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/InitializePasses.h"
89#include "llvm/Transforms/Utils/BasicBlockUtils.h"
90
91using namespace llvm;
92
93#define DEBUG_TYPE "wasm-eh-prepare"
94
95namespace {
96class WasmEHPrepareImpl {
97 friend class WasmEHPrepare;
98
99 Type *LPadContextTy = nullptr; // type of 'struct _Unwind_LandingPadContext'
100 GlobalVariable *LPadContextGV = nullptr; // __wasm_lpad_context
101
102 // Field addresses of struct _Unwind_LandingPadContext
103 Value *LPadIndexField = nullptr; // lpad_index field
104 Value *LSDAField = nullptr; // lsda field
105 Value *SelectorField = nullptr; // selector
106
107 Function *ThrowF = nullptr; // wasm.throw() intrinsic
108 Function *LPadIndexF = nullptr; // wasm.landingpad.index() intrinsic
109 Function *LSDAF = nullptr; // wasm.lsda() intrinsic
110 Function *GetExnF = nullptr; // wasm.get.exception() intrinsic
111 Function *CatchF = nullptr; // wasm.catch() intrinsic
112 Function *GetSelectorF = nullptr; // wasm.get.ehselector() intrinsic
113 FunctionCallee CallPersonalityF =
114 nullptr; // _Unwind_CallPersonality() wrapper
115
116 bool prepareThrows(Function &F);
117 bool prepareEHPads(Function &F);
118 void prepareEHPad(BasicBlock *BB, bool NeedPersonality, unsigned Index = 0);
119
120public:
121 WasmEHPrepareImpl() = default;
122 WasmEHPrepareImpl(Type *LPadContextTy_) : LPadContextTy(LPadContextTy_) {}
123 bool runOnFunction(Function &F);
124};
125
126class WasmEHPrepare : public FunctionPass {
127 WasmEHPrepareImpl P;
128
129public:
130 static char ID; // Pass identification, replacement for typeid
131
132 WasmEHPrepare() : FunctionPass(ID) {}
133 bool doInitialization(Module &M) override;
134 bool runOnFunction(Function &F) override { return P.runOnFunction(F); }
135
136 StringRef getPassName() const override {
137 return "WebAssembly Exception handling preparation";
138 }
139};
140
141} // end anonymous namespace
142
143PreservedAnalyses WasmEHPreparePass::run(Function &F,
144 FunctionAnalysisManager &) {
145 auto &Context = F.getContext();
146 auto *I32Ty = Type::getInt32Ty(C&: Context);
147 auto *PtrTy = PointerType::get(C&: Context, AddressSpace: 0);
148 auto *LPadContextTy =
149 StructType::get(elt1: I32Ty /*lpad_index*/, elts: PtrTy /*lsda*/, elts: I32Ty /*selector*/);
150 WasmEHPrepareImpl P(LPadContextTy);
151 bool Changed = P.runOnFunction(F);
152 return Changed ? PreservedAnalyses::none() : PreservedAnalyses ::all();
153}
154
155char WasmEHPrepare::ID = 0;
156INITIALIZE_PASS_BEGIN(WasmEHPrepare, DEBUG_TYPE,
157 "Prepare WebAssembly exceptions", false, false)
158INITIALIZE_PASS_END(WasmEHPrepare, DEBUG_TYPE, "Prepare WebAssembly exceptions",
159 false, false)
160
161FunctionPass *llvm::createWasmEHPass() { return new WasmEHPrepare(); }
162
163bool WasmEHPrepare::doInitialization(Module &M) {
164 IRBuilder<> IRB(M.getContext());
165 P.LPadContextTy = StructType::get(elt1: IRB.getInt32Ty(), // lpad_index
166 elts: IRB.getPtrTy(), // lsda
167 elts: IRB.getInt32Ty() // selector
168 );
169 return false;
170}
171
172// Erase the specified BBs if the BB does not have any remaining predecessors,
173// and also all its dead children.
