjLjEHPrepare.cpp - Eliminate Invoke & Unwind instructions ---------===//
      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 transformation is designed for use by code generators which use SjLj
     11 // based exception handling.
     12 //
     13 //===----------------------------------------------------------------------===//
     14 
     15 #include "llvm/CodeGen/Passes.h"
     16 #include "llvm/ADT/SetVector.h"
     17 #include "llvm/ADT/SmallPtrSet.h"
     18 #include "llvm/ADT/SmallVector.h"
     19 #include "llvm/ADT/Statistic.h"
     20 #include "llvm/IR/Constants.h"
     21 #include "llvm/IR/DataLayout.h"
     22 #include "llvm/IR/DerivedTypes.h"
     23 #include "llvm/IR/IRBuilder.h"
     24 #include "llvm/IR/Instructions.h"
     25 #include "llvm/IR/Intrinsics.h"
     26 #include "llvm/IR/Module.h"
     27 #include "llvm/Pass.h"
     28 #include "llvm/Support/Debug.h"
     29 #include "llvm/Support/raw_ostream.h"
     30 #include "llvm/Transforms/Utils/Local.h"
     31 using namespace llvm;
     32 
     33 #define DEBUG_TYPE "sjljehprepare"
     34 
     35 STATISTIC(NumInvokes, "Number of invokes replaced");
     36 STATISTIC(NumSpilled, "Number of registers live across unwind edges");
     37 
     38 namespace {
     39 class SjLjEHPrepare : public FunctionPass {
     40   Type *doubleUnderDataTy;
     41   Type *doubleUnderJBufTy;
     42   Type *FunctionContextTy;
     43   Constant *RegisterFn;
     44   Constant *UnregisterFn;
     45   Constant *BuiltinSetupDispatchFn;
     46   Constant *FrameAddrFn;
     47   Constant *StackAddrFn;
     48   Constant *StackRestoreFn;
     49   Constant *LSDAAddrFn;
     50   Constant *CallSiteFn;
     51   Constant *FuncCtxFn;
     52   AllocaInst *FuncCtx;
     53 
     54 public:
     55   static char ID; // Pass identification, replacement for typeid
     56   explicit SjLjEHPrepare() : FunctionPass(ID) {}
     57   bool doInitialization(Module &M) override;
     58   bool runOnFunction(Function &F) override;
     59 
     60   void getAnalysisUsage(AnalysisUsage &AU) const override {}
     61   const char *getPassName() const override {
     62     return "SJLJ Exception Handling preparation";
     63   }
     64 
     65 private:
     66   bool setupEntryBlockAndCallSites(Function &F);
     67   void substituteLPadValues(LandingPadInst *LPI, Value *ExnVal, Value *SelVal);
     68   Value *setupFunctionContext(Function &F, ArrayRef<LandingPadInst *> LPads);
     69   void lowerIncomingArguments(Function &F);
     70   void lowerAcrossUnwindEdges(Function &F, ArrayRef<InvokeInst *> Invokes);
     71   void insertCallSiteStore(Instruction *I, int Number);
     72 };
     73 } // end anonymous namespace
     74 
     75 char SjLjEHPrepare::ID = 0;
     76 INITIALIZE_PASS(SjLjEHPrepare, "sjljehprepare", "Prepare SjLj exceptions",
     77                 false, false)
     78 
     79 // Public Interface To the SjLjEHPrepare pass.
     80 FunctionPass *llvm::createSjLjEHPreparePass() { return new SjLjEHPrepare(); }
     81 // doInitialization - Set up decalarations and types needed to process
     82 // exceptions.
     83 bool SjLjEHPrepare::doInitialization(Module &M) {
     84   // Build the function context structure.
