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//=== WebAssemblyLowerEmscriptenEHSjLj.cpp - Lower exceptions for Emscripten =//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
///
/// \file
/// This file lowers exception-related instructions and setjmp/longjmp
/// function calls in order to use Emscripten's JavaScript try and catch
/// mechanism.
///
/// To handle exceptions and setjmp/longjmps, this scheme relies on JavaScript's
/// try and catch syntax and relevant exception-related libraries implemented
/// in JavaScript glue code that will be produced by Emscripten. This is similar
/// to the current Emscripten asm.js exception handling in fastcomp. For
/// fastcomp's EH / SjLj scheme, see these files in fastcomp LLVM branch:
/// (Location: https://github.com/kripken/emscripten-fastcomp)
/// lib/Target/JSBackend/NaCl/LowerEmExceptionsPass.cpp
/// lib/Target/JSBackend/NaCl/LowerEmSetjmp.cpp
/// lib/Target/JSBackend/JSBackend.cpp
/// lib/Target/JSBackend/CallHandlers.h
///
/// * Exception handling
/// This pass lowers invokes and landingpads into library functions in JS glue
/// code. Invokes are lowered into function wrappers called invoke wrappers that
/// exist in JS side, which wraps the original function call with JS try-catch.
/// If an exception occurred, cxa_throw() function in JS side sets some
/// variables (see below) so we can check whether an exception occurred from
/// wasm code and handle it appropriately.
///
/// * Setjmp-longjmp handling
/// This pass lowers setjmp to a reasonably-performant approach for emscripten.
/// The idea is that each block with a setjmp is broken up into two parts: the
/// part containing setjmp and the part right after the setjmp. The latter part
/// is either reached from the setjmp, or later from a longjmp. To handle the
/// longjmp, all calls that might longjmp are also called using invoke wrappers
/// and thus JS / try-catch. JS longjmp() function also sets some variables so
/// we can check / whether a longjmp occurred from wasm code. Each block with a
/// function call that might longjmp is also split up after the longjmp call.
/// After the longjmp call, we check whether a longjmp occurred, and if it did,
/// which setjmp it corresponds to, and jump to the right post-setjmp block.
/// We assume setjmp-longjmp handling always run after EH handling, which means
/// we don't expect any exception-related instructions when SjLj runs.
/// FIXME Currently this scheme does not support indirect call of setjmp,
/// because of the limitation of the scheme itself. fastcomp does not support it
/// either.
///
/// In detail, this pass does following things:
///
/// 1) Create three global variables: __THREW__, __threwValue, and __tempRet0.
/// __tempRet0 will be set within __cxa_find_matching_catch() function in
/// JS library, and __THREW__ and __threwValue will be set in invoke wrappers
/// in JS glue code. For what invoke wrappers are, refer to 3). These
/// variables are used for both exceptions and setjmp/longjmps.
/// __THREW__ indicates whether an exception or a longjmp occurred or not. 0
/// means nothing occurred, 1 means an exception occurred, and other numbers
/// mean a longjmp occurred. In the case of longjmp, __threwValue variable
/// indicates the corresponding setjmp buffer the longjmp corresponds to.
/// In exception handling, __tempRet0 indicates the type of an exception
/// caught, and in setjmp/longjmp, it means the second argument to longjmp
/// function.
///
/// * Exception handling
///
/// 2) Create setThrew and setTempRet0 functions.
/// The global variables created in 1) will exist in wasm address space,
/// but their values should be set in JS code, so we provide these functions
/// as interfaces to JS glue code. These functions are equivalent to the
/// following JS functions, which actually exist in asm.js version of JS
/// library.
///
/// function setThrew(threw, value) {
/// if (__THREW__ == 0) {
/// __THREW__ = threw;
/// __threwValue = value;
/// }
/// }
///
/// function setTempRet0(value) {
/// __tempRet0 = value;
/// }
///
/// 3) Lower
/// invoke @func(arg1, arg2) to label %invoke.cont unwind label %lpad
/// into
/// __THREW__ = 0;
/// call @__invoke_SIG(func, arg1, arg2)
/// %__THREW__.val = __THREW__;
/// __THREW__ = 0;
/// if (%__THREW__.val == 1)
/// goto %lpad
/// else
/// goto %invoke.cont
/// SIG is a mangled string generated based on the LLVM IR-level function
/// signature. After LLVM IR types are lowered to the target wasm types,
/// the names for these wrappers will change based on wasm types as well,
/// as in invoke_vi (function takes an int and returns void). The bodies of
/// these wrappers will be generated in JS glue code, and inside those
/// wrappers we use JS try-catch to generate actual exception effects. It
/// also calls the original callee function. An example wrapper in JS code
/// would look like this:
/// function invoke_vi(index,a1) {
/// try {
/// Module["dynCall_vi"](index,a1); // This calls original callee
/// } catch(e) {
/// if (typeof e !== 'number' && e !== 'longjmp') throw e;
/// asm["setThrew"](1, 0); // setThrew is called here
/// }
/// }
/// If an exception is thrown, __THREW__ will be set to true in a wrapper,
/// so we can jump to the right BB based on this value.
///
/// 4) Lower
/// %val = landingpad catch c1 catch c2 catch c3 ...
/// ... use %val ...
/// into
/// %fmc = call @__cxa_find_matching_catch_N(c1, c2, c3, ...)
/// %val = {%fmc, __tempRet0}
/// ... use %val ...
/// Here N is a number calculated based on the number of clauses.
/// Global variable __tempRet0 is set within __cxa_find_matching_catch() in
/// JS glue code.
///
/// 5) Lower
/// resume {%a, %b}
/// into
/// call @__resumeException(%a)
/// where __resumeException() is a function in JS glue code.
///
/// 6) Lower
/// call @llvm.eh.typeid.for(type) (intrinsic)
/// into
/// call @llvm_eh_typeid_for(type)
/// llvm_eh_typeid_for function will be generated in JS glue code.
///
/// * Setjmp / Longjmp handling
///
/// 7) In the function entry that calls setjmp, initialize setjmpTable and
/// sejmpTableSize as follows:
/// setjmpTableSize = 4;
/// setjmpTable = (int *) malloc(40);
/// setjmpTable[0] = 0;
/// setjmpTable and setjmpTableSize are used in saveSetjmp() function in JS
/// code.
