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1 //===-- WasmEHPrepare - Prepare excepton handling for WebAssembly --------===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This transformation is designed for use by code generators which use
11 // WebAssembly exception handling scheme.
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(0); // 0 is a tag for C++
27 //   wasm.landingpad.index(index);
28 //   // Only add below in case it's not a single catch (...)
29 //   __wasm_lpad_context.lpad_index = index;
30 //   __wasm_lpad_context.lsda = wasm.lsda();
31 //   _Unwind_CallPersonality(exn);
32 //   int selector = __wasm.landingpad_context.selector;
33 //   ...
34 //
35 // Also, does the following for a cleanuppad block with a call to
36 // __clang_call_terminate():
37 // - Before:
38 //   cleanuppad ...
39 //   exn = wasm.get.exception();
40 //   __clang_call_terminate(exn);
41 //
42 // - After:
43 //   cleanuppad ...
44 //   exn = wasm.catch(0); // 0 is a tag for C++
45 //   __clang_call_terminate(exn);
46 //
47 //
48 // * Background: WebAssembly EH instructions
49 // WebAssembly's try and catch instructions are structured as follows:
50 // try
51 //   instruction*
52 // catch (C++ tag)
53 //   instruction*
54 // ...
55 // catch_all
56 //   instruction*
57 // try_end
58 //
59 // A catch instruction in WebAssembly does not correspond to a C++ catch clause.
60 // In WebAssembly, there is a single catch instruction for all C++ exceptions.
61 // There can be more catch instructions for exceptions in other languages, but
62 // they are not generated for now. catch_all catches all exceptions including
63 // foreign exceptions (e.g. JavaScript). We turn catchpads into catch (C++ tag)
64 // and cleanuppads into catch_all, with one exception: cleanuppad with a call to
65 // __clang_call_terminate should be both in catch (C++ tag) and catch_all.
66 //
67 //
68 // * Background: Direct personality function call
69 // In WebAssembly EH, the VM is responsible for unwinding the stack once an
70 // exception is thrown. After the stack is unwound, the control flow is
71 // transfered to WebAssembly 'catch' instruction, which returns a caught
72 // exception object.
73 //
74 // Unwinding the stack is not done by libunwind but the VM, so the personality
75 // function in libcxxabi cannot be called from libunwind during the unwinding
76 // process. So after a catch instruction, we insert a call to a wrapper function
77 // in libunwind that in turn calls the real personality function.
78 //
79 // In Itanium EH, if the personality function decides there is no matching catch
80 // clause in a call frame and no cleanup action to perform, the unwinder doesn't
81 // stop there and continues unwinding. But in Wasm EH, the unwinder stops at
82 // every call frame with a catch intruction, after which the personality
83 // function is called from the compiler-generated user code here.
84 //
85 // In libunwind, we have this struct that serves as a communincation channel
86 // between the compiler-generated user code and the personality function in
87 // libcxxabi.
88 //
89 // struct _Unwind_LandingPadContext {
90 //   uintptr_t lpad_index;
91 //   uintptr_t lsda;
92 //   uintptr_t selector;
93 // };
94 // struct _Unwind_LandingPadContext __wasm_lpad_context = ...;
95 //
96 // And this wrapper in libunwind calls the personality function.
97 //
98 // _Unwind_Reason_Code _Unwind_CallPersonality(void *exception_ptr) {
99 //   struct _Unwind_Exception *exception_obj =
100 //       (struct _Unwind_Exception *)exception_ptr;
101 //   _Unwind_Reason_Code ret = __gxx_personality_v0(
102 //       1, _UA_CLEANUP_PHASE, exception_obj->exception_class, exception_obj,
103 //       (struct _Unwind_Context *)__wasm_lpad_context);
104 //   return ret;
105 // }
106 //
107 // We pass a landing pad index, and the address of LSDA for the current function
108 // to the wrapper function _Unwind_CallPersonality in libunwind, and we retrieve
109 // the selector after it returns.
