LLVM 17.0.0git
WasmEHPrepare.cpp
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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
81#include "llvm/CodeGen/Passes.h"
84#include "llvm/IR/IRBuilder.h"
85#include "llvm/IR/IntrinsicsWebAssembly.h"
88
89using namespace llvm;
90
91#define DEBUG_TYPE "wasmehprepare"
92
93namespace {
94class WasmEHPrepare : public FunctionPass {
95 Type *LPadContextTy = nullptr; // type of 'struct _Unwind_LandingPadContext'
96 GlobalVariable *LPadContextGV = nullptr; // __wasm_lpad_context
97
98 // Field addresses of struct _Unwind_LandingPadContext
99 Value *LPadIndexField = nullptr; // lpad_index field
100 Value *LSDAField = nullptr; // lsda field
101 Value *SelectorField = nullptr; // selector
102
103 Function *ThrowF = nullptr; // wasm.throw() intrinsic
104 Function *LPadIndexF = nullptr; // wasm.landingpad.index() intrinsic
105 Function *LSDAF = nullptr; // wasm.lsda() intrinsic
106 Function *GetExnF = nullptr; // wasm.get.exception() intrinsic
107 Function *CatchF = nullptr; // wasm.catch() intrinsic
108 Function *GetSelectorF = nullptr; // wasm.get.ehselector() intrinsic
109 FunctionCallee CallPersonalityF =
110 nullptr; // _Unwind_CallPersonality() wrapper
111
112 bool prepareThrows(Function &F);
113 bool prepareEHPads(Function &F);
114 void prepareEHPad(BasicBlock *BB, bool NeedPersonality, unsigned Index = 0);
115
116public:
117 static char ID; // Pass identification, replacement for typeid
118
119 WasmEHPrepare() : FunctionPass(ID) {}
120 bool doInitialization(Module &M) override;
121 bool runOnFunction(Function &F) override;
122
123 StringRef getPassName() const override {
124 return "WebAssembly Exception handling preparation";
125 }
126};
127} // end anonymous namespace
128
129char WasmEHPrepare::ID = 0;
131 "Prepare WebAssembly exceptions", false, false)
132INITIALIZE_PASS_END(WasmEHPrepare, DEBUG_TYPE, "Prepare WebAssembly exceptions",
134
135FunctionPass *llvm::createWasmEHPass() { return new WasmEHPrepare(); }
136
137bool WasmEHPrepare::doInitialization(Module &M) {
138 IRBuilder<> IRB(M.getContext());
139 LPadContextTy = StructType::get(IRB.getInt32Ty(), // lpad_index
140 IRB.getInt8PtrTy(), // lsda
141 IRB.getInt32Ty() // selector
142 );
143 return false;
144}
145
146// Erase the specified BBs if the BB does not have any remaining predecessors,
147// and also all its dead children.
148template <typename Container>
149static void eraseDeadBBsAndChildren(const Container &BBs) {
150 SmallVector<BasicBlock *, 8> WL(BBs.begin(), BBs.end());
151 while (!WL.empty()) {
152 auto *BB = WL.pop_back_val();
153 if (!pred_empty(BB))
154 continue;
155 WL.append(succ_begin(BB), succ_end(BB));
156 DeleteDeadBlock(BB);
157 }
158}
159
160bool WasmEHPrepare::runOnFunction(Function &F) {
161 bool Changed = false;
162 Changed |= prepareThrows(F);
163 Changed |= prepareEHPads(F);
164 return Changed;
165}
166
167bool WasmEHPrepare::prepareThrows(Function &F) {
168 Module &M = *F.getParent();
169 IRBuilder<> IRB(F.getContext());
170 bool Changed = false;
171
172 // wasm.throw() intinsic, which will be lowered to wasm 'throw' instruction.
173 ThrowF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_throw);
174 // Insert an unreachable instruction after a call to @llvm.wasm.throw and
175 // delete all following instructions within the BB, and delete all the dead
176 // children of the BB as well.
177 for (User *U : ThrowF->users()) {
178 // A call to @llvm.wasm.throw() is only generated from __cxa_throw()
179 // builtin call within libcxxabi, and cannot be an InvokeInst.