///
/// 8) Lower
/// setjmp(buf)
/// into
/// setjmpTable = saveSetjmp(buf, label, setjmpTable, setjmpTableSize);
/// setjmpTableSize = __tempRet0;
/// For each dynamic setjmp call, setjmpTable stores its ID (a number which
/// is incrementally assigned from 0) and its label (a unique number that
/// represents each callsite of setjmp). When we need more entries in
/// setjmpTable, it is reallocated in saveSetjmp() in JS code and it will
/// return the new table address, and assign the new table size in
/// __tempRet0. saveSetjmp also stores the setjmp's ID into the buffer buf.
/// A BB with setjmp is split into two after setjmp call in order to make the
/// post-setjmp BB the possible destination of longjmp BB.
///
/// 9) Lower
/// longjmp(buf, value)
/// into
/// emscripten_longjmp_jmpbuf(buf, value)
/// emscripten_longjmp_jmpbuf will be lowered to emscripten_longjmp later.
///
/// 10) Lower every call that might longjmp into
/// __THREW__ = 0;
/// call @__invoke_SIG(func, arg1, arg2)
/// %__THREW__.val = __THREW__;
/// __THREW__ = 0;
/// if (%__THREW__.val != 0 & __threwValue != 0) {
/// %label = testSetjmp(mem[%__THREW__.val], setjmpTable,
/// setjmpTableSize);
/// if (%label == 0)
/// emscripten_longjmp(%__THREW__.val, __threwValue);
/// __tempRet0 = __threwValue;
/// } else {
/// %label = -1;
/// }
/// longjmp_result = __tempRet0;
/// switch label {
/// label 1: goto post-setjmp BB 1
/// label 2: goto post-setjmp BB 2
/// ...
/// default: goto splitted next BB
/// }
/// testSetjmp examines setjmpTable to see if there is a matching setjmp
/// call. After calling an invoke wrapper, if a longjmp occurred, __THREW__
/// will be the address of matching jmp_buf buffer and __threwValue be the
/// second argument to longjmp. mem[__THREW__.val] is a setjmp ID that is
/// stored in saveSetjmp. testSetjmp returns a setjmp label, a unique ID to
/// each setjmp callsite. Label 0 means this longjmp buffer does not
/// correspond to one of the setjmp callsites in this function, so in this
/// case we just chain the longjmp to the caller. (Here we call
/// emscripten_longjmp, which is different from emscripten_longjmp_jmpbuf.
/// emscripten_longjmp_jmpbuf takes jmp_buf as its first argument, while
/// emscripten_longjmp takes an int. Both of them will eventually be lowered
/// to emscripten_longjmp in s2wasm, but here we need two signatures - we
/// can't translate an int value to a jmp_buf.)
/// Label -1 means no longjmp occurred. Otherwise we jump to the right
/// post-setjmp BB based on the label.
///
///===----------------------------------------------------------------------===//
#include "WebAssembly.h"
#include "llvm/IR/CallSite.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/SSAUpdater.h"
using namespace llvm;
#define DEBUG_TYPE "wasm-lower-em-ehsjlj"
static cl::list<std::string>
EHWhitelist("emscripten-cxx-exceptions-whitelist",
cl::desc("The list of function names in which Emscripten-style "
"exception handling is enabled (see emscripten "
"EMSCRIPTEN_CATCHING_WHITELIST options)"),
cl::CommaSeparated);
namespace {
class WebAssemblyLowerEmscriptenEHSjLj final : public ModulePass {
static const char *ThrewGVName;
static const char *ThrewValueGVName;
static const char *TempRet0GVName;
static const char *ResumeFName;
static const char *EHTypeIDFName;
static const char *SetThrewFName;
static const char *SetTempRet0FName;
static const char *EmLongjmpFName;
static const char *EmLongjmpJmpbufFName;
static const char *SaveSetjmpFName;
static const char *TestSetjmpFName;
static const char *FindMatchingCatchPrefix;
static const char *InvokePrefix;
bool EnableEH; // Enable exception handling
bool EnableSjLj; // Enable setjmp/longjmp handling
GlobalVariable *ThrewGV;
GlobalVariable *ThrewValueGV;
GlobalVariable *TempRet0GV;
Function *ResumeF;
Function *EHTypeIDF;
Function *EmLongjmpF;
Function *EmLongjmpJmpbufF;
Function *SaveSetjmpF;
Function *TestSetjmpF;
// __cxa_find_matching_catch_N functions.
// Indexed by the number of clauses in an original landingpad instruction.
DenseMap<int, Function *> FindMatchingCatches;
// Map of <function signature string, invoke_ wrappers>
StringMap<Function *> InvokeWrappers;
// Set of whitelisted function names for exception handling
std::set<std::string> EHWhitelistSet;
StringRef getPassName() const override {
return "WebAssembly Lower Emscripten Exceptions";
}
bool runEHOnFunction(Function &F);
bool runSjLjOnFunction(Function &F);
Function *getFindMatchingCatch(Module &M, unsigned NumClauses);
template <typename CallOrInvoke> Value *wrapInvoke(CallOrInvoke *CI);
void wrapTestSetjmp(BasicBlock *BB, Instruction *InsertPt, Value *Threw,
Value *SetjmpTable, Value *SetjmpTableSize, Value *&Label,
Value *&LongjmpResult, BasicBlock *&EndBB);
template <typename CallOrInvoke> Function *getInvokeWrapper(CallOrInvoke *CI);
bool areAllExceptionsAllowed() const { return EHWhitelistSet.empty(); }
bool canLongjmp(Module &M, const Value *Callee) const;
void createSetThrewFunction(Module &M);
void createSetTempRet0Function(Module &M);
void rebuildSSA(Function &F);
public:
static char ID;
WebAssemblyLowerEmscriptenEHSjLj(bool EnableEH = true, bool EnableSjLj = true)
: ModulePass(ID), EnableEH(EnableEH), EnableSjLj(EnableSjLj),
ThrewGV(nullptr), ThrewValueGV(nullptr), TempRet0GV(nullptr),
ResumeF(nullptr), EHTypeIDF(nullptr), EmLongjmpF(nullptr),
EmLongjmpJmpbufF(nullptr), SaveSetjmpF(nullptr), TestSetjmpF(nullptr) {
