LLVM 17.0.0git
WasmEHPrepare.cpp
Go to the documentation of this file.
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"
83#include "llvm/IR/IRBuilder.h"
84#include "llvm/IR/IntrinsicsWebAssembly.h"
87
88using namespace llvm;
89
90#define DEBUG_TYPE "wasmehprepare"
91
92namespace {
93class WasmEHPrepare : public FunctionPass {
94 Type *LPadContextTy = nullptr; // type of 'struct _Unwind_LandingPadContext'
95 GlobalVariable *LPadContextGV = nullptr; // __wasm_lpad_context
96
97 // Field addresses of struct _Unwind_LandingPadContext
98 Value *LPadIndexField = nullptr; // lpad_index field
99 Value *LSDAField = nullptr; // lsda field
100 Value *SelectorField = nullptr; // selector
101
102 Function *ThrowF = nullptr; // wasm.throw() intrinsic
103 Function *LPadIndexF = nullptr; // wasm.landingpad.index() intrinsic
104 Function *LSDAF = nullptr; // wasm.lsda() intrinsic
105 Function *GetExnF = nullptr; // wasm.get.exception() intrinsic
106 Function *CatchF = nullptr; // wasm.catch() intrinsic
107 Function *GetSelectorF = nullptr; // wasm.get.ehselector() intrinsic
108 FunctionCallee CallPersonalityF =
109 nullptr; // _Unwind_CallPersonality() wrapper
110
111 bool prepareThrows(Function &F);
112 bool prepareEHPads(Function &F);
113 void prepareEHPad(BasicBlock *BB, bool NeedPersonality, unsigned Index = 0);
114
115public:
116 static char ID; // Pass identification, replacement for typeid
117
118 WasmEHPrepare() : FunctionPass(ID) {}
119 bool doInitialization(Module &M) override;
120 bool runOnFunction(Function &F) override;
121
122 StringRef getPassName() const override {
123 return "WebAssembly Exception handling preparation";
124 }
125};
126} // end anonymous namespace
127
128char WasmEHPrepare::ID = 0;
130 "Prepare WebAssembly exceptions", false, false)
131INITIALIZE_PASS_END(WasmEHPrepare, DEBUG_TYPE, "Prepare WebAssembly exceptions",
133
134FunctionPass *llvm::createWasmEHPass() { return new WasmEHPrepare(); }
135
136bool WasmEHPrepare::doInitialization(Module &M) {
137 IRBuilder<> IRB(M.getContext());
138 LPadContextTy = StructType::get(IRB.getInt32Ty(), // lpad_index
139 IRB.getInt8PtrTy(), // lsda
140 IRB.getInt32Ty() // selector
141 );
142 return false;
143}
144
145// Erase the specified BBs if the BB does not have any remaining predecessors,
146// and also all its dead children.
147template <typename Container>
148static void eraseDeadBBsAndChildren(const Container &BBs) {
149 SmallVector<BasicBlock *, 8> WL(BBs.begin(), BBs.end());
150 while (!WL.empty()) {
151 auto *BB = WL.pop_back_val();
152 if (!pred_empty(BB))
153 continue;
154 WL.append(succ_begin(BB), succ_end(BB));
155 DeleteDeadBlock(BB);
156 }
157}
158
159bool WasmEHPrepare::runOnFunction(Function &F) {
160 bool Changed = false;
161 Changed |= prepareThrows(F);
162 Changed |= prepareEHPads(F);
163 return Changed;
164}
165
166bool WasmEHPrepare::prepareThrows(Function &F) {
167 Module &M = *F.getParent();
168 IRBuilder<> IRB(F.getContext());
169 bool Changed = false;
170
171 // wasm.throw() intinsic, which will be lowered to wasm 'throw' instruction.
172 ThrowF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_throw);
173 // Insert an unreachable instruction after a call to @llvm.wasm.throw and
174 // delete all following instructions within the BB, and delete all the dead
175 // children of the BB as well.
176 for (User *U : ThrowF->users()) {
177 // A call to @llvm.wasm.throw() is only generated from __cxa_throw()
178 // builtin call within libcxxabi, and cannot be an InvokeInst.
