1 //===-- WasmEHPrepare - Prepare excepton handling for WebAssembly --------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This transformation is designed for use by code generators which use
11 // WebAssembly exception handling scheme.
13 // WebAssembly exception handling uses Windows exception IR for the middle level
14 // representation. This pass does the following transformation for every
16 // (In C-style pseudocode)
20 // exn = wasm.get.exception();
21 // selector = wasm.get.selector();
26 // exn = wasm.catch(0); // 0 is a tag for C++
27 // wasm.landingpad.index(index);
28 // // Only add below in case it's not a single catch (...)
29 // __wasm_lpad_context.lpad_index = index;
30 // __wasm_lpad_context.lsda = wasm.lsda();
31 // _Unwind_CallPersonality(exn);
32 // int selector = __wasm.landingpad_context.selector;
35 // Also, does the following for a cleanuppad block with a call to
36 // __clang_call_terminate():
39 // exn = wasm.get.exception();
40 // __clang_call_terminate(exn);
44 // exn = wasm.catch(0); // 0 is a tag for C++
45 // __clang_call_terminate(exn);
48 // * Background: WebAssembly EH instructions
49 // WebAssembly's try and catch instructions are structured as follows:
59 // A catch instruction in WebAssembly does not correspond to a C++ catch clause.
60 // In WebAssembly, there is a single catch instruction for all C++ exceptions.
61 // There can be more catch instructions for exceptions in other languages, but
62 // they are not generated for now. catch_all catches all exceptions including
63 // foreign exceptions (e.g. JavaScript). We turn catchpads into catch (C++ tag)
64 // and cleanuppads into catch_all, with one exception: cleanuppad with a call to
65 // __clang_call_terminate should be both in catch (C++ tag) and catch_all.
68 // * Background: Direct personality function call
69 // In WebAssembly EH, the VM is responsible for unwinding the stack once an
70 // exception is thrown. After the stack is unwound, the control flow is
71 // transfered to WebAssembly 'catch' instruction, which returns a caught
74 // Unwinding the stack is not done by libunwind but the VM, so the personality
75 // function in libcxxabi cannot be called from libunwind during the unwinding
76 // process. So after a catch instruction, we insert a call to a wrapper function
77 // in libunwind that in turn calls the real personality function.
79 // In Itanium EH, if the personality function decides there is no matching catch
80 // clause in a call frame and no cleanup action to perform, the unwinder doesn't
81 // stop there and continues unwinding. But in Wasm EH, the unwinder stops at
82 // every call frame with a catch intruction, after which the personality
83 // function is called from the compiler-generated user code here.
85 // In libunwind, we have this struct that serves as a communincation channel
86 // between the compiler-generated user code and the personality function in
89 // struct _Unwind_LandingPadContext {
90 // uintptr_t lpad_index;
92 // uintptr_t selector;
94 // struct _Unwind_LandingPadContext __wasm_lpad_context = ...;
96 // And this wrapper in libunwind calls the personality function.
98 // _Unwind_Reason_Code _Unwind_CallPersonality(void *exception_ptr) {
99 // struct _Unwind_Exception *exception_obj =
100 // (struct _Unwind_Exception *)exception_ptr;
101 // _Unwind_Reason_Code ret = __gxx_personality_v0(
102 // 1, _UA_CLEANUP_PHASE, exception_obj->exception_class, exception_obj,
103 // (struct _Unwind_Context *)__wasm_lpad_context);
107 // We pass a landing pad index, and the address of LSDA for the current function
108 // to the wrapper function _Unwind_CallPersonality in libunwind, and we retrieve
109 // the selector after it returns.