110 //
111 //===----------------------------------------------------------------------===//
112 
113 #include "llvm/ADT/SetVector.h"
114 #include "llvm/ADT/Statistic.h"
115 #include "llvm/ADT/Triple.h"
116 #include "llvm/CodeGen/Passes.h"
117 #include "llvm/CodeGen/TargetLowering.h"
118 #include "llvm/CodeGen/TargetSubtargetInfo.h"
119 #include "llvm/CodeGen/WasmEHFuncInfo.h"
120 #include "llvm/IR/Dominators.h"
121 #include "llvm/IR/IRBuilder.h"
122 #include "llvm/IR/Intrinsics.h"
123 #include "llvm/Pass.h"
124 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
125 
126 using namespace llvm;
127 
128 #define DEBUG_TYPE "wasmehprepare"
129 
130 namespace {
131 class WasmEHPrepare : public FunctionPass {
132   Type *LPadContextTy = nullptr; // type of 'struct _Unwind_LandingPadContext'
133   GlobalVariable *LPadContextGV = nullptr; // __wasm_lpad_context
134 
135   // Field addresses of struct _Unwind_LandingPadContext
136   Value *LPadIndexField = nullptr; // lpad_index field
137   Value *LSDAField = nullptr;      // lsda field
138   Value *SelectorField = nullptr;  // selector
139 
140   Function *CatchF = nullptr;           // wasm.catch.extract() intrinsic
141   Function *LPadIndexF = nullptr;       // wasm.landingpad.index() intrinsic
142   Function *LSDAF = nullptr;            // wasm.lsda() intrinsic
143   Function *GetExnF = nullptr;          // wasm.get.exception() intrinsic
144   Function *GetSelectorF = nullptr;     // wasm.get.ehselector() intrinsic
145   Function *CallPersonalityF = nullptr; // _Unwind_CallPersonality() wrapper
146   Function *ClangCallTermF = nullptr;   // __clang_call_terminate() function
147 
148   void prepareEHPad(BasicBlock *BB, unsigned Index);
149   void prepareTerminateCleanupPad(BasicBlock *BB);
150 
151 public:
152   static char ID; // Pass identification, replacement for typeid
153 
WasmEHPrepare()154   WasmEHPrepare() : FunctionPass(ID) {}
155 
156   bool doInitialization(Module &M) override;
157   bool runOnFunction(Function &F) override;
158 
getPassName() const159   StringRef getPassName() const override {
160     return "WebAssembly Exception handling preparation";
161   }
162 };
163 } // end anonymous namespace
164 
165 char WasmEHPrepare::ID = 0;
166 INITIALIZE_PASS(WasmEHPrepare, DEBUG_TYPE, "Prepare WebAssembly exceptions",
167                 false, false)
168 
createWasmEHPass()169 FunctionPass *llvm::createWasmEHPass() { return new WasmEHPrepare(); }
170 
doInitialization(Module & M)171 bool WasmEHPrepare::doInitialization(Module &M) {
172   IRBuilder<> IRB(M.getContext());
173   LPadContextTy = StructType::get(IRB.getInt32Ty(),   // lpad_index
174                                   IRB.getInt8PtrTy(), // lsda
175                                   IRB.getInt32Ty()    // selector
176   );
177   return false;
178 }
179 
runOnFunction(Function & F)180 bool WasmEHPrepare::runOnFunction(Function &F) {
181   SmallVector<BasicBlock *, 16> CatchPads;
182   SmallVector<BasicBlock *, 16> CleanupPads;
183   for (BasicBlock &BB : F) {
184     if (!BB.isEHPad())
185       continue;
186     auto *Pad = BB.getFirstNonPHI();
187     if (isa<CatchPadInst>(Pad))
188       CatchPads.push_back(&BB);
189     else if (isa<CleanupPadInst>(Pad))
190       CleanupPads.push_back(&BB);
191   }
192 
193   if (CatchPads.empty() && CleanupPads.empty())
194     return false;
195   assert(F.hasPersonalityFn() && "Personality function not found");
196 
197   Module &M = *F.getParent();
198   IRBuilder<> IRB(F.getContext());
199 
200   // __wasm_lpad_context global variable
201   LPadContextGV = cast<GlobalVariable>(
202       M.getOrInsertGlobal("__wasm_lpad_context", LPadContextTy));
203   LPadIndexField = IRB.CreateConstGEP2_32(LPadContextTy, LPadContextGV, 0, 0,
204                                           "lpad_index_gep");
205   LSDAField =
206       IRB.CreateConstGEP2_32(LPadContextTy, LPadContextGV, 0, 1, "lsda_gep");
207   SelectorField = IRB.CreateConstGEP2_32(LPadContextTy, LPadContextGV, 0, 2,
208                                          "selector_gep");
209 
210   // wasm.catch() intinsic, which will be lowered to wasm 'catch' instruction.