     85   // builtin_setjmp uses a five word jbuf
     86   Type *VoidPtrTy = Type::getInt8PtrTy(M.getContext());
     87   Type *Int32Ty = Type::getInt32Ty(M.getContext());
     88   doubleUnderDataTy = ArrayType::get(Int32Ty, 4);
     89   doubleUnderJBufTy = ArrayType::get(VoidPtrTy, 5);
     90   FunctionContextTy = StructType::get(VoidPtrTy,         // __prev
     91                                       Int32Ty,           // call_site
     92                                       doubleUnderDataTy, // __data
     93                                       VoidPtrTy,         // __personality
     94                                       VoidPtrTy,         // __lsda
     95                                       doubleUnderJBufTy, // __jbuf
     96                                       nullptr);
     97 
     98   return true;
     99 }
    100 
    101 /// insertCallSiteStore - Insert a store of the call-site value to the
    102 /// function context
    103 void SjLjEHPrepare::insertCallSiteStore(Instruction *I, int Number) {
    104   IRBuilder<> Builder(I);
    105 
    106   // Get a reference to the call_site field.
    107   Type *Int32Ty = Type::getInt32Ty(I->getContext());
    108   Value *Zero = ConstantInt::get(Int32Ty, 0);
    109   Value *One = ConstantInt::get(Int32Ty, 1);
    110   Value *Idxs[2] = { Zero, One };
    111   Value *CallSite =
    112       Builder.CreateGEP(FunctionContextTy, FuncCtx, Idxs, "call_site");
    113 
    114   // Insert a store of the call-site number
    115   ConstantInt *CallSiteNoC =
    116       ConstantInt::get(Type::getInt32Ty(I->getContext()), Number);
    117   Builder.CreateStore(CallSiteNoC, CallSite, true /*volatile*/);
    118 }
    119 
    120 /// MarkBlocksLiveIn - Insert BB and all of its predecessors into LiveBBs until
    121 /// we reach blocks we've already seen.
    122 static void MarkBlocksLiveIn(BasicBlock *BB,
    123                              SmallPtrSetImpl<BasicBlock *> &LiveBBs) {
    124   if (!LiveBBs.insert(BB).second)
    125     return; // already been here.
    126 
    127   for (BasicBlock *PredBB : predecessors(BB))
    128     MarkBlocksLiveIn(PredBB, LiveBBs);
    129 }
    130 
    131 /// substituteLPadValues - Substitute the values returned by the landingpad
    132 /// instruction with those returned by the personality function.
    133 void SjLjEHPrepare::substituteLPadValues(LandingPadInst *LPI, Value *ExnVal,
    134                                          Value *SelVal) {
    135   SmallVector<Value *, 8> UseWorkList(LPI->user_begin(), LPI->user_end());
    136   while (!UseWorkList.empty()) {
    137     Value *Val = UseWorkList.pop_back_val();
    138     auto *EVI = dyn_cast<ExtractValueInst>(Val);
    139     if (!EVI)
    140       continue;
    141     if (EVI->getNumIndices() != 1)
    142       continue;
    143     if (*EVI->idx_begin() == 0)
    144       EVI->replaceAllUsesWith(ExnVal);
    145     else if (*EVI->idx_begin() == 1)
    146       EVI->replaceAllUsesWith(SelVal);
    147     if (EVI->use_empty())
    148       EVI->eraseFromParent();
    149   }
    150 
    151   if (LPI->use_empty())
    152     return;
    153 
    154   // There are still some uses of LPI. Construct an aggregate with the exception
    155   // values and replace the LPI with that aggregate.
    156   Type *LPadType = LPI->getType();
    157   Value *LPadVal = UndefValue::get(LPadType);
    158   auto *SelI = cast<Instruction>(SelVal);
    159   IRBuilder<> Builder(SelI->getParent(), std::next(SelI->getIterator()));
    160   LPadVal = Builder.CreateInsertValue(LPadVal, ExnVal, 0, "lpad.val");
    161   LPadVal = Builder.CreateInsertValue(LPadVal, SelVal, 1, "lpad.val");
    162 
    163   LPI->replaceAllUsesWith(LPadVal);
    164 }
    165 
    166 /// setupFunctionContext - Allocate the function context on the stack and fill
    167 /// it with all of the data that we know at this point.