174template <typename Container>
175static void eraseDeadBBsAndChildren(const Container &BBs) {
176 SmallVector<BasicBlock *, 8> WL(BBs.begin(), BBs.end());
177 while (!WL.empty()) {
178 auto *BB = WL.pop_back_val();
179 if (!pred_empty(BB))
180 continue;
181 WL.append(in_start: succ_begin(BB), in_end: succ_end(BB));
182 DeleteDeadBlock(BB);
183 }
184}
185
186bool WasmEHPrepareImpl::runOnFunction(Function &F) {
187 bool Changed = false;
188 Changed |= prepareThrows(F);
189 Changed |= prepareEHPads(F);
190 return Changed;
191}
192
193bool WasmEHPrepareImpl::prepareThrows(Function &F) {
194 Module &M = *F.getParent();
195 IRBuilder<> IRB(F.getContext());
196 bool Changed = false;
197
198 // wasm.throw() intinsic, which will be lowered to wasm 'throw' instruction.
199 ThrowF = Intrinsic::getOrInsertDeclaration(M: &M, id: Intrinsic::wasm_throw);
200 // Insert an unreachable instruction after a call to @llvm.wasm.throw and
201 // delete all following instructions within the BB, and delete all the dead
202 // children of the BB as well.
203 for (User *U : ThrowF->users()) {
204 auto *ThrowI = dyn_cast<CallInst>(Val: U);
205 if (!ThrowI || ThrowI->getFunction() != &F)
206 continue;
207 Changed = true;
208 auto *BB = ThrowI->getParent();
209 SmallVector<BasicBlock *, 4> Succs(successors(BB));
210 BB->erase(FromIt: std::next(x: BasicBlock::iterator(ThrowI)), ToIt: BB->end());
211 IRB.SetInsertPoint(BB);
212 IRB.CreateUnreachable();
213 eraseDeadBBsAndChildren(BBs: Succs);
214 }
215
216 return Changed;
217}
218
219bool WasmEHPrepareImpl::prepareEHPads(Function &F) {
220 Module &M = *F.getParent();
221 IRBuilder<> IRB(F.getContext());
222
223 SmallVector<BasicBlock *, 16> CatchPads;
224 SmallVector<BasicBlock *, 16> CleanupPads;
225 for (BasicBlock &BB : F) {
226 if (!BB.isEHPad())
227 continue;
228 BasicBlock::iterator Pad = BB.getFirstNonPHIIt();
229 if (isa<CatchPadInst>(Val: Pad))
230 CatchPads.push_back(Elt: &BB);
231 else if (isa<CleanupPadInst>(Val: Pad))
232 CleanupPads.push_back(Elt: &BB);
233 }
234 if (CatchPads.empty() && CleanupPads.empty())
235 return false;
236
237 if (!F.hasPersonalityFn() ||
238 !isScopedEHPersonality(Pers: classifyEHPersonality(Pers: F.getPersonalityFn()))) {
239 report_fatal_error(reason: "Function '" + F.getName() +
240 "' does not have a correct Wasm personality function "
241 "'__gxx_wasm_personality_v0'");
242 }
243 assert(F.hasPersonalityFn() && "Personality function not found");
244
245 // __wasm_lpad_context global variable.
246 // This variable should be thread local. If the target does not support TLS,
247 // we depend on CoalesceFeaturesAndStripAtomics to downgrade it to
248 // non-thread-local ones, in which case we don't allow this object to be
249 // linked with other objects using shared memory.
250 LPadContextGV = M.getOrInsertGlobal(Name: "__wasm_lpad_context", Ty: LPadContextTy);
251 LPadContextGV->setThreadLocalMode(GlobalValue::GeneralDynamicTLSModel);
252
253 LPadIndexField = LPadContextGV;
254 LSDAField = IRB.CreateConstInBoundsGEP2_32(Ty: LPadContextTy, Ptr: LPadContextGV, Idx0: 0, Idx1: 1,
255 Name: "lsda_gep");
256 SelectorField = IRB.CreateConstInBoundsGEP2_32(Ty: LPadContextTy, Ptr: LPadContextGV,
257 Idx0: 0, Idx1: 2, Name: "selector_gep");
258
259 // wasm.landingpad.index() intrinsic, which is to specify landingpad index
260 LPadIndexF =
261 Intrinsic::getOrInsertDeclaration(M: &M, id: Intrinsic::wasm_landingpad_index);
262 // wasm.lsda() intrinsic. Returns the address of LSDA table for the current
263 // function.