211   CatchF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_catch);
212   // wasm.landingpad.index() intrinsic, which is to specify landingpad index
213   LPadIndexF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_landingpad_index);
214   // wasm.lsda() intrinsic. Returns the address of LSDA table for the current
215   // function.
216   LSDAF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_lsda);
217   // wasm.get.exception() and wasm.get.ehselector() intrinsics. Calls to these
218   // are generated in clang.
219   GetExnF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_get_exception);
220   GetSelectorF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_get_ehselector);
221 
222   // _Unwind_CallPersonality() wrapper function, which calls the personality
223   CallPersonalityF = cast<Function>(M.getOrInsertFunction(
224       "_Unwind_CallPersonality", IRB.getInt32Ty(), IRB.getInt8PtrTy()));
225   CallPersonalityF->setDoesNotThrow();
226 
227   // __clang_call_terminate() function, which is inserted by clang in case a
228   // cleanup throws
229   ClangCallTermF = M.getFunction("__clang_call_terminate");
230 
231   unsigned Index = 0;
232   for (auto *BB : CatchPads) {
233     auto *CPI = cast<CatchPadInst>(BB->getFirstNonPHI());
234     // In case of a single catch (...), we don't need to emit LSDA
235     if (CPI->getNumArgOperands() == 1 &&
236         cast<Constant>(CPI->getArgOperand(0))->isNullValue())
237       prepareEHPad(BB, -1);
238     else
239       prepareEHPad(BB, Index++);
240   }
241 
242   if (!ClangCallTermF)
243     return !CatchPads.empty();
244 
245   // Cleanuppads will turn into catch_all later, but cleanuppads with a call to
246   // __clang_call_terminate() is a special case. __clang_call_terminate() takes
247   // an exception object, so we have to duplicate call in both 'catch <C++ tag>'
248   // and 'catch_all' clauses. Here we only insert a call to catch; the
249   // duplication will be done later. In catch_all, the exception object will be
250   // set to null.
251   for (auto *BB : CleanupPads)
252     for (auto &I : *BB)
253       if (auto *CI = dyn_cast<CallInst>(&I))
254         if (CI->getCalledValue() == ClangCallTermF)
255           prepareEHPad(BB, -1);
256 
257   return true;
258 }
259 
prepareEHPad(BasicBlock * BB,unsigned Index)260 void WasmEHPrepare::prepareEHPad(BasicBlock *BB, unsigned Index) {
261   assert(BB->isEHPad() && "BB is not an EHPad!");
262   IRBuilder<> IRB(BB->getContext());
263 
264   IRB.SetInsertPoint(&*BB->getFirstInsertionPt());
265   // The argument to wasm.catch() is the tag for C++ exceptions, which we set to
266   // 0 for this module.
267   // Pseudocode: void *exn = wasm.catch(0);
268   Instruction *Exn = IRB.CreateCall(CatchF, IRB.getInt32(0), "exn");
269   // Replace the return value of wasm.get.exception() with the return value from
270   // wasm.catch().
271   auto *FPI = cast<FuncletPadInst>(BB->getFirstNonPHI());
272   Instruction *GetExnCI = nullptr, *GetSelectorCI = nullptr;
273   for (auto &U : FPI->uses()) {
274     if (auto *CI = dyn_cast<CallInst>(U.getUser())) {
275       if (CI->getCalledValue() == GetExnF)
276         GetExnCI = CI;
277       else if (CI->getCalledValue() == GetSelectorF)
278         GetSelectorCI = CI;
279     }
280   }
281 
282   assert(GetExnCI && "wasm.get.exception() call does not exist");
283   GetExnCI->replaceAllUsesWith(Exn);
284   GetExnCI->eraseFromParent();
285 
286   // In case it is a catchpad with single catch (...) or a cleanuppad, we don't
287   // need to call personality function because we don't need a selector.
288   if (FPI->getNumArgOperands() == 0 ||
289       (FPI->getNumArgOperands() == 1 &&
290        cast<Constant>(FPI->getArgOperand(0))->isNullValue())) {
291     if (GetSelectorCI) {
292       assert(GetSelectorCI->use_empty() &&
293              "wasm.get.ehselector() still has uses!");
294       GetSelectorCI->eraseFromParent();
295     }
296     return;
297   }
298   IRB.SetInsertPoint(Exn->getNextNode());
299 
300   // This is to create a map of <landingpad EH label, landingpad index> in
301   // SelectionDAGISel, which is to be used in EHStreamer to emit LSDA tables.
