1//===-- WasmEHPrepare - Prepare excepton handling for WebAssembly --------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8//
9// This transformation is designed for use by code generators which use
10// WebAssembly exception handling scheme. This currently supports C++
11// exceptions.
12//
13// WebAssembly exception handling uses Windows exception IR for the middle level
14// representation. This pass does the following transformation for every
15// catchpad block:
16// (In C-style pseudocode)
17//
18// - Before:
19// catchpad ...
20// exn = wasm.get.exception();
21// selector = wasm.get.selector();
22// ...
23//
24// - After:
25// catchpad ...
26// exn = wasm.catch(WebAssembly::CPP_EXCEPTION);
27// // Only add below in case it's not a single catch (...)
28// wasm.landingpad.index(index);
29// __wasm_lpad_context.lpad_index = index;
30// __wasm_lpad_context.lsda = wasm.lsda();
31// _Unwind_CallPersonality(exn);
32// selector = __wasm_lpad_context.selector;
33// ...
34//
35//
36// * Background: Direct personality function call
37// In WebAssembly EH, the VM is responsible for unwinding the stack once an
38// exception is thrown. After the stack is unwound, the control flow is
39// transfered to WebAssembly 'catch' instruction.
40//
41// Unwinding the stack is not done by libunwind but the VM, so the personality
42// function in libcxxabi cannot be called from libunwind during the unwinding
43// process. So after a catch instruction, we insert a call to a wrapper function
44// in libunwind that in turn calls the real personality function.
45//
46// In Itanium EH, if the personality function decides there is no matching catch
47// clause in a call frame and no cleanup action to perform, the unwinder doesn't
48// stop there and continues unwinding. But in Wasm EH, the unwinder stops at
49// every call frame with a catch intruction, after which the personality
50// function is called from the compiler-generated user code here.
51//
52// In libunwind, we have this struct that serves as a communincation channel
53// between the compiler-generated user code and the personality function in
54// libcxxabi.
55//
56// struct _Unwind_LandingPadContext {
57// uintptr_t lpad_index;
58// uintptr_t lsda;
59// uintptr_t selector;
60// };
61// struct _Unwind_LandingPadContext __wasm_lpad_context = ...;
62//
63// And this wrapper in libunwind calls the personality function.
64//
65// _Unwind_Reason_Code _Unwind_CallPersonality(void *exception_ptr) {
66// struct _Unwind_Exception *exception_obj =
67// (struct _Unwind_Exception *)exception_ptr;
68// _Unwind_Reason_Code ret = __gxx_personality_v0(
69// 1, _UA_CLEANUP_PHASE, exception_obj->exception_class, exception_obj,
70// (struct _Unwind_Context *)__wasm_lpad_context);
71// return ret;
72// }
73//
74// We pass a landing pad index, and the address of LSDA for the current function
75// to the wrapper function _Unwind_CallPersonality in libunwind, and we retrieve
76// the selector after it returns.