264 LSDAF = Intrinsic::getOrInsertDeclaration(M: &M, id: Intrinsic::wasm_lsda);
265 // wasm.get.exception() and wasm.get.ehselector() intrinsics. Calls to these
266 // are generated in clang.
267 GetExnF =
268 Intrinsic::getOrInsertDeclaration(M: &M, id: Intrinsic::wasm_get_exception);
269 GetSelectorF =
270 Intrinsic::getOrInsertDeclaration(M: &M, id: Intrinsic::wasm_get_ehselector);
271
272 // wasm.catch() will be lowered down to wasm 'catch' instruction in
273 // instruction selection.
274 CatchF = Intrinsic::getOrInsertDeclaration(M: &M, id: Intrinsic::wasm_catch);
275
276 // _Unwind_CallPersonality() wrapper function, which calls the personality
277 CallPersonalityF = M.getOrInsertFunction(Name: "_Unwind_CallPersonality",
278 RetTy: IRB.getInt32Ty(), Args: IRB.getPtrTy());
279 if (Function *F = dyn_cast<Function>(Val: CallPersonalityF.getCallee()))
280 F->setDoesNotThrow();
281
282 unsigned Index = 0;
283 for (auto *BB : CatchPads) {
284 auto *CPI = cast<CatchPadInst>(Val: BB->getFirstNonPHIIt());
285 // In case of a single catch (...), we don't need to emit a personalify
286 // function call
287 if (CPI->arg_size() == 1 &&
288 cast<Constant>(Val: CPI->getArgOperand(i: 0))->isNullValue())
289 prepareEHPad(BB, NeedPersonality: false);
290 else
291 prepareEHPad(BB, NeedPersonality: true, Index: Index++);
292 }
293
294 // Cleanup pads don't need a personality function call.
295 for (auto *BB : CleanupPads)
296 prepareEHPad(BB, NeedPersonality: false);
297
298 return true;
299}
300
301// Prepare an EH pad for Wasm EH handling. If NeedPersonality is false, Index is
302// ignored.
303void WasmEHPrepareImpl::prepareEHPad(BasicBlock *BB, bool NeedPersonality,
304 unsigned Index) {
305 assert(BB->isEHPad() && "BB is not an EHPad!");
306 IRBuilder<> IRB(BB->getContext());
307 IRB.SetInsertPoint(TheBB: BB, IP: BB->getFirstInsertionPt());
308
309 auto *FPI = cast<FuncletPadInst>(Val: BB->getFirstNonPHIIt());
310 Instruction *GetExnCI = nullptr, *GetSelectorCI = nullptr;
311 for (auto &U : FPI->uses()) {
312 if (auto *CI = dyn_cast<CallInst>(Val: U.getUser())) {
313 if (CI->getCalledOperand() == GetExnF)
314 GetExnCI = CI;
315 if (CI->getCalledOperand() == GetSelectorF)
316 GetSelectorCI = CI;
317 }
318 }
319
320 // Cleanup pads do not have any of wasm.get.exception() or
321 // wasm.get.ehselector() calls. We need to do nothing.
322 if (!GetExnCI) {
323 assert(!GetSelectorCI &&
324 "wasm.get.ehselector() cannot exist w/o wasm.get.exception()");
325 return;
326 }
327
328 // Replace wasm.get.exception intrinsic with wasm.catch intrinsic, which will
329 // be lowered to wasm 'catch' instruction. We do this mainly because
330 // instruction selection cannot handle wasm.get.exception intrinsic's token
331 // argument.