EHWhitelistSet.insert(EHWhitelist.begin(), EHWhitelist.end());
}
bool runOnModule(Module &M) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<DominatorTreeWrapperPass>();
}
};
} // End anonymous namespace
const char *WebAssemblyLowerEmscriptenEHSjLj::ThrewGVName = "__THREW__";
const char *WebAssemblyLowerEmscriptenEHSjLj::ThrewValueGVName = "__threwValue";
const char *WebAssemblyLowerEmscriptenEHSjLj::TempRet0GVName = "__tempRet0";
const char *WebAssemblyLowerEmscriptenEHSjLj::ResumeFName = "__resumeException";
const char *WebAssemblyLowerEmscriptenEHSjLj::EHTypeIDFName =
"llvm_eh_typeid_for";
const char *WebAssemblyLowerEmscriptenEHSjLj::SetThrewFName = "setThrew";
const char *WebAssemblyLowerEmscriptenEHSjLj::SetTempRet0FName = "setTempRet0";
const char *WebAssemblyLowerEmscriptenEHSjLj::EmLongjmpFName =
"emscripten_longjmp";
const char *WebAssemblyLowerEmscriptenEHSjLj::EmLongjmpJmpbufFName =
"emscripten_longjmp_jmpbuf";
const char *WebAssemblyLowerEmscriptenEHSjLj::SaveSetjmpFName = "saveSetjmp";
const char *WebAssemblyLowerEmscriptenEHSjLj::TestSetjmpFName = "testSetjmp";
const char *WebAssemblyLowerEmscriptenEHSjLj::FindMatchingCatchPrefix =
"__cxa_find_matching_catch_";
const char *WebAssemblyLowerEmscriptenEHSjLj::InvokePrefix = "__invoke_";
char WebAssemblyLowerEmscriptenEHSjLj::ID = 0;
INITIALIZE_PASS(WebAssemblyLowerEmscriptenEHSjLj, DEBUG_TYPE,
"WebAssembly Lower Emscripten Exceptions / Setjmp / Longjmp",
false, false)
ModulePass *llvm::createWebAssemblyLowerEmscriptenEHSjLj(bool EnableEH,
bool EnableSjLj) {
return new WebAssemblyLowerEmscriptenEHSjLj(EnableEH, EnableSjLj);
}
static bool canThrow(const Value *V) {
if (const auto *F = dyn_cast<const Function>(V)) {
// Intrinsics cannot throw
if (F->isIntrinsic())
return false;
StringRef Name = F->getName();
// leave setjmp and longjmp (mostly) alone, we process them properly later
if (Name == "setjmp" || Name == "longjmp")
return false;
return !F->doesNotThrow();
}
// not a function, so an indirect call - can throw, we can't tell
return true;
}
// Returns an available name for a global value.
// If the proposed name already exists in the module, adds '_' at the end of
// the name until the name is available.
static inline std::string createGlobalValueName(const Module &M,
const std::string &Propose) {
std::string Name = Propose;
while (M.getNamedGlobal(Name))
Name += "_";
return Name;
}
// Simple function name mangler.
// This function simply takes LLVM's string representation of parameter types
// and concatenate them with '_'. There are non-alphanumeric characters but llc
// is ok with it, and we need to postprocess these names after the lowering
// phase anyway.
static std::string getSignature(FunctionType *FTy) {
std::string Sig;
raw_string_ostream OS(Sig);
OS << *FTy->getReturnType();
for (Type *ParamTy : FTy->params())
OS << "_" << *ParamTy;
if (FTy->isVarArg())
OS << "_...";
Sig = OS.str();
Sig.erase(remove_if(Sig, isspace), Sig.end());
// When s2wasm parses .s file, a comma means the end of an argument. So a
// mangled function name can contain any character but a comma.
std::replace(Sig.begin(), Sig.end(), ',', '.');
return Sig;
}
// Returns __cxa_find_matching_catch_N function, where N = NumClauses + 2.
// This is because a landingpad instruction contains two more arguments, a
// personality function and a cleanup bit, and __cxa_find_matching_catch_N
// functions are named after the number of arguments in the original landingpad
// instruction.
Function *
WebAssemblyLowerEmscriptenEHSjLj::getFindMatchingCatch(Module &M,
unsigned NumClauses) {
if (FindMatchingCatches.count(NumClauses))
return FindMatchingCatches[NumClauses];
PointerType *Int8PtrTy = Type::getInt8PtrTy(M.getContext());
SmallVector<Type *, 16> Args(NumClauses, Int8PtrTy);
FunctionType *FTy = FunctionType::get(Int8PtrTy, Args, false);
Function *F =
Function::Create(FTy, GlobalValue::ExternalLinkage,
FindMatchingCatchPrefix + Twine(NumClauses + 2), &M);
FindMatchingCatches[NumClauses] = F;
return F;
}
// Generate invoke wrapper seqence with preamble and postamble
// Preamble:
// __THREW__ = 0;
// Postamble:
// %__THREW__.val = __THREW__; __THREW__ = 0;
// Returns %__THREW__.val, which indicates whether an exception is thrown (or
// whether longjmp occurred), for future use.
template <typename CallOrInvoke>
Value *WebAssemblyLowerEmscriptenEHSjLj::wrapInvoke(CallOrInvoke *CI) {
LLVMContext &C = CI->getModule()->getContext();
// If we are calling a function that is noreturn, we must remove that
// attribute. The code we insert here does expect it to return, after we
// catch the exception.
if (CI->doesNotReturn()) {
if (auto *F = dyn_cast<Function>(CI->getCalledValue()))
F->removeFnAttr(Attribute::NoReturn);
CI->removeAttribute(AttributeList::FunctionIndex, Attribute::NoReturn);
}
IRBuilder<> IRB(C);
IRB.SetInsertPoint(CI);
// Pre-invoke
// __THREW__ = 0;
IRB.CreateStore(IRB.getInt32(0), ThrewGV);
// Invoke function wrapper in JavaScript
SmallVector<Value *, 16> Args;
// Put the pointer to the callee as first argument, so it can be called
// within the invoke wrapper later
Args.push_back(CI->getCalledValue());
Args.append(CI->arg_begin(), CI->arg_end());
CallInst *NewCall = IRB.CreateCall(getInvokeWrapper(CI), Args);
NewCall->takeName(CI);
NewCall->setCallingConv(CI->getCallingConv());
NewCall->setDebugLoc(CI->getDebugLoc());
// Because we added the pointer to the callee as first argument, all
// argument attribute indices have to be incremented by one.