111 //===----------------------------------------------------------------------===//
113 #include "llvm/ADT/SetVector.h"
114 #include "llvm/ADT/Statistic.h"
115 #include "llvm/ADT/Triple.h"
116 #include "llvm/CodeGen/Passes.h"
117 #include "llvm/CodeGen/TargetLowering.h"
118 #include "llvm/CodeGen/TargetSubtargetInfo.h"
119 #include "llvm/CodeGen/WasmEHFuncInfo.h"
120 #include "llvm/IR/Dominators.h"
121 #include "llvm/IR/IRBuilder.h"
122 #include "llvm/IR/Intrinsics.h"
123 #include "llvm/Pass.h"
124 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
126 using namespace llvm;
128 #define DEBUG_TYPE "wasmehprepare"
131 class WasmEHPrepare : public FunctionPass {
132 Type *LPadContextTy = nullptr; // type of 'struct _Unwind_LandingPadContext'
133 GlobalVariable *LPadContextGV = nullptr; // __wasm_lpad_context
135 // Field addresses of struct _Unwind_LandingPadContext
136 Value *LPadIndexField = nullptr; // lpad_index field
137 Value *LSDAField = nullptr; // lsda field
138 Value *SelectorField = nullptr; // selector
140 Function *ThrowF = nullptr; // wasm.throw() intrinsic
141 Function *CatchF = nullptr; // wasm.catch.extract() intrinsic
142 Function *LPadIndexF = nullptr; // wasm.landingpad.index() intrinsic
143 Function *LSDAF = nullptr; // wasm.lsda() intrinsic
144 Function *GetExnF = nullptr; // wasm.get.exception() intrinsic
145 Function *GetSelectorF = nullptr; // wasm.get.ehselector() intrinsic
146 Function *CallPersonalityF = nullptr; // _Unwind_CallPersonality() wrapper
147 Function *ClangCallTermF = nullptr; // __clang_call_terminate() function
149 bool prepareEHPads(Function &F);
150 bool prepareThrows(Function &F);
152 void prepareEHPad(BasicBlock *BB, unsigned Index);
153 void prepareTerminateCleanupPad(BasicBlock *BB);
156 static char ID; // Pass identification, replacement for typeid
158 WasmEHPrepare() : FunctionPass(ID) {}
160 bool doInitialization(Module &M) override;
161 bool runOnFunction(Function &F) override;
163 StringRef getPassName() const override {
164 return "WebAssembly Exception handling preparation";
167 } // end anonymous namespace
169 char WasmEHPrepare::ID = 0;
170 INITIALIZE_PASS(WasmEHPrepare, DEBUG_TYPE, "Prepare WebAssembly exceptions",
173 FunctionPass *llvm::createWasmEHPass() { return new WasmEHPrepare(); }
175 bool WasmEHPrepare::doInitialization(Module &M) {
176 IRBuilder<> IRB(M.getContext());
177 LPadContextTy = StructType::get(IRB.getInt32Ty(), // lpad_index
178 IRB.getInt8PtrTy(), // lsda
179 IRB.getInt32Ty() // selector
184 // Erase the specified BBs if the BB does not have any remaining predecessors,
185 // and also all its dead children.
186 template <typename Container>
187 static void eraseDeadBBsAndChildren(const Container &BBs) {
188 SmallVector<BasicBlock *, 8> WL(BBs.begin(), BBs.end());
189 while (!WL.empty()) {
190 auto *BB = WL.pop_back_val();
191 if (pred_begin(BB) != pred_end(BB))
193 WL.append(succ_begin(BB), succ_end(BB));
198 bool WasmEHPrepare::runOnFunction(Function &F) {
199 bool Changed = false;
200 Changed |= prepareThrows(F);
201 Changed |= prepareEHPads(F);
205 bool WasmEHPrepare::prepareThrows(Function &F) {
206 Module &M = *F.getParent();
207 IRBuilder<> IRB(F.getContext());
208 bool Changed = false;
210 // wasm.throw() intinsic, which will be lowered to wasm 'throw' instruction.
211 ThrowF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_throw);
213 // Insert an unreachable instruction after a call to @llvm.wasm.throw and
214 // delete all following instructions within the BB, and delete all the dead
215 // children of the BB as well.