    168 Value *SjLjEHPrepare::setupFunctionContext(Function &F,
    169                                            ArrayRef<LandingPadInst *> LPads) {
    170   BasicBlock *EntryBB = &F.front();
    171 
    172   // Create an alloca for the incoming jump buffer ptr and the new jump buffer
    173   // that needs to be restored on all exits from the function. This is an alloca
    174   // because the value needs to be added to the global context list.
    175   auto &DL = F.getParent()->getDataLayout();
    176   unsigned Align = DL.getPrefTypeAlignment(FunctionContextTy);
    177   FuncCtx = new AllocaInst(FunctionContextTy, nullptr, Align, "fn_context",
    178                            &EntryBB->front());
    179 
    180   // Fill in the function context structure.
    181   for (LandingPadInst *LPI : LPads) {
    182     IRBuilder<> Builder(LPI->getParent(),
    183                         LPI->getParent()->getFirstInsertionPt());
    184 
    185     // Reference the __data field.
    186     Value *FCData =
    187         Builder.CreateConstGEP2_32(FunctionContextTy, FuncCtx, 0, 2, "__data");
    188 
    189     // The exception values come back in context->__data[0].
    190     Value *ExceptionAddr = Builder.CreateConstGEP2_32(doubleUnderDataTy, FCData,
    191                                                       0, 0, "exception_gep");
    192     Value *ExnVal = Builder.CreateLoad(ExceptionAddr, true, "exn_val");
    193     ExnVal = Builder.CreateIntToPtr(ExnVal, Builder.getInt8PtrTy());
    194 
    195     Value *SelectorAddr = Builder.CreateConstGEP2_32(doubleUnderDataTy, FCData,
    196                                                      0, 1, "exn_selector_gep");
    197     Value *SelVal = Builder.CreateLoad(SelectorAddr, true, "exn_selector_val");
    198 
    199     substituteLPadValues(LPI, ExnVal, SelVal);
    200   }
    201 
    202   // Personality function
    203   IRBuilder<> Builder(EntryBB->getTerminator());
    204   Value *PersonalityFn = F.getPersonalityFn();
    205   Value *PersonalityFieldPtr = Builder.CreateConstGEP2_32(
    206       FunctionContextTy, FuncCtx, 0, 3, "pers_fn_gep");
    207   Builder.CreateStore(
    208       Builder.CreateBitCast(PersonalityFn, Builder.getInt8PtrTy()),
    209       PersonalityFieldPtr, /*isVolatile=*/true);
    210 
    211   // LSDA address
    212   Value *LSDA = Builder.CreateCall(LSDAAddrFn, {}, "lsda_addr");
    213   Value *LSDAFieldPtr =
    214       Builder.CreateConstGEP2_32(FunctionContextTy, FuncCtx, 0, 4, "lsda_gep");
    215   Builder.CreateStore(LSDA, LSDAFieldPtr, /*isVolatile=*/true);
    216 
    217   return FuncCtx;
    218 }
    219 
    220 /// lowerIncomingArguments - To avoid having to handle incoming arguments
    221 /// specially, we lower each arg to a copy instruction in the entry block. This
    222 /// ensures that the argument value itself cannot be live out of the entry
    223 /// block.
    224 void SjLjEHPrepare::lowerIncomingArguments(Function &F) {
    225   BasicBlock::iterator AfterAllocaInsPt = F.begin()->begin();
    226   while (isa<AllocaInst>(AfterAllocaInsPt) &&
    227          cast<AllocaInst>(AfterAllocaInsPt)->isStaticAlloca())
    228     ++AfterAllocaInsPt;
    229   assert(AfterAllocaInsPt != F.front().end());
    230 
    231   for (auto &AI : F.args()) {
    232     Type *Ty = AI.getType();
    233 
    234     // Use 'select i8 true, %arg, undef' to simulate a 'no-op' instruction.