77//
78//===----------------------------------------------------------------------===//
79
80#include "llvm/CodeGen/WasmEHPrepare.h"
81#include "llvm/CodeGen/MachineBasicBlock.h"
82#include "llvm/CodeGen/Passes.h"
83#include "llvm/CodeGen/WasmEHFuncInfo.h"
84#include "llvm/IR/EHPersonalities.h"
85#include "llvm/IR/IRBuilder.h"
86#include "llvm/IR/IntrinsicsWebAssembly.h"
87#include "llvm/IR/Module.h"
88#include "llvm/IR/RuntimeLibcalls.h"
89#include "llvm/InitializePasses.h"
90#include "llvm/Transforms/Utils/BasicBlockUtils.h"
91
92using namespace llvm;
93
94#define DEBUG_TYPE "wasm-eh-prepare"
95
96namespace {
97class WasmEHPrepareImpl {
98 friend class WasmEHPrepare;
99
100 Type *LPadContextTy = nullptr; // type of 'struct _Unwind_LandingPadContext'
101 GlobalVariable *LPadContextGV = nullptr; // __wasm_lpad_context
102
103 // Field addresses of struct _Unwind_LandingPadContext
104 Value *LPadIndexField = nullptr; // lpad_index field
105 Value *LSDAField = nullptr; // lsda field
106 Value *SelectorField = nullptr; // selector
107
108 Function *ThrowF = nullptr; // wasm.throw() intrinsic
109 Function *LPadIndexF = nullptr; // wasm.landingpad.index() intrinsic
110 Function *LSDAF = nullptr; // wasm.lsda() intrinsic
111 Function *GetExnF = nullptr; // wasm.get.exception() intrinsic
112 Function *CatchF = nullptr; // wasm.catch() intrinsic
113 Function *GetSelectorF = nullptr; // wasm.get.ehselector() intrinsic
114 FunctionCallee CallPersonalityF =
115 nullptr; // _Unwind_CallPersonality() wrapper
116
117 bool prepareThrows(Function &F);
118 bool prepareEHPads(Function &F);
119 void prepareEHPad(BasicBlock *BB, bool NeedPersonality, unsigned Index = 0);
120
121public:
122 WasmEHPrepareImpl() = default;
123 WasmEHPrepareImpl(Type *LPadContextTy_) : LPadContextTy(LPadContextTy_) {}
124 bool runOnFunction(Function &F);
125};
126
127class WasmEHPrepare : public FunctionPass {
128 WasmEHPrepareImpl P;
129
130public:
131 static char ID; // Pass identification, replacement for typeid
132
133 WasmEHPrepare() : FunctionPass(ID) {}
134 bool doInitialization(Module &M) override;
135 bool runOnFunction(Function &F) override { return P.runOnFunction(F); }
136
137 StringRef getPassName() const override {
138 return "WebAssembly Exception handling preparation";
139 }
140};
141
142} // end anonymous namespace
143
144PreservedAnalyses WasmEHPreparePass::run(Function &F,
145 FunctionAnalysisManager &) {
146 auto &Context = F.getContext();
147 auto *I32Ty = Type::getInt32Ty(C&: Context);
148 auto *PtrTy = PointerType::get(C&: Context, AddressSpace: 0);
149 auto *LPadContextTy =
150 StructType::get(elt1: I32Ty /*lpad_index*/, elts: PtrTy /*lsda*/, elts: I32Ty /*selector*/);
151 WasmEHPrepareImpl P(LPadContextTy);
152 bool Changed = P.runOnFunction(F);
153 return Changed ? PreservedAnalyses::none() : PreservedAnalyses ::all();
154}
155
156char WasmEHPrepare::ID = 0;
157INITIALIZE_PASS_BEGIN(WasmEHPrepare, DEBUG_TYPE,
158 "Prepare WebAssembly exceptions", false, false)
159INITIALIZE_PASS_END(WasmEHPrepare, DEBUG_TYPE, "Prepare WebAssembly exceptions",
160 false, false)
161
162FunctionPass *llvm::createWasmEHPass() { return new WasmEHPrepare(); }
163
164bool WasmEHPrepare::doInitialization(Module &M) {
165 IRBuilder<> IRB(M.getContext());
166 P.LPadContextTy = StructType::get(elt1: IRB.getInt32Ty(), // lpad_index
167 elts: IRB.getPtrTy(), // lsda
168 elts: IRB.getInt32Ty() // selector
169 );
170 return false;
171}
172
173// Erase the specified BBs if the BB does not have any remaining predecessors,
174// and also all its dead children.