SmallVector<AttributeSet, 8> ArgAttributes;
const AttributeList &InvokeAL = CI->getAttributes();
// No attributes for the callee pointer.
ArgAttributes.push_back(AttributeSet());
// Copy the argument attributes from the original
for (unsigned i = 0, e = CI->getNumArgOperands(); i < e; ++i)
ArgAttributes.push_back(InvokeAL.getParamAttributes(i));
// Reconstruct the AttributesList based on the vector we constructed.
AttributeList NewCallAL =
AttributeList::get(C, InvokeAL.getFnAttributes(),
InvokeAL.getRetAttributes(), ArgAttributes);
NewCall->setAttributes(NewCallAL);
CI->replaceAllUsesWith(NewCall);
// Post-invoke
// %__THREW__.val = __THREW__; __THREW__ = 0;
Value *Threw = IRB.CreateLoad(ThrewGV, ThrewGV->getName() + ".val");
IRB.CreateStore(IRB.getInt32(0), ThrewGV);
return Threw;
}
// Get matching invoke wrapper based on callee signature
template <typename CallOrInvoke>
Function *WebAssemblyLowerEmscriptenEHSjLj::getInvokeWrapper(CallOrInvoke *CI) {
Module *M = CI->getModule();
SmallVector<Type *, 16> ArgTys;
Value *Callee = CI->getCalledValue();
FunctionType *CalleeFTy;
if (auto *F = dyn_cast<Function>(Callee))
CalleeFTy = F->getFunctionType();
else {
auto *CalleeTy = cast<PointerType>(Callee->getType())->getElementType();
CalleeFTy = dyn_cast<FunctionType>(CalleeTy);
}
std::string Sig = getSignature(CalleeFTy);
if (InvokeWrappers.find(Sig) != InvokeWrappers.end())
return InvokeWrappers[Sig];
// Put the pointer to the callee as first argument
ArgTys.push_back(PointerType::getUnqual(CalleeFTy));
// Add argument types
ArgTys.append(CalleeFTy->param_begin(), CalleeFTy->param_end());
FunctionType *FTy = FunctionType::get(CalleeFTy->getReturnType(), ArgTys,
CalleeFTy->isVarArg());
Function *F = Function::Create(FTy, GlobalValue::ExternalLinkage,
InvokePrefix + Sig, M);
InvokeWrappers[Sig] = F;
return F;
}
bool WebAssemblyLowerEmscriptenEHSjLj::canLongjmp(Module &M,
const Value *Callee) const {
if (auto *CalleeF = dyn_cast<Function>(Callee))
if (CalleeF->isIntrinsic())
return false;
// The reason we include malloc/free here is to exclude the malloc/free
// calls generated in setjmp prep / cleanup routines.
Function *SetjmpF = M.getFunction("setjmp");
Function *MallocF = M.getFunction("malloc");
Function *FreeF = M.getFunction("free");
if (Callee == SetjmpF || Callee == MallocF || Callee == FreeF)
return false;
// There are functions in JS glue code
if (Callee == ResumeF || Callee == EHTypeIDF || Callee == SaveSetjmpF ||
Callee == TestSetjmpF)
return false;
// __cxa_find_matching_catch_N functions cannot longjmp
if (Callee->getName().startswith(FindMatchingCatchPrefix))
return false;
// Exception-catching related functions
Function *BeginCatchF = M.getFunction("__cxa_begin_catch");
Function *EndCatchF = M.getFunction("__cxa_end_catch");
Function *AllocExceptionF = M.getFunction("__cxa_allocate_exception");
Function *ThrowF = M.getFunction("__cxa_throw");
Function *TerminateF = M.getFunction("__clang_call_terminate");
if (Callee == BeginCatchF || Callee == EndCatchF ||
Callee == AllocExceptionF || Callee == ThrowF || Callee == TerminateF)
return false;
// Otherwise we don't know
return true;
}
// Generate testSetjmp function call seqence with preamble and postamble.
// The code this generates is equivalent to the following JavaScript code:
// if (%__THREW__.val != 0 & threwValue != 0) {
// %label = _testSetjmp(mem[%__THREW__.val], setjmpTable, setjmpTableSize);
// if (%label == 0)
// emscripten_longjmp(%__THREW__.val, threwValue);
// __tempRet0 = threwValue;
// } else {
// %label = -1;
// }
// %longjmp_result = __tempRet0;
//
// As output parameters. returns %label, %longjmp_result, and the BB the last
// instruction (%longjmp_result = ...) is in.
void WebAssemblyLowerEmscriptenEHSjLj::wrapTestSetjmp(
BasicBlock *BB, Instruction *InsertPt, Value *Threw, Value *SetjmpTable,
Value *SetjmpTableSize, Value *&Label, Value *&LongjmpResult,
BasicBlock *&EndBB) {
Function *F = BB->getParent();
LLVMContext &C = BB->getModule()->getContext();
IRBuilder<> IRB(C);
IRB.SetInsertPoint(InsertPt);
// if (%__THREW__.val != 0 & threwValue != 0)
IRB.SetInsertPoint(BB);
BasicBlock *ThenBB1 = BasicBlock::Create(C, "if.then1", F);
BasicBlock *ElseBB1 = BasicBlock::Create(C, "if.else1", F);
BasicBlock *EndBB1 = BasicBlock::Create(C, "if.end", F);
Value *ThrewCmp = IRB.CreateICmpNE(Threw, IRB.getInt32(0));
Value *ThrewValue =
IRB.CreateLoad(ThrewValueGV, ThrewValueGV->getName() + ".val");
Value *ThrewValueCmp = IRB.CreateICmpNE(ThrewValue, IRB.getInt32(0));
Value *Cmp1 = IRB.CreateAnd(ThrewCmp, ThrewValueCmp, "cmp1");
IRB.CreateCondBr(Cmp1, ThenBB1, ElseBB1);
// %label = _testSetjmp(mem[%__THREW__.val], _setjmpTable, _setjmpTableSize);
// if (%label == 0)
IRB.SetInsertPoint(ThenBB1);
BasicBlock *ThenBB2 = BasicBlock::Create(C, "if.then2", F);
BasicBlock *EndBB2 = BasicBlock::Create(C, "if.end2", F);
Value *ThrewInt = IRB.CreateIntToPtr(Threw, Type::getInt32PtrTy(C),
Threw->getName() + ".i32p");
Value *LoadedThrew =
IRB.CreateLoad(ThrewInt, ThrewInt->getName() + ".loaded");
Value *ThenLabel = IRB.CreateCall(
TestSetjmpF, {LoadedThrew, SetjmpTable, SetjmpTableSize}, "label");
Value *Cmp2 = IRB.CreateICmpEQ(ThenLabel, IRB.getInt32(0));