    235     Value *TrueValue = ConstantInt::getTrue(F.getContext());
    236     Value *UndefValue = UndefValue::get(Ty);
    237     Instruction *SI = SelectInst::Create(
    238         TrueValue, &AI, UndefValue, AI.getName() + ".tmp", &*AfterAllocaInsPt);
    239     AI.replaceAllUsesWith(SI);
    240 
    241     // Reset the operand, because it  was clobbered by the RAUW above.
    242     SI->setOperand(1, &AI);
    243   }
    244 }
    245 
    246 /// lowerAcrossUnwindEdges - Find all variables which are alive across an unwind
    247 /// edge and spill them.
    248 void SjLjEHPrepare::lowerAcrossUnwindEdges(Function &F,
    249                                            ArrayRef<InvokeInst *> Invokes) {
    250   // Finally, scan the code looking for instructions with bad live ranges.
    251   for (BasicBlock &BB : F) {
    252     for (Instruction &Inst : BB) {
    253       // Ignore obvious cases we don't have to handle. In particular, most
    254       // instructions either have no uses or only have a single use inside the
    255       // current block. Ignore them quickly.
    256       if (Inst.use_empty())
    257         continue;
    258       if (Inst.hasOneUse() &&
    259           cast<Instruction>(Inst.user_back())->getParent() == &BB &&
    260           !isa<PHINode>(Inst.user_back()))
    261         continue;
    262 
    263       // If this is an alloca in the entry block, it's not a real register
    264       // value.
    265       if (auto *AI = dyn_cast<AllocaInst>(&Inst))
    266         if (AI->isStaticAlloca())
    267           continue;
    268 
    269       // Avoid iterator invalidation by copying users to a temporary vector.
    270       SmallVector<Instruction *, 16> Users;
    271       for (User *U : Inst.users()) {
    272         Instruction *UI = cast<Instruction>(U);
    273         if (UI->getParent() != &BB || isa<PHINode>(UI))
    274           Users.push_back(UI);
    275       }
    276 
    277       // Find all of the blocks that this value is live in.
    278       SmallPtrSet<BasicBlock *, 32> LiveBBs;
    279       LiveBBs.insert(&BB);
    280       while (!Users.empty()) {
    281         Instruction *U = Users.pop_back_val();
    282 
    283         if (!isa<PHINode>(U)) {
    284           MarkBlocksLiveIn(U->getParent(), LiveBBs);
    285         } else {
    286           // Uses for a PHI node occur in their predecessor block.
    287           PHINode *PN = cast<PHINode>(U);
    288           for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
    289             if (PN->getIncomingValue(i) == &Inst)
    290               MarkBlocksLiveIn(PN->getIncomingBlock(i), LiveBBs);
    291         }
    292       }
    293 
    294       // Now that we know all of the blocks that this thing is live in, see if
    295       // it includes any of the unwind locations.
    296       bool NeedsSpill = false;
    297       for (InvokeInst *Invoke : Invokes) {
    298         BasicBlock *UnwindBlock = Invoke->getUnwindDest();
    299         if (UnwindBlock != &BB && LiveBBs.count(UnwindBlock)) {
    300           DEBUG(dbgs() << "SJLJ Spill: " << Inst << " around "
    301                        << UnwindBlock->getName() << "\n");
    302           NeedsSpill = true;
    303           break;
    304         }
    305       }
    306 
    307       // If we decided we need a spill, do it.
    308       // FIXME: Spilling this way is overkill, as it forces all uses of
    309       // the value to be reloaded from the stack slot, even those that aren't
    310       // in the unwind blocks. We should be more selective.
    311       if (NeedsSpill) {
    312         DemoteRegToStack(Inst, true);
    313         ++NumSpilled;
    314       }
    315     }
    316   }
    317 
    318   // Go through the landing pads and remove any PHIs there.
    319   for (InvokeInst *Invoke : Invokes) {
    320     BasicBlock *UnwindBlock = Invoke->getUnwindDest();
    321     LandingPadInst *LPI = UnwindBlock->getLandingPadInst();
    322 
    323     // Place PHIs into a set to avoid invalidating the iterator.