302   // Pseudocode: wasm.landingpad.index(Index);
303   IRB.CreateCall(LPadIndexF, IRB.getInt32(Index));
304 
305   // Pseudocode: __wasm_lpad_context.lpad_index = index;
306   IRB.CreateStore(IRB.getInt32(Index), LPadIndexField);
307 
308   // Store LSDA address only if this catchpad belongs to a top-level
309   // catchswitch. If there is another catchpad that dominates this pad, we don't
310   // need to store LSDA address again, because they are the same throughout the
311   // function and have been already stored before.
312   // TODO Can we not store LSDA address in user function but make libcxxabi
313   // compute it?
314   auto *CPI = cast<CatchPadInst>(FPI);
315   if (isa<ConstantTokenNone>(CPI->getCatchSwitch()->getParentPad()))
316     // Pseudocode: __wasm_lpad_context.lsda = wasm.lsda();
317     IRB.CreateStore(IRB.CreateCall(LSDAF), LSDAField);
318 
319   // Pseudocode: _Unwind_CallPersonality(exn);
320   CallInst *PersCI =
321       IRB.CreateCall(CallPersonalityF, Exn, OperandBundleDef("funclet", CPI));
322   PersCI->setDoesNotThrow();
323 
324   // Pseudocode: int selector = __wasm.landingpad_context.selector;
325   Instruction *Selector = IRB.CreateLoad(SelectorField, "selector");
326 
327   // Replace the return value from wasm.get.ehselector() with the selector value
328   // loaded from __wasm_lpad_context.selector.
329   assert(GetSelectorCI && "wasm.get.ehselector() call does not exist");
330   GetSelectorCI->replaceAllUsesWith(Selector);
331   GetSelectorCI->eraseFromParent();
332 }
333 
calculateWasmEHInfo(const Function * F,WasmEHFuncInfo & EHInfo)334 void llvm::calculateWasmEHInfo(const Function *F, WasmEHFuncInfo &EHInfo) {
335   for (const auto &BB : *F) {
336     if (!BB.isEHPad())
337       continue;
338     const Instruction *Pad = BB.getFirstNonPHI();
339 
340     // If an exception is not caught by a catchpad (i.e., it is a foreign
341     // exception), it will unwind to its parent catchswitch's unwind
342     // destination. We don't record an unwind destination for cleanuppads
343     // because every exception should be caught by it.
344     if (const auto *CatchPad = dyn_cast<CatchPadInst>(Pad)) {
345       const auto *UnwindBB = CatchPad->getCatchSwitch()->getUnwindDest();
346       if (!UnwindBB)
347         continue;
348       const Instruction *UnwindPad = UnwindBB->getFirstNonPHI();
349       if (const auto *CatchSwitch = dyn_cast<CatchSwitchInst>(UnwindPad))
350         // Currently there should be only one handler per a catchswitch.
351         EHInfo.setEHPadUnwindDest(&BB, *CatchSwitch->handlers().begin());
352       else // cleanuppad
353         EHInfo.setEHPadUnwindDest(&BB, UnwindBB);
354     }
355   }
356 
357   // Record the unwind destination for invoke and cleanupret instructions.
358   for (const auto &BB : *F) {
359     const Instruction *TI = BB.getTerminator();
360     BasicBlock *UnwindBB = nullptr;
361     if (const auto *Invoke = dyn_cast<InvokeInst>(TI))
362       UnwindBB = Invoke->getUnwindDest();
363     else if (const auto *CleanupRet = dyn_cast<CleanupReturnInst>(TI))
364       UnwindBB = CleanupRet->getUnwindDest();
365     if (!UnwindBB)
366       continue;
367     const Instruction *UnwindPad = UnwindBB->getFirstNonPHI();
368     if (const auto *CatchSwitch = dyn_cast<CatchSwitchInst>(UnwindPad))
369       // Currently there should be only one handler per a catchswitch.
370       EHInfo.setThrowUnwindDest(&BB, *CatchSwitch->handlers().begin());
371     else // cleanuppad
372       EHInfo.setThrowUnwindDest(&BB, UnwindBB);
373   }
374 }
375