180 auto *ThrowI = cast<CallInst>(U);
181 if (ThrowI->getFunction() != &F)
182 continue;
183 Changed = true;
184 auto *BB = ThrowI->getParent();
186 BB->erase(std::next(BasicBlock::iterator(ThrowI)), BB->end());
187 IRB.SetInsertPoint(BB);
188 IRB.CreateUnreachable();
190 }
191
192 return Changed;
193}
194
195bool WasmEHPrepare::prepareEHPads(Function &F) {
196 Module &M = *F.getParent();
197 IRBuilder<> IRB(F.getContext());
198
201 for (BasicBlock &BB : F) {
202 if (!BB.isEHPad())
203 continue;
204 auto *Pad = BB.getFirstNonPHI();
205 if (isa<CatchPadInst>(Pad))
206 CatchPads.push_back(&BB);
207 else if (isa<CleanupPadInst>(Pad))
208 CleanupPads.push_back(&BB);
209 }
210 if (CatchPads.empty() && CleanupPads.empty())
211 return false;
212
213 if (!F.hasPersonalityFn() ||
214 !isScopedEHPersonality(classifyEHPersonality(F.getPersonalityFn()))) {
215 report_fatal_error("Function '" + F.getName() +
216 "' does not have a correct Wasm personality function "
217 "'__gxx_wasm_personality_v0'");
218 }
219 assert(F.hasPersonalityFn() && "Personality function not found");
220
221 // __wasm_lpad_context global variable.
222 // This variable should be thread local. If the target does not support TLS,
223 // we depend on CoalesceFeaturesAndStripAtomics to downgrade it to
224 // non-thread-local ones, in which case we don't allow this object to be
225 // linked with other objects using shared memory.
226 LPadContextGV = cast<GlobalVariable>(
227 M.getOrInsertGlobal("__wasm_lpad_context", LPadContextTy));
228 LPadContextGV->setThreadLocalMode(GlobalValue::GeneralDynamicTLSModel);
229
230 LPadIndexField = IRB.CreateConstGEP2_32(LPadContextTy, LPadContextGV, 0, 0,
231 "lpad_index_gep");
232 LSDAField =
233 IRB.CreateConstGEP2_32(LPadContextTy, LPadContextGV, 0, 1, "lsda_gep");
234 SelectorField = IRB.CreateConstGEP2_32(LPadContextTy, LPadContextGV, 0, 2,
235 "selector_gep");
236
237 // wasm.landingpad.index() intrinsic, which is to specify landingpad index
238 LPadIndexF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_landingpad_index);
239 // wasm.lsda() intrinsic. Returns the address of LSDA table for the current
240 // function.
241 LSDAF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_lsda);
242 // wasm.get.exception() and wasm.get.ehselector() intrinsics. Calls to these
243 // are generated in clang.
244 GetExnF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_get_exception);
245 GetSelectorF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_get_ehselector);
246
247 // wasm.catch() will be lowered down to wasm 'catch' instruction in
248 // instruction selection.
249 CatchF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_catch);
250
251 // _Unwind_CallPersonality() wrapper function, which calls the personality
252 CallPersonalityF = M.getOrInsertFunction(
253 "_Unwind_CallPersonality", IRB.getInt32Ty(), IRB.getInt8PtrTy());
254 if (Function *F = dyn_cast<Function>(CallPersonalityF.getCallee()))
255 F->setDoesNotThrow();
256
257 unsigned Index = 0;
258 for (auto *BB : CatchPads) {
259 auto *CPI = cast<CatchPadInst>(BB->getFirstNonPHI());
260 // In case of a single catch (...), we don't need to emit a personalify
261 // function call
262 if (CPI->arg_size() == 1 &&
263 cast<Constant>(CPI->getArgOperand(0))->isNullValue())
264 prepareEHPad(BB, false);
265 else
266 prepareEHPad(BB, true, Index++);
267 }
268
269 // Cleanup pads don't need a personality function call.
270 for (auto *BB : CleanupPads)
271 prepareEHPad(BB, false);
272
273 return true;
274}
275
276// Prepare an EH pad for Wasm EH handling. If NeedPersonality is false, Index is
277// ignored.
278void WasmEHPrepare::prepareEHPad(BasicBlock *BB, bool NeedPersonality,
279 unsigned Index) {
280 assert(BB->isEHPad() && "BB is not an EHPad!");
281 IRBuilder<> IRB(BB->getContext());
282 IRB.SetInsertPoint(&*BB->getFirstInsertionPt());
283
284 auto *FPI = cast<FuncletPadInst>(BB->getFirstNonPHI());
285 Instruction *GetExnCI = nullptr, *GetSelectorCI = nullptr;
286 for (auto &U : FPI->uses()) {
287 if (auto *CI = dyn_cast<CallInst>(U.getUser())) {
288 if (CI->getCalledOperand() == GetExnF)
289 GetExnCI = CI;
290 if (CI->getCalledOperand() == GetSelectorF)
291 GetSelectorCI = CI;
292 }
293 }
294
295 // Cleanup pads do not have any of wasm.get.exception() or
296 // wasm.get.ehselector() calls. We need to do nothing.
