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.landingpad_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/ADT/SetVector.h"
81#include "llvm/ADT/Statistic.h"
82#include "llvm/ADT/Triple.h"
83#include "llvm/Analysis/DomTreeUpdater.h"
84#include "llvm/CodeGen/Passes.h"
85#include "llvm/CodeGen/TargetLowering.h"
86#include "llvm/CodeGen/TargetSubtargetInfo.h"
87#include "llvm/CodeGen/WasmEHFuncInfo.h"
88#include "llvm/IR/Dominators.h"
89#include "llvm/IR/IRBuilder.h"
90#include "llvm/IR/Intrinsics.h"
91#include "llvm/IR/IntrinsicsWebAssembly.h"
92#include "llvm/InitializePasses.h"
93#include "llvm/Pass.h"
94#include "llvm/Transforms/Utils/BasicBlockUtils.h"
95
96using namespace llvm;
97
98#define DEBUG_TYPE "wasmehprepare"
99
100namespace {
101class WasmEHPrepare : public FunctionPass {
102  Type *LPadContextTy = nullptr; // type of 'struct _Unwind_LandingPadContext'
103  GlobalVariable *LPadContextGV = nullptr; // __wasm_lpad_context
104
105  // Field addresses of struct _Unwind_LandingPadContext
106  Value *LPadIndexField = nullptr; // lpad_index field
107  Value *LSDAField = nullptr;      // lsda field
108  Value *SelectorField = nullptr;  // selector
109
110  Function *ThrowF = nullptr;       // wasm.throw() intrinsic
111  Function *LPadIndexF = nullptr;   // wasm.landingpad.index() intrinsic
112  Function *LSDAF = nullptr;        // wasm.lsda() intrinsic
113  Function *GetExnF = nullptr;      // wasm.get.exception() intrinsic
114  Function *CatchF = nullptr;       // wasm.catch() intrinsic
115  Function *GetSelectorF = nullptr; // wasm.get.ehselector() intrinsic
116  FunctionCallee CallPersonalityF =
117      nullptr; // _Unwind_CallPersonality() wrapper
118
119  bool prepareThrows(Function &F);
120  bool prepareEHPads(Function &F);
121  void prepareEHPad(BasicBlock *BB, bool NeedPersonality, unsigned Index = 0);
122
123public:
124  static char ID; // Pass identification, replacement for typeid
125
126  WasmEHPrepare() : FunctionPass(ID) {}
127  void getAnalysisUsage(AnalysisUsage &AU) const override;
128  bool doInitialization(Module &M) override;
129  bool runOnFunction(Function &F) override;
130
131  StringRef getPassName() const override {
132    return "WebAssembly Exception handling preparation";
133  }
134};
135} // end anonymous namespace
136
137char WasmEHPrepare::ID = 0;
138INITIALIZE_PASS_BEGIN(WasmEHPrepare, DEBUG_TYPE,
139                      "Prepare WebAssembly exceptions", false, false)
140INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
141INITIALIZE_PASS_END(WasmEHPrepare, DEBUG_TYPE, "Prepare WebAssembly exceptions",
142                    false, false)
143
144FunctionPass *llvm::createWasmEHPass() { return new WasmEHPrepare(); }
145
146void WasmEHPrepare::getAnalysisUsage(AnalysisUsage &AU) const {
147  AU.addRequired<DominatorTreeWrapperPass>();
148}
149
150bool WasmEHPrepare::doInitialization(Module &M) {
151  IRBuilder<> IRB(M.getContext());
152  LPadContextTy = StructType::get(IRB.getInt32Ty(),   // lpad_index
153                                  IRB.getInt8PtrTy(), // lsda
154                                  IRB.getInt32Ty()    // selector
155  );
156  return false;
157}
158
159// Erase the specified BBs if the BB does not have any remaining predecessors,
160// and also all its dead children.
161template <typename Container>
162static void eraseDeadBBsAndChildren(const Container &BBs, DomTreeUpdater *DTU) {
163  SmallVector<BasicBlock *, 8> WL(BBs.begin(), BBs.end());
164  while (!WL.empty()) {
165    auto *BB = WL.pop_back_val();
166    if (!pred_empty(BB))
167      continue;
168    WL.append(succ_begin(BB), succ_end(BB));
169    DeleteDeadBlock(BB, DTU);
170  }
171}
172
173bool WasmEHPrepare::runOnFunction(Function &F) {
174  bool Changed = false;
175  Changed |= prepareThrows(F);
176  Changed |= prepareEHPads(F);
177  return Changed;
178}
179
180bool WasmEHPrepare::prepareThrows(Function &F) {
181  auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
182  DomTreeUpdater DTU(&DT, /*PostDominatorTree*/ nullptr,
183                     DomTreeUpdater::UpdateStrategy::Eager);
184  Module &M = *F.getParent();
185  IRBuilder<> IRB(F.getContext());
186  bool Changed = false;
187
188  // wasm.throw() intinsic, which will be lowered to wasm 'throw' instruction.