179 auto *ThrowI = cast<CallInst>(U);
180 if (ThrowI->getFunction() != &F)
181 continue;
182 Changed = true;
183 auto *BB = ThrowI->getParent();
185 BB->erase(std::next(BasicBlock::iterator(ThrowI)), BB->end());
186 IRB.SetInsertPoint(BB);
187 IRB.CreateUnreachable();
189 }
190
191 return Changed;
192}
193
194bool WasmEHPrepare::prepareEHPads(Function &F) {
195 Module &M = *F.getParent();
196 IRBuilder<> IRB(F.getContext());
197
200 for (BasicBlock &BB : F) {
201 if (!BB.isEHPad())
202 continue;
203 auto *Pad = BB.getFirstNonPHI();
204 if (isa<CatchPadInst>(Pad))
205 CatchPads.push_back(&BB);
206 else if (isa<CleanupPadInst>(Pad))
207 CleanupPads.push_back(&BB);
208 }
209 if (CatchPads.empty() && CleanupPads.empty())
210 return false;
211
212 assert(F.hasPersonalityFn() && "Personality function not found");
213
214 // __wasm_lpad_context global variable.
215 // This variable should be thread local. If the target does not support TLS,
216 // we depend on CoalesceFeaturesAndStripAtomics to downgrade it to
217 // non-thread-local ones, in which case we don't allow this object to be
218 // linked with other objects using shared memory.
219 LPadContextGV = cast<GlobalVariable>(
220 M.getOrInsertGlobal("__wasm_lpad_context", LPadContextTy));
221 LPadContextGV->setThreadLocalMode(GlobalValue::GeneralDynamicTLSModel);
222
223 LPadIndexField = IRB.CreateConstGEP2_32(LPadContextTy, LPadContextGV, 0, 0,
224 "lpad_index_gep");
225 LSDAField =
226 IRB.CreateConstGEP2_32(LPadContextTy, LPadContextGV, 0, 1, "lsda_gep");
227 SelectorField = IRB.CreateConstGEP2_32(LPadContextTy, LPadContextGV, 0, 2,
228 "selector_gep");
229
230 // wasm.landingpad.index() intrinsic, which is to specify landingpad index
231 LPadIndexF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_landingpad_index);
232 // wasm.lsda() intrinsic. Returns the address of LSDA table for the current
233 // function.
234 LSDAF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_lsda);
235 // wasm.get.exception() and wasm.get.ehselector() intrinsics. Calls to these
236 // are generated in clang.
237 GetExnF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_get_exception);
238 GetSelectorF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_get_ehselector);
239
240 // wasm.catch() will be lowered down to wasm 'catch' instruction in
241 // instruction selection.
242 CatchF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_catch);
243
244 // _Unwind_CallPersonality() wrapper function, which calls the personality
245 CallPersonalityF = M.getOrInsertFunction(
246 "_Unwind_CallPersonality", IRB.getInt32Ty(), IRB.getInt8PtrTy());
247 if (Function *F = dyn_cast<Function>(CallPersonalityF.getCallee()))
248 F->setDoesNotThrow();
249
250 unsigned Index = 0;
251 for (auto *BB : CatchPads) {
252 auto *CPI = cast<CatchPadInst>(BB->getFirstNonPHI());
253 // In case of a single catch (...), we don't need to emit a personalify
254 // function call
255 if (CPI->arg_size() == 1 &&
256 cast<Constant>(CPI->getArgOperand(0))->isNullValue())
257 prepareEHPad(BB, false);
258 else
259 prepareEHPad(BB, true, Index++);
260 }
261
262 // Cleanup pads don't need a personality function call.
263 for (auto *BB : CleanupPads)
264 prepareEHPad(BB, false);
265
266 return true;
267}
268
269// Prepare an EH pad for Wasm EH handling. If NeedPersonality is false, Index is
270// ignored.
271void WasmEHPrepare::prepareEHPad(BasicBlock *BB, bool NeedPersonality,
272 unsigned Index) {
273 assert(BB->isEHPad() && "BB is not an EHPad!");
274 IRBuilder<> IRB(BB->getContext());
275 IRB.SetInsertPoint(&*BB->getFirstInsertionPt());
276
277 auto *FPI = cast<FuncletPadInst>(BB->getFirstNonPHI());
278 Instruction *GetExnCI = nullptr, *GetSelectorCI = nullptr;
279 for (auto &U : FPI->uses()) {
280 if (auto *CI = dyn_cast<CallInst>(U.getUser())) {
281 if (CI->getCalledOperand() == GetExnF)
282 GetExnCI = CI;
283 if (CI->getCalledOperand() == GetSelectorF)
284 GetSelectorCI = CI;
285 }
286 }
287
288 // Cleanup pads do not have any of wasm.get.exception() or
289 // wasm.get.ehselector() calls. We need to do nothing.