297 if (!GetExnCI) {
298 assert(!GetSelectorCI &&
299 "wasm.get.ehselector() cannot exist w/o wasm.get.exception()");
300 return;
301 }
302
303 // Replace wasm.get.exception intrinsic with wasm.catch intrinsic, which will
304 // be lowered to wasm 'catch' instruction. We do this mainly because
305 // instruction selection cannot handle wasm.get.exception intrinsic's token
306 // argument.
307 Instruction *CatchCI =
308 IRB.CreateCall(CatchF, {IRB.getInt32(WebAssembly::CPP_EXCEPTION)}, "exn");
309 GetExnCI->replaceAllUsesWith(CatchCI);
310 GetExnCI->eraseFromParent();
311
312 // In case it is a catchpad with single catch (...) or a cleanuppad, we don't
313 // need to call personality function because we don't need a selector.
314 if (!NeedPersonality) {
315 if (GetSelectorCI) {
316 assert(GetSelectorCI->use_empty() &&
317 "wasm.get.ehselector() still has uses!");
318 GetSelectorCI->eraseFromParent();
319 }
320 return;
321 }
322 IRB.SetInsertPoint(CatchCI->getNextNode());
323
324 // This is to create a map of <landingpad EH label, landingpad index> in
325 // SelectionDAGISel, which is to be used in EHStreamer to emit LSDA tables.
326 // Pseudocode: wasm.landingpad.index(Index);
327 IRB.CreateCall(LPadIndexF, {FPI, IRB.getInt32(Index)});
328
329 // Pseudocode: __wasm_lpad_context.lpad_index = index;
330 IRB.CreateStore(IRB.getInt32(Index), LPadIndexField);
331
332 auto *CPI = cast<CatchPadInst>(FPI);
333 // TODO Sometimes storing the LSDA address every time is not necessary, in
334 // case it is already set in a dominating EH pad and there is no function call
335 // between from that EH pad to here. Consider optimizing those cases.
336 // Pseudocode: __wasm_lpad_context.lsda = wasm.lsda();
337 IRB.CreateStore(IRB.CreateCall(LSDAF), LSDAField);
338
339 // Pseudocode: _Unwind_CallPersonality(exn);
340 CallInst *PersCI = IRB.CreateCall(CallPersonalityF, CatchCI,
341 OperandBundleDef("funclet", CPI));
342 PersCI->setDoesNotThrow();
343
344 // Pseudocode: int selector = __wasm_lpad_context.selector;
345 Instruction *Selector =
346 IRB.CreateLoad(IRB.getInt32Ty(), SelectorField, "selector");
347
348 // Replace the return value from wasm.get.ehselector() with the selector value
349 // loaded from __wasm_lpad_context.selector.
350 assert(GetSelectorCI && "wasm.get.ehselector() call does not exist");
351 GetSelectorCI->replaceAllUsesWith(Selector);
352 GetSelectorCI->eraseFromParent();
353}
354
356 // If an exception is not caught by a catchpad (i.e., it is a foreign
357 // exception), it will unwind to its parent catchswitch's unwind destination.
358 // We don't record an unwind destination for cleanuppads because every
359 // exception should be caught by it.
360 for (const auto &BB : *F) {
361 if (!BB.isEHPad())
362 continue;
363 const Instruction *Pad = BB.getFirstNonPHI();
364
365 if (const auto *CatchPad = dyn_cast<CatchPadInst>(Pad)) {
366 const auto *UnwindBB = CatchPad->getCatchSwitch()->getUnwindDest();
367 if (!UnwindBB)
368 continue;
369 const Instruction *UnwindPad = UnwindBB->getFirstNonPHI();
370 if (const auto *CatchSwitch = dyn_cast<CatchSwitchInst>(UnwindPad))
371 // Currently there should be only one handler per a catchswitch.
372 EHInfo.setUnwindDest(&BB, *CatchSwitch->handlers().begin());
373 else // cleanuppad
374 EHInfo.setUnwindDest(&BB, UnwindBB);
375 }
376 }
377}
#define F(x, y, z)
Definition: MD5.cpp:55
#define INITIALIZE_PASS_END(passName, arg, name, cfg, analysis)
Definition: PassSupport.h:59
#define INITIALIZE_PASS_BEGIN(passName, arg, name, cfg, analysis)
Definition: PassSupport.h:52
assert(ImpDefSCC.getReg()==AMDGPU::SCC &&ImpDefSCC.isDef())
Prepare WebAssembly exceptions
static void eraseDeadBBsAndChildren(const Container &BBs)
#define DEBUG_TYPE
LLVM Basic Block Representation.
Definition: BasicBlock.h:56
iterator begin()
Instruction iterator methods.