189  ThrowF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_throw);
190  // Insert an unreachable instruction after a call to @llvm.wasm.throw and
191  // delete all following instructions within the BB, and delete all the dead
192  // children of the BB as well.
193  for (User *U : ThrowF->users()) {
194    // A call to @llvm.wasm.throw() is only generated from __cxa_throw()
195    // builtin call within libcxxabi, and cannot be an InvokeInst.
196    auto *ThrowI = cast<CallInst>(U);
197    if (ThrowI->getFunction() != &F)
198      continue;
199    Changed = true;
200    auto *BB = ThrowI->getParent();
201    SmallVector<BasicBlock *, 4> Succs(successors(BB));
202    auto &InstList = BB->getInstList();
203    InstList.erase(std::next(BasicBlock::iterator(ThrowI)), InstList.end());
204    IRB.SetInsertPoint(BB);
205    IRB.CreateUnreachable();
206    eraseDeadBBsAndChildren(Succs, &DTU);
207  }
208
209  return Changed;
210}
211
212bool WasmEHPrepare::prepareEHPads(Function &F) {
213  Module &M = *F.getParent();
214  IRBuilder<> IRB(F.getContext());
215
216  SmallVector<BasicBlock *, 16> CatchPads;
217  SmallVector<BasicBlock *, 16> CleanupPads;
218  for (BasicBlock &BB : F) {
219    if (!BB.isEHPad())
220      continue;
221    auto *Pad = BB.getFirstNonPHI();
222    if (isa<CatchPadInst>(Pad))
223      CatchPads.push_back(&BB);
224    else if (isa<CleanupPadInst>(Pad))
225      CleanupPads.push_back(&BB);
226  }
227  if (CatchPads.empty() && CleanupPads.empty())
228    return false;
229
230  assert(F.hasPersonalityFn() && "Personality function not found");
231
232  // __wasm_lpad_context global variable
233  LPadContextGV = cast<GlobalVariable>(
234      M.getOrInsertGlobal("__wasm_lpad_context", LPadContextTy));
235  LPadIndexField = IRB.CreateConstGEP2_32(LPadContextTy, LPadContextGV, 0, 0,
236                                          "lpad_index_gep");
237  LSDAField =
238      IRB.CreateConstGEP2_32(LPadContextTy, LPadContextGV, 0, 1, "lsda_gep");
239  SelectorField = IRB.CreateConstGEP2_32(LPadContextTy, LPadContextGV, 0, 2,
240                                         "selector_gep");
241
242  // wasm.landingpad.index() intrinsic, which is to specify landingpad index
243  LPadIndexF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_landingpad_index);
244  // wasm.lsda() intrinsic. Returns the address of LSDA table for the current
245  // function.
246  LSDAF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_lsda);
247  // wasm.get.exception() and wasm.get.ehselector() intrinsics. Calls to these
248  // are generated in clang.
249  GetExnF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_get_exception);
250  GetSelectorF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_get_ehselector);
251
252  // wasm.catch() will be lowered down to wasm 'catch' instruction in
253  // instruction selection.
254  CatchF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_catch);
255
256  // _Unwind_CallPersonality() wrapper function, which calls the personality
257  CallPersonalityF = M.getOrInsertFunction(
258      "_Unwind_CallPersonality", IRB.getInt32Ty(), IRB.getInt8PtrTy());
259  if (Function *F = dyn_cast<Function>(CallPersonalityF.getCallee()))
260    F->setDoesNotThrow();
261
262  unsigned Index = 0;
263  for (auto *BB : CatchPads) {
264    auto *CPI = cast<CatchPadInst>(BB->getFirstNonPHI());
265    // In case of a single catch (...), we don't need to emit a personalify
266    // function call
267    if (CPI->getNumArgOperands() == 1 &&
268        cast<Constant>(CPI->getArgOperand(0))->isNullValue())
269      prepareEHPad(BB, false);
270    else
271      prepareEHPad(BB, true, Index++);
272  }
273
274  // Cleanup pads don't need a personality function call.
275  for (auto *BB : CleanupPads)
276    prepareEHPad(BB, false);
277
278  return true;
279}
280
281// Prepare an EH pad for Wasm EH handling. If NeedPersonality is false, Index is
282// ignored.