    324     SmallPtrSet<PHINode *, 8> PHIsToDemote;
    325     for (BasicBlock::iterator PN = UnwindBlock->begin(); isa<PHINode>(PN); ++PN)
    326       PHIsToDemote.insert(cast<PHINode>(PN));
    327     if (PHIsToDemote.empty())
    328       continue;
    329 
    330     // Demote the PHIs to the stack.
    331     for (PHINode *PN : PHIsToDemote)
    332       DemotePHIToStack(PN);
    333 
    334     // Move the landingpad instruction back to the top of the landing pad block.
    335     LPI->moveBefore(&UnwindBlock->front());
    336   }
    337 }
    338 
    339 /// setupEntryBlockAndCallSites - Setup the entry block by creating and filling
    340 /// the function context and marking the call sites with the appropriate
    341 /// values. These values are used by the DWARF EH emitter.
    342 bool SjLjEHPrepare::setupEntryBlockAndCallSites(Function &F) {
    343   SmallVector<ReturnInst *, 16> Returns;
    344   SmallVector<InvokeInst *, 16> Invokes;
    345   SmallSetVector<LandingPadInst *, 16> LPads;
    346 
    347   // Look through the terminators of the basic blocks to find invokes.
    348   for (BasicBlock &BB : F)
    349     if (auto *II = dyn_cast<InvokeInst>(BB.getTerminator())) {
    350       if (Function *Callee = II->getCalledFunction())
    351         if (Callee->getIntrinsicID() == Intrinsic::donothing) {
    352           // Remove the NOP invoke.
    353           BranchInst::Create(II->getNormalDest(), II);
    354           II->eraseFromParent();
    355           continue;
    356         }
    357 
    358       Invokes.push_back(II);
    359       LPads.insert(II->getUnwindDest()->getLandingPadInst());
    360     } else if (auto *RI = dyn_cast<ReturnInst>(BB.getTerminator())) {
    361       Returns.push_back(RI);
    362     }
    363 
    364   if (Invokes.empty())
    365     return false;
    366 
    367   NumInvokes += Invokes.size();
    368 
    369   lowerIncomingArguments(F);
    370   lowerAcrossUnwindEdges(F, Invokes);
    371 
    372   Value *FuncCtx =
    373       setupFunctionContext(F, makeArrayRef(LPads.begin(), LPads.end()));
    374   BasicBlock *EntryBB = &F.front();
    375   IRBuilder<> Builder(EntryBB->getTerminator());
    376 
    377   // Get a reference to the jump buffer.
    378   Value *JBufPtr =
    379       Builder.CreateConstGEP2_32(FunctionContextTy, FuncCtx, 0, 5, "jbuf_gep");
    380 
    381   // Save the frame pointer.
    382   Value *FramePtr = Builder.CreateConstGEP2_32(doubleUnderJBufTy, JBufPtr, 0, 0,
    383                                                "jbuf_fp_gep");
    384 
    385   Value *Val = Builder.CreateCall(FrameAddrFn, Builder.getInt32(0), "fp");
    386   Builder.CreateStore(Val, FramePtr, /*isVolatile=*/true);
    387 
    388   // Save the stack pointer.
    389   Value *StackPtr = Builder.CreateConstGEP2_32(doubleUnderJBufTy, JBufPtr, 0, 2,
    390                                                "jbuf_sp_gep");
    391 
    392   Val = Builder.CreateCall(StackAddrFn, {}, "sp");
    393   Builder.CreateStore(Val, StackPtr, /*isVolatile=*/true);
    394 
    395   // Call the setup_dispatch instrinsic. It fills in the rest of the jmpbuf.
    396   Builder.CreateCall(BuiltinSetupDispatchFn, {});
    397 
    398   // Store a pointer to the function context so that the back-end will know
    399   // where to look for it.
    400   Value *FuncCtxArg = Builder.CreateBitCast(FuncCtx, Builder.getInt8PtrTy());
    401   Builder.CreateCall(FuncCtxFn, FuncCtxArg);
    402 
    403   // At this point, we are all set up, update the invoke instructions to mark
    404   // their call_site values.