290 if (!GetExnCI) {
291 assert(!GetSelectorCI &&
292 "wasm.get.ehselector() cannot exist w/o wasm.get.exception()");
293 return;
294 }
295
296 // Replace wasm.get.exception intrinsic with wasm.catch intrinsic, which will
297 // be lowered to wasm 'catch' instruction. We do this mainly because
298 // instruction selection cannot handle wasm.get.exception intrinsic's token
299 // argument.
300 Instruction *CatchCI =
301 IRB.CreateCall(CatchF, {IRB.getInt32(WebAssembly::CPP_EXCEPTION)}, "exn");
302 GetExnCI->replaceAllUsesWith(CatchCI);
303 GetExnCI->eraseFromParent();
304
305 // In case it is a catchpad with single catch (...) or a cleanuppad, we don't
306 // need to call personality function because we don't need a selector.
307 if (!NeedPersonality) {
308 if (GetSelectorCI) {
309 assert(GetSelectorCI->use_empty() &&
310 "wasm.get.ehselector() still has uses!");
311 GetSelectorCI->eraseFromParent();
312 }
313 return;
314 }
315 IRB.SetInsertPoint(CatchCI->getNextNode());
316
317 // This is to create a map of <landingpad EH label, landingpad index> in
318 // SelectionDAGISel, which is to be used in EHStreamer to emit LSDA tables.
319 // Pseudocode: wasm.landingpad.index(Index);
320 IRB.CreateCall(LPadIndexF, {FPI, IRB.getInt32(Index)});
321
322 // Pseudocode: __wasm_lpad_context.lpad_index = index;
323 IRB.CreateStore(IRB.getInt32(Index), LPadIndexField);
324
325 auto *CPI = cast<CatchPadInst>(FPI);
326 // TODO Sometimes storing the LSDA address every time is not necessary, in
327 // case it is already set in a dominating EH pad and there is no function call
328 // between from that EH pad to here. Consider optimizing those cases.
329 // Pseudocode: __wasm_lpad_context.lsda = wasm.lsda();
330 IRB.CreateStore(IRB.CreateCall(LSDAF), LSDAField);
331
332 // Pseudocode: _Unwind_CallPersonality(exn);
333 CallInst *PersCI = IRB.CreateCall(CallPersonalityF, CatchCI,
334 OperandBundleDef("funclet", CPI));
335 PersCI->setDoesNotThrow();
336
337 // Pseudocode: int selector = __wasm_lpad_context.selector;
338 Instruction *Selector =
339 IRB.CreateLoad(IRB.getInt32Ty(), SelectorField, "selector");
340
341 // Replace the return value from wasm.get.ehselector() with the selector value
342 // loaded from __wasm_lpad_context.selector.
343 assert(GetSelectorCI && "wasm.get.ehselector() call does not exist");
344 GetSelectorCI->replaceAllUsesWith(Selector);
345 GetSelectorCI->eraseFromParent();
346}
347
349 // If an exception is not caught by a catchpad (i.e., it is a foreign
350 // exception), it will unwind to its parent catchswitch's unwind destination.
351 // We don't record an unwind destination for cleanuppads because every
352 // exception should be caught by it.
353 for (const auto &BB : *F) {
354 if (!BB.isEHPad())
355 continue;
356 const Instruction *Pad = BB.getFirstNonPHI();
357
358 if (const auto *CatchPad = dyn_cast<CatchPadInst>(Pad)) {
359 const auto *UnwindBB = CatchPad->getCatchSwitch()->getUnwindDest();
360 if (!UnwindBB)
361 continue;
362 const Instruction *UnwindPad = UnwindBB->getFirstNonPHI();
363 if (const auto *CatchSwitch = dyn_cast<CatchSwitchInst>(UnwindPad))
364 // Currently there should be only one handler per a catchswitch.
365 EHInfo.setUnwindDest(&BB, *CatchSwitch->handlers().begin());
366 else // cleanuppad
367 EHInfo.setUnwindDest(&BB, UnwindBB);
368 }
369 }
370}
#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:314
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:245
const Instruction * getFirstNonPHI() const
Returns a pointer to the first instruction in this block that is not a PHINode instruction.
Definition: BasicBlock.cpp:208
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:512
void setDoesNotThrow()
Definition: InstrTypes.h:1909
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:308
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:2550
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:116
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:408
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:532
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:1502
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.
OperandBundleDefT< Value * > OperandBundleDef
Definition: AutoUpgrade.h:33
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)