216 for (User *U : ThrowF->users()) {
217 // A call to @llvm.wasm.throw() is only generated from
218 // __builtin_wasm_throw() builtin call within libcxxabi, and cannot be an
220 auto *ThrowI = cast<CallInst>(U);
221 if (ThrowI->getFunction() != &F)
224 auto *BB = ThrowI->getParent();
225 SmallVector<BasicBlock *, 4> Succs(succ_begin(BB), succ_end(BB));
226 auto &InstList = BB->getInstList();
227 InstList.erase(std::next(BasicBlock::iterator(ThrowI)), InstList.end());
228 IRB.SetInsertPoint(BB);
229 IRB.CreateUnreachable();
230 eraseDeadBBsAndChildren(Succs);
236 bool WasmEHPrepare::prepareEHPads(Function &F) {
237 Module &M = *F.getParent();
238 IRBuilder<> IRB(F.getContext());
240 SmallVector<BasicBlock *, 16> CatchPads;
241 SmallVector<BasicBlock *, 16> CleanupPads;
242 for (BasicBlock &BB : F) {
245 auto *Pad = BB.getFirstNonPHI();
246 if (isa<CatchPadInst>(Pad))
247 CatchPads.push_back(&BB);
248 else if (isa<CleanupPadInst>(Pad))
249 CleanupPads.push_back(&BB);
252 if (CatchPads.empty() && CleanupPads.empty())
254 assert(F.hasPersonalityFn() && "Personality function not found");
256 // __wasm_lpad_context global variable
257 LPadContextGV = cast<GlobalVariable>(
258 M.getOrInsertGlobal("__wasm_lpad_context", LPadContextTy));
259 LPadIndexField = IRB.CreateConstGEP2_32(LPadContextTy, LPadContextGV, 0, 0,
262 IRB.CreateConstGEP2_32(LPadContextTy, LPadContextGV, 0, 1, "lsda_gep");
263 SelectorField = IRB.CreateConstGEP2_32(LPadContextTy, LPadContextGV, 0, 2,
266 // wasm.catch() intinsic, which will be lowered to wasm 'catch' instruction.
267 CatchF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_catch);
268 // wasm.landingpad.index() intrinsic, which is to specify landingpad index
269 LPadIndexF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_landingpad_index);
270 // wasm.lsda() intrinsic. Returns the address of LSDA table for the current
272 LSDAF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_lsda);
273 // wasm.get.exception() and wasm.get.ehselector() intrinsics. Calls to these
274 // are generated in clang.
275 GetExnF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_get_exception);
276 GetSelectorF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_get_ehselector);
278 // _Unwind_CallPersonality() wrapper function, which calls the personality
279 CallPersonalityF = cast<Function>(M.getOrInsertFunction(
280 "_Unwind_CallPersonality", IRB.getInt32Ty(), IRB.getInt8PtrTy()));
281 CallPersonalityF->setDoesNotThrow();
283 // __clang_call_terminate() function, which is inserted by clang in case a
285 ClangCallTermF = M.getFunction("__clang_call_terminate");
288 for (auto *BB : CatchPads) {
289 auto *CPI = cast<CatchPadInst>(BB->getFirstNonPHI());
290 // In case of a single catch (...), we don't need to emit LSDA
291 if (CPI->getNumArgOperands() == 1 &&
292 cast<Constant>(CPI->getArgOperand(0))->isNullValue())
293 prepareEHPad(BB, -1);
295 prepareEHPad(BB, Index++);
299 return !CatchPads.empty();
301 // Cleanuppads will turn into catch_all later, but cleanuppads with a call to
302 // __clang_call_terminate() is a special case. __clang_call_terminate() takes
303 // an exception object, so we have to duplicate call in both 'catch <C++ tag>'
304 // and 'catch_all' clauses. Here we only insert a call to catch; the
305 // duplication will be done later. In catch_all, the exception object will be
307 for (auto *BB : CleanupPads)
309 if (auto *CI = dyn_cast<CallInst>(&I))
310 if (CI->getCalledValue() == ClangCallTermF)
311 prepareEHPad(BB, -1);
316 void WasmEHPrepare::prepareEHPad(BasicBlock *BB, unsigned Index) {
317 assert(BB->isEHPad() && "BB is not an EHPad!");
318 IRBuilder<> IRB(BB->getContext());
320 IRB.SetInsertPoint(&*BB->getFirstInsertionPt());
321 // The argument to wasm.catch() is the tag for C++ exceptions, which we set to
322 // 0 for this module.
323 // Pseudocode: void *exn = wasm.catch(0);
324 Instruction *Exn = IRB.CreateCall(CatchF, IRB.getInt32(0), "exn");
325 // Replace the return value of wasm.get.exception() with the return value from
327 auto *FPI = cast<FuncletPadInst>(BB->getFirstNonPHI());
328 Instruction *GetExnCI = nullptr, *GetSelectorCI = nullptr;
329 for (auto &U : FPI->uses()) {
330 if (auto *CI = dyn_cast<CallInst>(U.getUser())) {
331 if (CI->getCalledValue() == GetExnF)
333 else if (CI->getCalledValue() == GetSelectorF)
338 assert(GetExnCI && "wasm.get.exception() call does not exist");
339 GetExnCI->replaceAllUsesWith(Exn);
340 GetExnCI->eraseFromParent();
342 // In case it is a catchpad with single catch (...) or a cleanuppad, we don't
343 // need to call personality function because we don't need a selector.