283void WasmEHPrepare::prepareEHPad(BasicBlock *BB, bool NeedPersonality,
284                                 unsigned Index) {
285  assert(BB->isEHPad() && "BB is not an EHPad!");
286  IRBuilder<> IRB(BB->getContext());
287  IRB.SetInsertPoint(&*BB->getFirstInsertionPt());
288
289  auto *FPI = cast<FuncletPadInst>(BB->getFirstNonPHI());
290  Instruction *GetExnCI = nullptr, *GetSelectorCI = nullptr;
291  for (auto &U : FPI->uses()) {
292    if (auto *CI = dyn_cast<CallInst>(U.getUser())) {
293      if (CI->getCalledOperand() == GetExnF)
294        GetExnCI = CI;
295      if (CI->getCalledOperand() == GetSelectorF)
296        GetSelectorCI = CI;
297    }
298  }
299
300  // Cleanup pads do not have any of wasm.get.exception() or
301  // wasm.get.ehselector() calls. We need to do nothing.
302  if (!GetExnCI) {
303    assert(!GetSelectorCI &&
304           "wasm.get.ehselector() cannot exist w/o wasm.get.exception()");
305    return;
306  }
307
308  // Replace wasm.get.exception intrinsic with wasm.catch intrinsic, which will
309  // be lowered to wasm 'catch' instruction. We do this mainly because
310  // instruction selection cannot handle wasm.get.exception intrinsic's token
311  // argument.
312  Instruction *CatchCI =
313      IRB.CreateCall(CatchF, {IRB.getInt32(WebAssembly::CPP_EXCEPTION)}, "exn");
314  GetExnCI->replaceAllUsesWith(CatchCI);
315  GetExnCI->eraseFromParent();
316
317  // In case it is a catchpad with single catch (...) or a cleanuppad, we don't
318  // need to call personality function because we don't need a selector.
319  if (!NeedPersonality) {
320    if (GetSelectorCI) {
321      assert(GetSelectorCI->use_empty() &&
322             "wasm.get.ehselector() still has uses!");
323      GetSelectorCI->eraseFromParent();
324    }
325    return;
326  }
327  IRB.SetInsertPoint(CatchCI->getNextNode());
328
329  // This is to create a map of <landingpad EH label, landingpad index> in
330  // SelectionDAGISel, which is to be used in EHStreamer to emit LSDA tables.
331  // Pseudocode: wasm.landingpad.index(Index);
332  IRB.CreateCall(LPadIndexF, {FPI, IRB.getInt32(Index)});
333
334  // Pseudocode: __wasm_lpad_context.lpad_index = index;
335  IRB.CreateStore(IRB.getInt32(Index), LPadIndexField);
336
337  auto *CPI = cast<CatchPadInst>(FPI);
338  // TODO Sometimes storing the LSDA address every time is not necessary, in
339  // case it is already set in a dominating EH pad and there is no function call
340  // between from that EH pad to here. Consider optimizing those cases.
341  // Pseudocode: __wasm_lpad_context.lsda = wasm.lsda();
342  IRB.CreateStore(IRB.CreateCall(LSDAF), LSDAField);
343
344  // Pseudocode: _Unwind_CallPersonality(exn);
345  CallInst *PersCI = IRB.CreateCall(CallPersonalityF, CatchCI,
346                                    OperandBundleDef("funclet", CPI));
347  PersCI->setDoesNotThrow();
348
349  // Pseudocode: int selector = __wasm.landingpad_context.selector;
350  Instruction *Selector =
351      IRB.CreateLoad(IRB.getInt32Ty(), SelectorField, "selector");
352
353  // Replace the return value from wasm.get.ehselector() with the selector value
354  // loaded from __wasm_lpad_context.selector.
355  assert(GetSelectorCI && "wasm.get.ehselector() call does not exist");
356  GetSelectorCI->replaceAllUsesWith(Selector);
357  GetSelectorCI->eraseFromParent();
358}
359
360void llvm::calculateWasmEHInfo(const Function *F, WasmEHFuncInfo &EHInfo) {
361  // If an exception is not caught by a catchpad (i.e., it is a foreign
362  // exception), it will unwind to its parent catchswitch's unwind destination.
363  // We don't record an unwind destination for cleanuppads because every
364  // exception should be caught by it.
365  for (const auto &BB : *F) {
366    if (!BB.isEHPad())
367      continue;
368    const Instruction *Pad = BB.getFirstNonPHI();
369
370    if (const auto *CatchPad = dyn_cast<CatchPadInst>(Pad)) {
371      const auto *UnwindBB = CatchPad->getCatchSwitch()->getUnwindDest();
372      if (!UnwindBB)
373        continue;
374      const Instruction *UnwindPad = UnwindBB->getFirstNonPHI();
375      if (const auto *CatchSwitch = dyn_cast<CatchSwitchInst>(UnwindPad))
376        // Currently there should be only one handler per a catchswitch.
377        EHInfo.setUnwindDest(&BB, *CatchSwitch->handlers().begin());
378      else // cleanuppad
379        EHInfo.setUnwindDest(&BB, UnwindBB);
380    }
381  }
382}
383