IRB.CreateCondBr(Cmp2, ThenBB2, EndBB2);
// emscripten_longjmp(%__THREW__.val, threwValue);
IRB.SetInsertPoint(ThenBB2);
IRB.CreateCall(EmLongjmpF, {Threw, ThrewValue});
IRB.CreateUnreachable();
// __tempRet0 = threwValue;
IRB.SetInsertPoint(EndBB2);
IRB.CreateStore(ThrewValue, TempRet0GV);
IRB.CreateBr(EndBB1);
IRB.SetInsertPoint(ElseBB1);
IRB.CreateBr(EndBB1);
// longjmp_result = __tempRet0;
IRB.SetInsertPoint(EndBB1);
PHINode *LabelPHI = IRB.CreatePHI(IRB.getInt32Ty(), 2, "label");
LabelPHI->addIncoming(ThenLabel, EndBB2);
LabelPHI->addIncoming(IRB.getInt32(-1), ElseBB1);
// Output parameter assignment
Label = LabelPHI;
EndBB = EndBB1;
LongjmpResult = IRB.CreateLoad(TempRet0GV, "longjmp_result");
}
// Create setThrew function
// function setThrew(threw, value) {
// if (__THREW__ == 0) {
// __THREW__ = threw;
// __threwValue = value;
// }
// }
void WebAssemblyLowerEmscriptenEHSjLj::createSetThrewFunction(Module &M) {
LLVMContext &C = M.getContext();
IRBuilder<> IRB(C);
assert(!M.getNamedGlobal(SetThrewFName) && "setThrew already exists");
Type *Params[] = {IRB.getInt32Ty(), IRB.getInt32Ty()};
FunctionType *FTy = FunctionType::get(IRB.getVoidTy(), Params, false);
Function *F =
Function::Create(FTy, GlobalValue::ExternalLinkage, SetThrewFName, &M);
Argument *Arg1 = &*(F->arg_begin());
Argument *Arg2 = &*std::next(F->arg_begin());
Arg1->setName("threw");
Arg2->setName("value");
BasicBlock *EntryBB = BasicBlock::Create(C, "entry", F);
BasicBlock *ThenBB = BasicBlock::Create(C, "if.then", F);
BasicBlock *EndBB = BasicBlock::Create(C, "if.end", F);
IRB.SetInsertPoint(EntryBB);
Value *Threw = IRB.CreateLoad(ThrewGV, ThrewGV->getName() + ".val");
Value *Cmp = IRB.CreateICmpEQ(Threw, IRB.getInt32(0), "cmp");
IRB.CreateCondBr(Cmp, ThenBB, EndBB);
IRB.SetInsertPoint(ThenBB);
IRB.CreateStore(Arg1, ThrewGV);
IRB.CreateStore(Arg2, ThrewValueGV);
IRB.CreateBr(EndBB);
IRB.SetInsertPoint(EndBB);
IRB.CreateRetVoid();
}
// Create setTempRet0 function
// function setTempRet0(value) {
// __tempRet0 = value;
// }
void WebAssemblyLowerEmscriptenEHSjLj::createSetTempRet0Function(Module &M) {
LLVMContext &C = M.getContext();
IRBuilder<> IRB(C);
assert(!M.getNamedGlobal(SetTempRet0FName) && "setTempRet0 already exists");
Type *Params[] = {IRB.getInt32Ty()};
FunctionType *FTy = FunctionType::get(IRB.getVoidTy(), Params, false);
Function *F =
Function::Create(FTy, GlobalValue::ExternalLinkage, SetTempRet0FName, &M);
F->arg_begin()->setName("value");
BasicBlock *EntryBB = BasicBlock::Create(C, "entry", F);
IRB.SetInsertPoint(EntryBB);
IRB.CreateStore(&*F->arg_begin(), TempRet0GV);
IRB.CreateRetVoid();
}
void WebAssemblyLowerEmscriptenEHSjLj::rebuildSSA(Function &F) {
DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>(F).getDomTree();
DT.recalculate(F); // CFG has been changed
SSAUpdater SSA;
for (BasicBlock &BB : F) {
for (Instruction &I : BB) {
for (auto UI = I.use_begin(), UE = I.use_end(); UI != UE;) {
Use &U = *UI;
++UI;
SSA.Initialize(I.getType(), I.getName());
SSA.AddAvailableValue(&BB, &I);
Instruction *User = cast<Instruction>(U.getUser());
if (User->getParent() == &BB)
continue;
if (PHINode *UserPN = dyn_cast<PHINode>(User))
if (UserPN->getIncomingBlock(U) == &BB)
continue;
if (DT.dominates(&I, User))
continue;
SSA.RewriteUseAfterInsertions(U);
}
}
}
}
bool WebAssemblyLowerEmscriptenEHSjLj::runOnModule(Module &M) {
LLVMContext &C = M.getContext();
IRBuilder<> IRB(C);
Function *SetjmpF = M.getFunction("setjmp");
Function *LongjmpF = M.getFunction("longjmp");
bool SetjmpUsed = SetjmpF && !SetjmpF->use_empty();
bool LongjmpUsed = LongjmpF && !LongjmpF->use_empty();
bool DoSjLj = EnableSjLj && (SetjmpUsed || LongjmpUsed);
// Create global variables __THREW__, threwValue, and __tempRet0, which are
// used in common for both exception handling and setjmp/longjmp handling
ThrewGV = new GlobalVariable(M, IRB.getInt32Ty(), false,
GlobalValue::ExternalLinkage, IRB.getInt32(0),
createGlobalValueName(M, ThrewGVName));
ThrewValueGV = new GlobalVariable(
M, IRB.getInt32Ty(), false, GlobalValue::ExternalLinkage, IRB.getInt32(0),
createGlobalValueName(M, ThrewValueGVName));
TempRet0GV = new GlobalVariable(M, IRB.getInt32Ty(), false,
GlobalValue::ExternalLinkage, IRB.getInt32(0),
createGlobalValueName(M, TempRet0GVName));
bool Changed = false;
// Exception handling
if (EnableEH) {
// Register __resumeException function
FunctionType *ResumeFTy =
FunctionType::get(IRB.getVoidTy(), IRB.getInt8PtrTy(), false);
ResumeF = Function::Create(ResumeFTy, GlobalValue::ExternalLinkage,
ResumeFName, &M);
// Register llvm_eh_typeid_for function
FunctionType *EHTypeIDTy =
FunctionType::get(IRB.getInt32Ty(), IRB.getInt8PtrTy(), false);
EHTypeIDF = Function::Create(EHTypeIDTy, GlobalValue::ExternalLinkage,
EHTypeIDFName, &M);
for (Function &F : M) {
if (F.isDeclaration())
continue;
Changed |= runEHOnFunction(F);
}
}
// Setjmp/longjmp handling
if (DoSjLj) {
Changed = true; // We have setjmp or longjmp somewhere
Function *MallocF = M.getFunction("malloc");
Function *FreeF = M.getFunction("free");
if (!MallocF || !FreeF)
report_fatal_error(
"malloc and free must be linked into the module if setjmp is used");