175template <typename Container>
176static void eraseDeadBBsAndChildren(const Container &BBs) {
177 SmallVector<BasicBlock *, 8> WL(BBs.begin(), BBs.end());
178 while (!WL.empty()) {
179 auto *BB = WL.pop_back_val();
180 if (!pred_empty(BB))
181 continue;
182 WL.append(in_start: succ_begin(BB), in_end: succ_end(BB));
183 DeleteDeadBlock(BB);
184 }
185}
186
187bool WasmEHPrepareImpl::runOnFunction(Function &F) {
188 bool Changed = false;
189 Changed |= prepareThrows(F);
190 Changed |= prepareEHPads(F);
191 return Changed;
192}
193
194bool WasmEHPrepareImpl::prepareThrows(Function &F) {
195 Module &M = *F.getParent();
196 IRBuilder<> IRB(F.getContext());
197 bool Changed = false;
198
199 // wasm.throw() intinsic, which will be lowered to wasm 'throw' instruction.
200 ThrowF = Intrinsic::getOrInsertDeclaration(M: &M, id: Intrinsic::wasm_throw);
201 // Insert an unreachable instruction after a call to @llvm.wasm.throw and
202 // delete all following instructions within the BB, and delete all the dead
203 // children of the BB as well.
204 for (User *U : ThrowF->users()) {
205 auto *ThrowI = dyn_cast<CallInst>(Val: U);
206 if (!ThrowI || ThrowI->getFunction() != &F)
207 continue;
208 Changed = true;
209 auto *BB = ThrowI->getParent();
210 SmallVector<BasicBlock *, 4> Succs(successors(BB));
211 BB->erase(FromIt: std::next(x: BasicBlock::iterator(ThrowI)), ToIt: BB->end());
212 IRB.SetInsertPoint(BB);
213 IRB.CreateUnreachable();
214 eraseDeadBBsAndChildren(BBs: Succs);
215 }
216
217 return Changed;
218}
219
220bool WasmEHPrepareImpl::prepareEHPads(Function &F) {
221 Module &M = *F.getParent();
222 IRBuilder<> IRB(F.getContext());
223
224 SmallVector<BasicBlock *, 16> CatchPads;
225 SmallVector<BasicBlock *, 16> CleanupPads;
226 for (BasicBlock &BB : F) {
227 if (!BB.isEHPad())
228 continue;
229 BasicBlock::iterator Pad = BB.getFirstNonPHIIt();
230 if (isa<CatchPadInst>(Val: Pad))
231 CatchPads.push_back(Elt: &BB);
232 else if (isa<CleanupPadInst>(Val: Pad))
233 CleanupPads.push_back(Elt: &BB);
234 }
235 if (CatchPads.empty() && CleanupPads.empty())
236 return false;
237
238 if (!F.hasPersonalityFn() ||
239 !isScopedEHPersonality(Pers: classifyEHPersonality(Pers: F.getPersonalityFn()))) {
240 report_fatal_error(reason: "Function '" + F.getName() +
241 "' does not have a correct Wasm personality function "
242 "'__gxx_wasm_personality_v0'");
243 }
244 assert(F.hasPersonalityFn() && "Personality function not found");
245
246 // __wasm_lpad_context global variable.
247 // This variable should be thread local. If the target does not support TLS,
248 // we depend on CoalesceFeaturesAndStripAtomics to downgrade it to
249 // non-thread-local ones, in which case we don't allow this object to be
250 // linked with other objects using shared memory.
251 LPadContextGV = M.getOrInsertGlobal(Name: "__wasm_lpad_context", Ty: LPadContextTy);
252 LPadContextGV->setThreadLocalMode(GlobalValue::GeneralDynamicTLSModel);
253
254 LPadIndexField = LPadContextGV;
255 LSDAField = IRB.CreateConstInBoundsGEP2_32(Ty: LPadContextTy, Ptr: LPadContextGV, Idx0: 0, Idx1: 1,
256 Name: "lsda_gep");
257 SelectorField = IRB.CreateConstInBoundsGEP2_32(Ty: LPadContextTy, Ptr: LPadContextGV,
258 Idx0: 0, Idx1: 2, Name: "selector_gep");
259
260 // wasm.landingpad.index() intrinsic, which is to specify landingpad index
261 LPadIndexF =
262 Intrinsic::getOrInsertDeclaration(M: &M, id: Intrinsic::wasm_landingpad_index);
263 // wasm.lsda() intrinsic. Returns the address of LSDA table for the current
264 // function.