344 if (FPI->getNumArgOperands() == 0 ||
345 (FPI->getNumArgOperands() == 1 &&
346 cast<Constant>(FPI->getArgOperand(0))->isNullValue())) {
348 assert(GetSelectorCI->use_empty() &&
349 "wasm.get.ehselector() still has uses!");
350 GetSelectorCI->eraseFromParent();
354 IRB.SetInsertPoint(Exn->getNextNode());
356 // This is to create a map of <landingpad EH label, landingpad index> in
357 // SelectionDAGISel, which is to be used in EHStreamer to emit LSDA tables.
358 // Pseudocode: wasm.landingpad.index(Index);
359 IRB.CreateCall(LPadIndexF, {FPI, IRB.getInt32(Index)});
361 // Pseudocode: __wasm_lpad_context.lpad_index = index;
362 IRB.CreateStore(IRB.getInt32(Index), LPadIndexField);
364 // Store LSDA address only if this catchpad belongs to a top-level
365 // catchswitch. If there is another catchpad that dominates this pad, we don't
366 // need to store LSDA address again, because they are the same throughout the
367 // function and have been already stored before.
368 // TODO Can we not store LSDA address in user function but make libcxxabi
370 auto *CPI = cast<CatchPadInst>(FPI);
371 if (isa<ConstantTokenNone>(CPI->getCatchSwitch()->getParentPad()))
372 // Pseudocode: __wasm_lpad_context.lsda = wasm.lsda();
373 IRB.CreateStore(IRB.CreateCall(LSDAF), LSDAField);
375 // Pseudocode: _Unwind_CallPersonality(exn);
377 IRB.CreateCall(CallPersonalityF, Exn, OperandBundleDef("funclet", CPI));
378 PersCI->setDoesNotThrow();
380 // Pseudocode: int selector = __wasm.landingpad_context.selector;
381 Instruction *Selector = IRB.CreateLoad(SelectorField, "selector");
383 // Replace the return value from wasm.get.ehselector() with the selector value
384 // loaded from __wasm_lpad_context.selector.
385 assert(GetSelectorCI && "wasm.get.ehselector() call does not exist");
386 GetSelectorCI->replaceAllUsesWith(Selector);
387 GetSelectorCI->eraseFromParent();
390 void llvm::calculateWasmEHInfo(const Function *F, WasmEHFuncInfo &EHInfo) {
391 for (const auto &BB : *F) {
394 const Instruction *Pad = BB.getFirstNonPHI();
396 // If an exception is not caught by a catchpad (i.e., it is a foreign
397 // exception), it will unwind to its parent catchswitch's unwind
398 // destination. We don't record an unwind destination for cleanuppads
399 // because every exception should be caught by it.
400 if (const auto *CatchPad = dyn_cast<CatchPadInst>(Pad)) {
401 const auto *UnwindBB = CatchPad->getCatchSwitch()->getUnwindDest();
404 const Instruction *UnwindPad = UnwindBB->getFirstNonPHI();
405 if (const auto *CatchSwitch = dyn_cast<CatchSwitchInst>(UnwindPad))
406 // Currently there should be only one handler per a catchswitch.
407 EHInfo.setEHPadUnwindDest(&BB, *CatchSwitch->handlers().begin());
409 EHInfo.setEHPadUnwindDest(&BB, UnwindBB);
413 // Record the unwind destination for invoke and cleanupret instructions.
414 for (const auto &BB : *F) {
415 const Instruction *TI = BB.getTerminator();
416 BasicBlock *UnwindBB = nullptr;
417 if (const auto *Invoke = dyn_cast<InvokeInst>(TI))
418 UnwindBB = Invoke->getUnwindDest();
419 else if (const auto *CleanupRet = dyn_cast<CleanupReturnInst>(TI))
420 UnwindBB = CleanupRet->getUnwindDest();
423 const Instruction *UnwindPad = UnwindBB->getFirstNonPHI();
424 if (const auto *CatchSwitch = dyn_cast<CatchSwitchInst>(UnwindPad))
425 // Currently there should be only one handler per a catchswitch.
426 EHInfo.setThrowUnwindDest(&BB, *CatchSwitch->handlers().begin());
428 EHInfo.setThrowUnwindDest(&BB, UnwindBB);