// Register saveSetjmp function
FunctionType *SetjmpFTy = SetjmpF->getFunctionType();
SmallVector<Type *, 4> Params = {SetjmpFTy->getParamType(0),
IRB.getInt32Ty(), Type::getInt32PtrTy(C),
IRB.getInt32Ty()};
FunctionType *FTy =
FunctionType::get(Type::getInt32PtrTy(C), Params, false);
SaveSetjmpF = Function::Create(FTy, GlobalValue::ExternalLinkage,
SaveSetjmpFName, &M);
// Register testSetjmp function
Params = {IRB.getInt32Ty(), Type::getInt32PtrTy(C), IRB.getInt32Ty()};
FTy = FunctionType::get(IRB.getInt32Ty(), Params, false);
TestSetjmpF = Function::Create(FTy, GlobalValue::ExternalLinkage,
TestSetjmpFName, &M);
if (LongjmpF) {
// Replace all uses of longjmp with emscripten_longjmp_jmpbuf, which is
// defined in JS code
EmLongjmpJmpbufF = Function::Create(LongjmpF->getFunctionType(),
GlobalValue::ExternalLinkage,
EmLongjmpJmpbufFName, &M);
LongjmpF->replaceAllUsesWith(EmLongjmpJmpbufF);
}
FTy = FunctionType::get(IRB.getVoidTy(),
{IRB.getInt32Ty(), IRB.getInt32Ty()}, false);
EmLongjmpF =
Function::Create(FTy, GlobalValue::ExternalLinkage, EmLongjmpFName, &M);
// Only traverse functions that uses setjmp in order not to insert
// unnecessary prep / cleanup code in every function
SmallPtrSet<Function *, 8> SetjmpUsers;
for (User *U : SetjmpF->users()) {
auto *UI = cast<Instruction>(U);
SetjmpUsers.insert(UI->getFunction());
}
for (Function *F : SetjmpUsers)
runSjLjOnFunction(*F);
}
if (!Changed) {
// Delete unused global variables and functions
ThrewGV->eraseFromParent();
ThrewValueGV->eraseFromParent();
TempRet0GV->eraseFromParent();
if (ResumeF)
ResumeF->eraseFromParent();
if (EHTypeIDF)
EHTypeIDF->eraseFromParent();
if (EmLongjmpF)
EmLongjmpF->eraseFromParent();
if (SaveSetjmpF)
SaveSetjmpF->eraseFromParent();
if (TestSetjmpF)
TestSetjmpF->eraseFromParent();
return false;
}
// If we have made any changes while doing exception handling or
// setjmp/longjmp handling, we have to create these functions for JavaScript
// to call.
createSetThrewFunction(M);
createSetTempRet0Function(M);
return true;
}
bool WebAssemblyLowerEmscriptenEHSjLj::runEHOnFunction(Function &F) {
Module &M = *F.getParent();
LLVMContext &C = F.getContext();
IRBuilder<> IRB(C);
bool Changed = false;
SmallVector<Instruction *, 64> ToErase;
SmallPtrSet<LandingPadInst *, 32> LandingPads;
bool AllowExceptions =
areAllExceptionsAllowed() || EHWhitelistSet.count(F.getName());
for (BasicBlock &BB : F) {
auto *II = dyn_cast<InvokeInst>(BB.getTerminator());
if (!II)
continue;
Changed = true;
LandingPads.insert(II->getLandingPadInst());
IRB.SetInsertPoint(II);
bool NeedInvoke = AllowExceptions && canThrow(II->getCalledValue());
if (NeedInvoke) {
// Wrap invoke with invoke wrapper and generate preamble/postamble
Value *Threw = wrapInvoke(II);
ToErase.push_back(II);
// Insert a branch based on __THREW__ variable
Value *Cmp = IRB.CreateICmpEQ(Threw, IRB.getInt32(1), "cmp");
IRB.CreateCondBr(Cmp, II->getUnwindDest(), II->getNormalDest());
} else {
// This can't throw, and we don't need this invoke, just replace it with a
// call+branch
SmallVector<Value *, 16> Args(II->arg_begin(), II->arg_end());
CallInst *NewCall = IRB.CreateCall(II->getCalledValue(), Args);
NewCall->takeName(II);
NewCall->setCallingConv(II->getCallingConv());
NewCall->setDebugLoc(II->getDebugLoc());
NewCall->setAttributes(II->getAttributes());
II->replaceAllUsesWith(NewCall);
ToErase.push_back(II);
IRB.CreateBr(II->getNormalDest());
// Remove any PHI node entries from the exception destination
II->getUnwindDest()->removePredecessor(&BB);
}
}
// Process resume instructions
for (BasicBlock &BB : F) {
// Scan the body of the basic block for resumes
for (Instruction &I : BB) {
auto *RI = dyn_cast<ResumeInst>(&I);
if (!RI)
continue;
// Split the input into legal values
Value *Input = RI->getValue();
IRB.SetInsertPoint(RI);
Value *Low = IRB.CreateExtractValue(Input, 0, "low");
// Create a call to __resumeException function
IRB.CreateCall(ResumeF, {Low});
// Add a terminator to the block
IRB.CreateUnreachable();
ToErase.push_back(RI);
}
}
// Process llvm.eh.typeid.for intrinsics
for (BasicBlock &BB : F) {
for (Instruction &I : BB) {
auto *CI = dyn_cast<CallInst>(&I);
if (!CI)
continue;
const Function *Callee = CI->getCalledFunction();
if (!Callee)
continue;
if (Callee->getIntrinsicID() != Intrinsic::eh_typeid_for)
continue;
IRB.SetInsertPoint(CI);
CallInst *NewCI =
IRB.CreateCall(EHTypeIDF, CI->getArgOperand(0), "typeid");
CI->replaceAllUsesWith(NewCI);
ToErase.push_back(CI);
}
}
// Look for orphan landingpads, can occur in blocks with no predecessors
for (BasicBlock &BB : F) {
Instruction *I = BB.getFirstNonPHI();
if (auto *LPI = dyn_cast<LandingPadInst>(I))
LandingPads.insert(LPI);
}
// Handle all the landingpad for this function together, as multiple invokes
// may share a single lp
for (LandingPadInst *LPI : LandingPads) {
IRB.SetInsertPoint(LPI);
SmallVector<Value *, 16> FMCArgs;
for (unsigned i = 0, e = LPI->getNumClauses(); i < e; ++i) {
Constant *Clause = LPI->getClause(i);
// As a temporary workaround for the lack of aggregate varargs support
// in the interface between JS and wasm, break out filter operands into
// their component elements.