265 LSDAF = Intrinsic::getOrInsertDeclaration(M: &M, id: Intrinsic::wasm_lsda);
266 // wasm.get.exception() and wasm.get.ehselector() intrinsics. Calls to these
267 // are generated in clang.
268 GetExnF =
269 Intrinsic::getOrInsertDeclaration(M: &M, id: Intrinsic::wasm_get_exception);
270 GetSelectorF =
271 Intrinsic::getOrInsertDeclaration(M: &M, id: Intrinsic::wasm_get_ehselector);
272
273 // wasm.catch() will be lowered down to wasm 'catch' instruction in
274 // instruction selection.
275 CatchF = Intrinsic::getOrInsertDeclaration(M: &M, id: Intrinsic::wasm_catch);
276
277 // FIXME: Verify this is really supported for current module.
278 StringRef UnwindCallPersonalityName =
279 RTLIB::RuntimeLibcallsInfo::getLibcallImplName(
280 CallImpl: RTLIB::impl__Unwind_CallPersonality);
281
282 // _Unwind_CallPersonality() wrapper function, which calls the personality
283 CallPersonalityF = M.getOrInsertFunction(Name: UnwindCallPersonalityName,
284 RetTy: IRB.getInt32Ty(), Args: IRB.getPtrTy());
285 if (Function *F = dyn_cast<Function>(Val: CallPersonalityF.getCallee()))
286 F->setDoesNotThrow();
287
288 unsigned Index = 0;
289 for (auto *BB : CatchPads) {
290 auto *CPI = cast<CatchPadInst>(Val: BB->getFirstNonPHIIt());
291 // In case of a single catch (...), we don't need to emit a personalify
292 // function call
293 if (CPI->arg_size() == 1 &&
294 cast<Constant>(Val: CPI->getArgOperand(i: 0))->isNullValue())
295 prepareEHPad(BB, NeedPersonality: false);
296 else
297 prepareEHPad(BB, NeedPersonality: true, Index: Index++);
298 }
299
300 // Cleanup pads don't need a personality function call.
301 for (auto *BB : CleanupPads)
302 prepareEHPad(BB, NeedPersonality: false);
303
304 return true;
305}
306
307// Prepare an EH pad for Wasm EH handling. If NeedPersonality is false, Index is
308// ignored.
309void WasmEHPrepareImpl::prepareEHPad(BasicBlock *BB, bool NeedPersonality,
310 unsigned Index) {
311 assert(BB->isEHPad() && "BB is not an EHPad!");
312 IRBuilder<> IRB(BB->getContext());
313 IRB.SetInsertPoint(TheBB: BB, IP: BB->getFirstInsertionPt());
314
315 auto *FPI = cast<FuncletPadInst>(Val: BB->getFirstNonPHIIt());
316 Instruction *GetExnCI = nullptr, *GetSelectorCI = nullptr;
317 for (auto &U : FPI->uses()) {
318 if (auto *CI = dyn_cast<CallInst>(Val: U.getUser())) {
319 if (CI->getCalledOperand() == GetExnF)
320 GetExnCI = CI;
321 if (CI->getCalledOperand() == GetSelectorF)
322 GetSelectorCI = CI;
323 }
324 }
325
326 // Cleanup pads do not have any of wasm.get.exception() or
327 // wasm.get.ehselector() calls. We need to do nothing.
328 if (!GetExnCI) {
329 assert(!GetSelectorCI &&
330 "wasm.get.ehselector() cannot exist w/o wasm.get.exception()");
331 return;
332 }
333
334 // Replace wasm.get.exception intrinsic with wasm.catch intrinsic, which will
335 // be lowered to wasm 'catch' instruction. We do this mainly because
336 // instruction selection cannot handle wasm.get.exception intrinsic's token
337 // argument.