Definition: BasicBlock.h:326
const_iterator getFirstInsertionPt() const
Returns an iterator to the first instruction in this block that is suitable for inserting a non-PHI i...
Definition: BasicBlock.cpp:253
const Instruction * getFirstNonPHI() const
Returns a pointer to the first instruction in this block that is not a PHINode instruction.
Definition: BasicBlock.cpp:216
InstListType::iterator iterator
Instruction iterators...
Definition: BasicBlock.h:87
LLVMContext & getContext() const
Get the context in which this basic block lives.
Definition: BasicBlock.cpp:35
bool isEHPad() const
Return true if this basic block is an exception handling block.
Definition: BasicBlock.h:524
void setDoesNotThrow()
Definition: InstrTypes.h:1928
This class represents a function call, abstracting a target machine's calling convention.
A handy container for a FunctionType+Callee-pointer pair, which can be passed around as a single enti...
Definition: DerivedTypes.h:165
FunctionPass class - This class is used to implement most global optimizations.
Definition: Pass.h:311
virtual bool runOnFunction(Function &F)=0
runOnFunction - Virtual method overriden by subclasses to do the per-function processing of the pass.
This provides a uniform API for creating instructions and inserting them into a basic block: either a...
Definition: IRBuilder.h:2570
SymbolTableList< Instruction >::iterator eraseFromParent()
This method unlinks 'this' from the containing basic block and deletes it.
Definition: Instruction.cpp:82
A Module instance is used to store all the information related to an LLVM module.
Definition: Module.h:65
virtual bool doInitialization(Module &)
doInitialization - Virtual method overridden by subclasses to do any necessary initialization before ...
Definition: Pass.h:119
virtual StringRef getPassName() const
getPassName - Return a nice clean name for a pass.
Definition: Pass.cpp:81
bool empty() const
Definition: SmallVector.h:94
void append(ItTy in_start, ItTy in_end)
Add the specified range to the end of the SmallVector.
Definition: SmallVector.h:687
void push_back(const T &Elt)
Definition: SmallVector.h:416
This is a 'vector' (really, a variable-sized array), optimized for the case when the array is small.
Definition: SmallVector.h:1200
StringRef - Represent a constant reference to a string, i.e.
Definition: StringRef.h:50
static StructType * get(LLVMContext &Context, ArrayRef< Type * > Elements, bool isPacked=false)
This static method is the primary way to create a literal StructType.
Definition: Type.cpp:434
The instances of the Type class are immutable: once they are created, they are never changed.
Definition: Type.h:45
LLVM Value Representation.
Definition: Value.h:74
void replaceAllUsesWith(Value *V)
Change all uses of this to point to a new Value.
Definition: Value.cpp:535
iterator_range< user_iterator > users()
Definition: Value.h:421
NodeTy * getNextNode()
Get the next node, or nullptr for the list tail.
Definition: ilist_node.h:289
unsigned ID
LLVM IR allows to use arbitrary numbers as calling convention identifiers.
Definition: CallingConv.h:24
Function * getDeclaration(Module *M, ID id, ArrayRef< Type * > Tys=std::nullopt)
Create or insert an LLVM Function declaration for an intrinsic, and return it.
Definition: Function.cpp:1465
This is an optimization pass for GlobalISel generic memory operations.
Definition: AddressRanges.h:18
Interval::succ_iterator succ_end(Interval *I)
Definition: Interval.h:102
auto successors(const MachineBasicBlock *BB)
FunctionPass * createWasmEHPass()
createWasmEHPass - This pass adapts exception handling code to use WebAssembly's exception handling s...
Interval::succ_iterator succ_begin(Interval *I)
succ_begin/succ_end - define methods so that Intervals may be used just like BasicBlocks can with the...
Definition: Interval.h:99
void DeleteDeadBlock(BasicBlock *BB, DomTreeUpdater *DTU=nullptr, bool KeepOneInputPHIs=false)
Delete the specified block, which must have no predecessors.
bool isScopedEHPersonality(EHPersonality Pers)
Returns true if this personality uses scope-style EH IR instructions: catchswitch,...
OperandBundleDefT< Value * > OperandBundleDef
Definition: AutoUpgrade.h:33
void report_fatal_error(Error Err, bool gen_crash_diag=true)
Report a serious error, calling any installed error handler.
Definition: Error.cpp:145
EHPersonality classifyEHPersonality(const Value *Pers)
See if the given exception handling personality function is one that we understand.
void calculateWasmEHInfo(const Function *F, WasmEHFuncInfo &EHInfo)
bool pred_empty(const BasicBlock *BB)
Definition: CFG.h:118
void setUnwindDest(const BasicBlock *BB, const BasicBlock *Dest)