if (LPI->isFilter(i)) {
auto *ATy = cast<ArrayType>(Clause->getType());
for (unsigned j = 0, e = ATy->getNumElements(); j < e; ++j) {
Value *EV = IRB.CreateExtractValue(Clause, makeArrayRef(j), "filter");
FMCArgs.push_back(EV);
}
} else
FMCArgs.push_back(Clause);
}
// Create a call to __cxa_find_matching_catch_N function
Function *FMCF = getFindMatchingCatch(M, FMCArgs.size());
CallInst *FMCI = IRB.CreateCall(FMCF, FMCArgs, "fmc");
Value *Undef = UndefValue::get(LPI->getType());
Value *Pair0 = IRB.CreateInsertValue(Undef, FMCI, 0, "pair0");
Value *TempRet0 =
IRB.CreateLoad(TempRet0GV, TempRet0GV->getName() + ".val");
Value *Pair1 = IRB.CreateInsertValue(Pair0, TempRet0, 1, "pair1");
LPI->replaceAllUsesWith(Pair1);
ToErase.push_back(LPI);
}
// Erase everything we no longer need in this function
for (Instruction *I : ToErase)
I->eraseFromParent();
return Changed;
}
bool WebAssemblyLowerEmscriptenEHSjLj::runSjLjOnFunction(Function &F) {
Module &M = *F.getParent();
LLVMContext &C = F.getContext();
IRBuilder<> IRB(C);
SmallVector<Instruction *, 64> ToErase;
// Vector of %setjmpTable values
std::vector<Instruction *> SetjmpTableInsts;
// Vector of %setjmpTableSize values
std::vector<Instruction *> SetjmpTableSizeInsts;
// Setjmp preparation
// This instruction effectively means %setjmpTableSize = 4.
// We create this as an instruction intentionally, and we don't want to fold
// this instruction to a constant 4, because this value will be used in
// SSAUpdater.AddAvailableValue(...) later.
BasicBlock &EntryBB = F.getEntryBlock();
BinaryOperator *SetjmpTableSize = BinaryOperator::Create(
Instruction::Add, IRB.getInt32(4), IRB.getInt32(0), "setjmpTableSize",
&*EntryBB.getFirstInsertionPt());
// setjmpTable = (int *) malloc(40);
Instruction *SetjmpTable = CallInst::CreateMalloc(
SetjmpTableSize, IRB.getInt32Ty(), IRB.getInt32Ty(), IRB.getInt32(40),
nullptr, nullptr, "setjmpTable");
// setjmpTable[0] = 0;
IRB.SetInsertPoint(SetjmpTableSize);
IRB.CreateStore(IRB.getInt32(0), SetjmpTable);
SetjmpTableInsts.push_back(SetjmpTable);
SetjmpTableSizeInsts.push_back(SetjmpTableSize);
// Setjmp transformation
std::vector<PHINode *> SetjmpRetPHIs;
Function *SetjmpF = M.getFunction("setjmp");
for (User *U : SetjmpF->users()) {
auto *CI = dyn_cast<CallInst>(U);
if (!CI)
report_fatal_error("Does not support indirect calls to setjmp");
BasicBlock *BB = CI->getParent();
if (BB->getParent() != &F) // in other function
continue;
// The tail is everything right after the call, and will be reached once
// when setjmp is called, and later when longjmp returns to the setjmp
BasicBlock *Tail = SplitBlock(BB, CI->getNextNode());
// Add a phi to the tail, which will be the output of setjmp, which
// indicates if this is the first call or a longjmp back. The phi directly
// uses the right value based on where we arrive from
IRB.SetInsertPoint(Tail->getFirstNonPHI());
PHINode *SetjmpRet = IRB.CreatePHI(IRB.getInt32Ty(), 2, "setjmp.ret");
// setjmp initial call returns 0
SetjmpRet->addIncoming(IRB.getInt32(0), BB);
// The proper output is now this, not the setjmp call itself
CI->replaceAllUsesWith(SetjmpRet);
// longjmp returns to the setjmp will add themselves to this phi
SetjmpRetPHIs.push_back(SetjmpRet);
// Fix call target
// Our index in the function is our place in the array + 1 to avoid index
// 0, because index 0 means the longjmp is not ours to handle.
IRB.SetInsertPoint(CI);
Value *Args[] = {CI->getArgOperand(0), IRB.getInt32(SetjmpRetPHIs.size()),
SetjmpTable, SetjmpTableSize};
Instruction *NewSetjmpTable =
IRB.CreateCall(SaveSetjmpF, Args, "setjmpTable");
Instruction *NewSetjmpTableSize =
IRB.CreateLoad(TempRet0GV, "setjmpTableSize");
SetjmpTableInsts.push_back(NewSetjmpTable);
SetjmpTableSizeInsts.push_back(NewSetjmpTableSize);
ToErase.push_back(CI);
}
// Update each call that can longjmp so it can return to a setjmp where
// relevant.