    405   for (unsigned I = 0, E = Invokes.size(); I != E; ++I) {
    406     insertCallSiteStore(Invokes[I], I + 1);
    407 
    408     ConstantInt *CallSiteNum =
    409         ConstantInt::get(Type::getInt32Ty(F.getContext()), I + 1);
    410 
    411     // Record the call site value for the back end so it stays associated with
    412     // the invoke.
    413     CallInst::Create(CallSiteFn, CallSiteNum, "", Invokes[I]);
    414   }
    415 
    416   // Mark call instructions that aren't nounwind as no-action (call_site ==
    417   // -1). Skip the entry block, as prior to then, no function context has been
    418   // created for this function and any unexpected exceptions thrown will go
    419   // directly to the caller's context, which is what we want anyway, so no need
    420   // to do anything here.
    421   for (BasicBlock &BB : F) {
    422     if (&BB == &F.front())
    423       continue;
    424     for (Instruction &I : BB)
    425       if (I.mayThrow())
    426         insertCallSiteStore(&I, -1);
    427   }
    428 
    429   // Register the function context and make sure it's known to not throw
    430   CallInst *Register =
    431       CallInst::Create(RegisterFn, FuncCtx, "", EntryBB->getTerminator());
    432   Register->setDoesNotThrow();
    433 
    434   // Following any allocas not in the entry block, update the saved SP in the
    435   // jmpbuf to the new value.
    436   for (BasicBlock &BB : F) {
    437     if (&BB == &F.front())
    438       continue;
    439     for (Instruction &I : BB) {
    440       if (auto *CI = dyn_cast<CallInst>(&I)) {
    441         if (CI->getCalledFunction() != StackRestoreFn)
    442           continue;
    443       } else if (!isa<AllocaInst>(&I)) {
    444         continue;
    445       }
    446       Instruction *StackAddr = CallInst::Create(StackAddrFn, "sp");
    447       StackAddr->insertAfter(&I);
    448       Instruction *StoreStackAddr = new StoreInst(StackAddr, StackPtr, true);
    449       StoreStackAddr->insertAfter(StackAddr);
    450     }
    451   }
    452 
    453   // Finally, for any returns from this function, if this function contains an
    454   // invoke, add a call to unregister the function context.
    455   for (ReturnInst *Return : Returns)
    456     CallInst::Create(UnregisterFn, FuncCtx, "", Return);
    457 
    458   return true;
    459 }
    460 
    461 bool SjLjEHPrepare::runOnFunction(Function &F) {
    462   Module &M = *F.getParent();
    463   RegisterFn = M.getOrInsertFunction(
    464       "_Unwind_SjLj_Register", Type::getVoidTy(M.getContext()),
    465       PointerType::getUnqual(FunctionContextTy), nullptr);
    466   UnregisterFn = M.getOrInsertFunction(
    467       "_Unwind_SjLj_Unregister", Type::getVoidTy(M.getContext()),
    468       PointerType::getUnqual(FunctionContextTy), nullptr);
    469   FrameAddrFn = Intrinsic::getDeclaration(&M, Intrinsic::frameaddress);
    470   StackAddrFn = Intrinsic::getDeclaration(&M, Intrinsic::stacksave);
    471   StackRestoreFn = Intrinsic::getDeclaration(&M, Intrinsic::stackrestore);
    472   BuiltinSetupDispatchFn =
    473     Intrinsic::getDeclaration(&M, Intrinsic::eh_sjlj_setup_dispatch);
    474   LSDAAddrFn = Intrinsic::getDeclaration(&M, Intrinsic::eh_sjlj_lsda);
    475   CallSiteFn = Intrinsic::getDeclaration(&M, Intrinsic::eh_sjlj_callsite);
    476   FuncCtxFn = Intrinsic::getDeclaration(&M, Intrinsic::eh_sjlj_functioncontext);
    477 
    478   bool Res = setupEntryBlockAndCallSites(F);
    479   return Res;
    480 }
    481