338 Instruction *CatchCI =
339 IRB.CreateCall(Callee: CatchF, Args: {IRB.getInt32(C: WebAssembly::CPP_EXCEPTION)}, Name: "exn");
340 GetExnCI->replaceAllUsesWith(V: CatchCI);
341 GetExnCI->eraseFromParent();
342
343 // In case it is a catchpad with single catch (...) or a cleanuppad, we don't
344 // need to call personality function because we don't need a selector.
345 if (!NeedPersonality) {
346 if (GetSelectorCI) {
347 assert(GetSelectorCI->use_empty() &&
348 "wasm.get.ehselector() still has uses!");
349 GetSelectorCI->eraseFromParent();
350 }
351 return;
352 }
353 IRB.SetInsertPoint(CatchCI->getNextNode());
354
355 // This is to create a map of <landingpad EH label, landingpad index> in
356 // SelectionDAGISel, which is to be used in EHStreamer to emit LSDA tables.
357 // Pseudocode: wasm.landingpad.index(Index);
358 IRB.CreateCall(Callee: LPadIndexF, Args: {FPI, IRB.getInt32(C: Index)});
359
360 // Pseudocode: __wasm_lpad_context.lpad_index = index;
361 IRB.CreateStore(Val: IRB.getInt32(C: Index), Ptr: LPadIndexField);
362
363 auto *CPI = cast<CatchPadInst>(Val: FPI);
364 // TODO Sometimes storing the LSDA address every time is not necessary, in
365 // case it is already set in a dominating EH pad and there is no function call
366 // between from that EH pad to here. Consider optimizing those cases.
367 // Pseudocode: __wasm_lpad_context.lsda = wasm.lsda();
368 IRB.CreateStore(Val: IRB.CreateCall(Callee: LSDAF), Ptr: LSDAField);
369
370 // Pseudocode: _Unwind_CallPersonality(exn);
371 CallInst *PersCI = IRB.CreateCall(Callee: CallPersonalityF, Args: CatchCI,
372 OpBundles: OperandBundleDef("funclet", CPI));
373 PersCI->setDoesNotThrow();
374
375 // Pseudocode: int selector = __wasm_lpad_context.selector;
376 Instruction *Selector =
377 IRB.CreateLoad(Ty: IRB.getInt32Ty(), Ptr: SelectorField, Name: "selector");
378
379 // Replace the return value from wasm.get.ehselector() with the selector value
380 // loaded from __wasm_lpad_context.selector.
381 assert(GetSelectorCI && "wasm.get.ehselector() call does not exist");
382 GetSelectorCI->replaceAllUsesWith(V: Selector);
383 GetSelectorCI->eraseFromParent();
384}
385
386void llvm::calculateWasmEHInfo(const Function *F, WasmEHFuncInfo &EHInfo) {
387 // If an exception is not caught by a catchpad (i.e., it is a foreign
388 // exception), it will unwind to its parent catchswitch's unwind destination.
389 // We don't record an unwind destination for cleanuppads because every
390 // exception should be caught by it.
391 for (const auto &BB : *F) {
392 if (!BB.isEHPad())
393 continue;
394 const Instruction *Pad = &*BB.getFirstNonPHIIt();
395
396 if (const auto *CatchPad = dyn_cast<CatchPadInst>(Val: Pad)) {
397 const auto *UnwindBB = CatchPad->getCatchSwitch()->getUnwindDest();
398 if (!UnwindBB)
399 continue;
400 const Instruction *UnwindPad = &*UnwindBB->getFirstNonPHIIt();
401 if (const auto *CatchSwitch = dyn_cast<CatchSwitchInst>(Val: UnwindPad))
402 // Currently there should be only one handler per a catchswitch.
403 EHInfo.setUnwindDest(BB: &BB, Dest: *CatchSwitch->handlers().begin());
404 else // cleanuppad
405 EHInfo.setUnwindDest(BB: &BB, Dest: UnwindBB);
406 }
407 }
408}
409