// Because we are creating new BBs while processing and don't want to make
// all these newly created BBs candidates again for longjmp processing, we
// first make the vector of candidate BBs.
std::vector<BasicBlock *> BBs;
for (BasicBlock &BB : F)
BBs.push_back(&BB);
// BBs.size() will change within the loop, so we query it every time
for (unsigned i = 0; i < BBs.size(); i++) {
BasicBlock *BB = BBs[i];
for (Instruction &I : *BB) {
assert(!isa<InvokeInst>(&I));
auto *CI = dyn_cast<CallInst>(&I);
if (!CI)
continue;
const Value *Callee = CI->getCalledValue();
if (!canLongjmp(M, Callee))
continue;
Value *Threw = nullptr;
BasicBlock *Tail;
if (Callee->getName().startswith(InvokePrefix)) {
// If invoke wrapper has already been generated for this call in
// previous EH phase, search for the load instruction
// %__THREW__.val = __THREW__;
// in postamble after the invoke wrapper call
LoadInst *ThrewLI = nullptr;
StoreInst *ThrewResetSI = nullptr;
for (auto I = std::next(BasicBlock::iterator(CI)), IE = BB->end();
I != IE; ++I) {
if (auto *LI = dyn_cast<LoadInst>(I))
if (auto *GV = dyn_cast<GlobalVariable>(LI->getPointerOperand()))
if (GV == ThrewGV) {
Threw = ThrewLI = LI;
break;
}
}
// Search for the store instruction after the load above
// __THREW__ = 0;
for (auto I = std::next(BasicBlock::iterator(ThrewLI)), IE = BB->end();
I != IE; ++I) {
if (auto *SI = dyn_cast<StoreInst>(I))
if (auto *GV = dyn_cast<GlobalVariable>(SI->getPointerOperand()))
if (GV == ThrewGV && SI->getValueOperand() == IRB.getInt32(0)) {
ThrewResetSI = SI;
break;
}
}
assert(Threw && ThrewLI && "Cannot find __THREW__ load after invoke");
assert(ThrewResetSI && "Cannot find __THREW__ store after invoke");
Tail = SplitBlock(BB, ThrewResetSI->getNextNode());
} else {
// Wrap call with invoke wrapper and generate preamble/postamble
Threw = wrapInvoke(CI);
ToErase.push_back(CI);
Tail = SplitBlock(BB, CI->getNextNode());
}
// We need to replace the terminator in Tail - SplitBlock makes BB go
// straight to Tail, we need to check if a longjmp occurred, and go to the
// right setjmp-tail if so
ToErase.push_back(BB->getTerminator());
// Generate a function call to testSetjmp function and preamble/postamble
// code to figure out (1) whether longjmp occurred (2) if longjmp
// occurred, which setjmp it corresponds to
Value *Label = nullptr;
Value *LongjmpResult = nullptr;
BasicBlock *EndBB = nullptr;
wrapTestSetjmp(BB, CI, Threw, SetjmpTable, SetjmpTableSize, Label,
LongjmpResult, EndBB);
assert(Label && LongjmpResult && EndBB);
// Create switch instruction
IRB.SetInsertPoint(EndBB);
SwitchInst *SI = IRB.CreateSwitch(Label, Tail, SetjmpRetPHIs.size());
// -1 means no longjmp happened, continue normally (will hit the default
// switch case). 0 means a longjmp that is not ours to handle, needs a
// rethrow. Otherwise the index is the same as the index in P+1 (to avoid
// 0).
for (unsigned i = 0; i < SetjmpRetPHIs.size(); i++) {
SI->addCase(IRB.getInt32(i + 1), SetjmpRetPHIs[i]->getParent());
SetjmpRetPHIs[i]->addIncoming(LongjmpResult, EndBB);
}
// We are splitting the block here, and must continue to find other calls
// in the block - which is now split. so continue to traverse in the Tail
BBs.push_back(Tail);
}
}
// Erase everything we no longer need in this function
for (Instruction *I : ToErase)
I->eraseFromParent();
// Free setjmpTable buffer before each return instruction
for (BasicBlock &BB : F) {
TerminatorInst *TI = BB.getTerminator();
if (isa<ReturnInst>(TI))
CallInst::CreateFree(SetjmpTable, TI);
}
// Every call to saveSetjmp can change setjmpTable and setjmpTableSize
// (when buffer reallocation occurs)
// entry:
// setjmpTableSize = 4;
// setjmpTable = (int *) malloc(40);
// setjmpTable[0] = 0;
// ...
// somebb:
// setjmpTable = saveSetjmp(buf, label, setjmpTable, setjmpTableSize);
// setjmpTableSize = __tempRet0;
// So we need to make sure the SSA for these variables is valid so that every
// saveSetjmp and testSetjmp calls have the correct arguments.
SSAUpdater SetjmpTableSSA;
SSAUpdater SetjmpTableSizeSSA;
SetjmpTableSSA.Initialize(Type::getInt32PtrTy(C), "setjmpTable");
SetjmpTableSizeSSA.Initialize(Type::getInt32Ty(C), "setjmpTableSize");
for (Instruction *I : SetjmpTableInsts)
SetjmpTableSSA.AddAvailableValue(I->getParent(), I);
for (Instruction *I : SetjmpTableSizeInsts)
SetjmpTableSizeSSA.AddAvailableValue(I->getParent(), I);
for (auto UI = SetjmpTable->use_begin(), UE = SetjmpTable->use_end();
UI != UE;) {
// Grab the use before incrementing the iterator.
Use &U = *UI;
// Increment the iterator before removing the use from the list.
++UI;
if (Instruction *I = dyn_cast<Instruction>(U.getUser()))
if (I->getParent() != &EntryBB)
SetjmpTableSSA.RewriteUse(U);
}
for (auto UI = SetjmpTableSize->use_begin(), UE = SetjmpTableSize->use_end();
UI != UE;) {
Use &U = *UI;
++UI;
if (Instruction *I = dyn_cast<Instruction>(U.getUser()))
if (I->getParent() != &EntryBB)
SetjmpTableSizeSSA.RewriteUse(U);
}
// Finally, our modifications to the cfg can break dominance of SSA variables.
// For example, in this code,
// if (x()) { .. setjmp() .. }
// if (y()) { .. longjmp() .. }
// We must split the longjmp block, and it can jump into the block splitted
// from setjmp one. But that means that when we split the setjmp block, it's
// first part no longer dominates its second part - there is a theoretically
// possible control flow path where x() is false, then y() is true and we
// reach the second part of the setjmp block, without ever reaching the first
// part. So, we rebuild SSA form here.
rebuildSSA(F);
return true;
}
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