332 Instruction *CatchCI =
333 IRB.CreateCall(Callee: CatchF, Args: {IRB.getInt32(C: WebAssembly::CPP_EXCEPTION)}, Name: "exn");
334 GetExnCI->replaceAllUsesWith(V: CatchCI);
335 GetExnCI->eraseFromParent();
336
337 // In case it is a catchpad with single catch (...) or a cleanuppad, we don't
338 // need to call personality function because we don't need a selector.
339 if (!NeedPersonality) {
340 if (GetSelectorCI) {
341 assert(GetSelectorCI->use_empty() &&
342 "wasm.get.ehselector() still has uses!");
343 GetSelectorCI->eraseFromParent();
344 }
345 return;
346 }
347 IRB.SetInsertPoint(CatchCI->getNextNode());
348
349 // This is to create a map of <landingpad EH label, landingpad index> in
350 // SelectionDAGISel, which is to be used in EHStreamer to emit LSDA tables.
351 // Pseudocode: wasm.landingpad.index(Index);
352 IRB.CreateCall(Callee: LPadIndexF, Args: {FPI, IRB.getInt32(C: Index)});
353
354 // Pseudocode: __wasm_lpad_context.lpad_index = index;
355 IRB.CreateStore(Val: IRB.getInt32(C: Index), Ptr: LPadIndexField);
356
357 auto *CPI = cast<CatchPadInst>(Val: FPI);
358 // TODO Sometimes storing the LSDA address every time is not necessary, in
359 // case it is already set in a dominating EH pad and there is no function call
360 // between from that EH pad to here. Consider optimizing those cases.
361 // Pseudocode: __wasm_lpad_context.lsda = wasm.lsda();
362 IRB.CreateStore(Val: IRB.CreateCall(Callee: LSDAF), Ptr: LSDAField);
363
364 // Pseudocode: _Unwind_CallPersonality(exn);
365 CallInst *PersCI = IRB.CreateCall(Callee: CallPersonalityF, Args: CatchCI,
366 OpBundles: OperandBundleDef("funclet", CPI));
367 PersCI->setDoesNotThrow();
368
369 // Pseudocode: int selector = __wasm_lpad_context.selector;
370 Instruction *Selector =
371 IRB.CreateLoad(Ty: IRB.getInt32Ty(), Ptr: SelectorField, Name: "selector");
372
373 // Replace the return value from wasm.get.ehselector() with the selector value
374 // loaded from __wasm_lpad_context.selector.
375 assert(GetSelectorCI && "wasm.get.ehselector() call does not exist");
376 GetSelectorCI->replaceAllUsesWith(V: Selector);
377 GetSelectorCI->eraseFromParent();
378}
379
380void llvm::calculateWasmEHInfo(const Function *F, WasmEHFuncInfo &EHInfo) {
381 // If an exception is not caught by a catchpad (i.e., it is a foreign
382 // exception), it will unwind to its parent catchswitch's unwind destination.
383 // We don't record an unwind destination for cleanuppads because every
384 // exception should be caught by it.
385 for (const auto &BB : *F) {
386 if (!BB.isEHPad())
387 continue;
388 const Instruction *Pad = &*BB.getFirstNonPHIIt();
389
390 if (const auto *CatchPad = dyn_cast<CatchPadInst>(Val: Pad)) {
391 const auto *UnwindBB = CatchPad->getCatchSwitch()->getUnwindDest();
392 if (!UnwindBB)
393 continue;
394 const Instruction *UnwindPad = &*UnwindBB->getFirstNonPHIIt();
395 if (const auto *CatchSwitch = dyn_cast<CatchSwitchInst>(Val: UnwindPad))
396 // Currently there should be only one handler per a catchswitch.
397 EHInfo.setUnwindDest(BB: &BB, Dest: *CatchSwitch->handlers().begin());
398 else // cleanuppad
399 EHInfo.setUnwindDest(BB: &BB, Dest: UnwindBB);
400 }
401 }
402}
403