1 //===-- CodeGen/AsmPrinter/WasmException.cpp - Wasm Exception Impl --------===//
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 file contains support for writing WebAssembly exception info into asm
10 // files.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #include "WasmException.h"
15 #include "llvm/CodeGen/AsmPrinter.h"
16 #include "llvm/CodeGen/MachineFunction.h"
17 #include "llvm/IR/Mangler.h"
18 #include "llvm/MC/MCContext.h"
19 #include "llvm/MC/MCStreamer.h"
20 using namespace llvm;
21 
22 void WasmException::endModule() {
23   // These are symbols used to throw/catch C++ exceptions and C longjmps. These
24   // symbols have to be emitted somewhere once in the module. Check if each of
25   // the symbols has already been created, i.e., we have at least one 'throw' or
26   // 'catch' instruction with the symbol in the module, and emit the symbol only
27   // if so.
28   //
29   // But in dynamic linking, it is in general not possible to come up with a
30   // module instantiating order in which tag-defining modules are loaded before
31   // the importing modules. So we make them undefined symbols here, define tags
32   // in the JS side, and feed them to each importing module.
33   if (!Asm->isPositionIndependent()) {
34     for (const char *SymName : {"__cpp_exception", "__c_longjmp"}) {
35       SmallString<60> NameStr;
36       Mangler::getNameWithPrefix(NameStr, SymName, Asm->getDataLayout());
37       if (Asm->OutContext.lookupSymbol(NameStr)) {
38         MCSymbol *ExceptionSym = Asm->GetExternalSymbolSymbol(SymName);
39         Asm->OutStreamer->emitLabel(ExceptionSym);
40       }
41     }
42   }
43 }
44 
45 void WasmException::endFunction(const MachineFunction *MF) {
46   bool ShouldEmitExceptionTable = false;
47   for (const LandingPadInfo &Info : MF->getLandingPads()) {
48     if (MF->hasWasmLandingPadIndex(Info.LandingPadBlock)) {
49       ShouldEmitExceptionTable = true;
50       break;
51     }
52   }
53   if (!ShouldEmitExceptionTable)
54     return;
55   MCSymbol *LSDALabel = emitExceptionTable();
56   assert(LSDALabel && ".GCC_exception_table has not been emitted!");
57 
58   // Wasm requires every data section symbol to have a .size set. So we emit an
59   // end marker and set the size as the difference between the start end the end
60   // marker.
61   MCSymbol *LSDAEndLabel = Asm->createTempSymbol("GCC_except_table_end");
62   Asm->OutStreamer->emitLabel(LSDAEndLabel);
63   MCContext &OutContext = Asm->OutStreamer->getContext();
64   const MCExpr *SizeExp = MCBinaryExpr::createSub(
65       MCSymbolRefExpr::create(LSDAEndLabel, OutContext),
66       MCSymbolRefExpr::create(LSDALabel, OutContext), OutContext);
67   Asm->OutStreamer->emitELFSize(LSDALabel, SizeExp);
68 }
69 
70 // Compute the call-site table for wasm EH. Even though we use the same function
71 // name to share the common routines, a call site entry in the table corresponds
72 // to not a call site for possibly-throwing functions but a landing pad. In wasm
73 // EH the VM is responsible for stack unwinding. After an exception occurs and
74 // the stack is unwound, the control flow is transferred to wasm 'catch'
75 // instruction by the VM, after which the personality function is called from
76 // the compiler-generated code. Refer to WasmEHPrepare pass for more
77 // information.
78 void WasmException::computeCallSiteTable(
79     SmallVectorImpl<CallSiteEntry> &CallSites,
80     SmallVectorImpl<CallSiteRange> &CallSiteRanges,
81     const SmallVectorImpl<const LandingPadInfo *> &LandingPads,
82     const SmallVectorImpl<unsigned> &FirstActions) {
83   MachineFunction &MF = *Asm->MF;
84   for (unsigned I = 0, N = LandingPads.size(); I < N; ++I) {
85     const LandingPadInfo *Info = LandingPads[I];
86     MachineBasicBlock *LPad = Info->LandingPadBlock;
87     // We don't emit LSDA for single catch (...).
88     if (!MF.hasWasmLandingPadIndex(LPad))
89       continue;
90     // Wasm EH must maintain the EH pads in the order assigned to them by the
91     // WasmEHPrepare pass.
92     unsigned LPadIndex = MF.getWasmLandingPadIndex(LPad);
93     CallSiteEntry Site = {nullptr, nullptr, Info, FirstActions[I]};
94     if (CallSites.size() < LPadIndex + 1)
95       CallSites.resize(LPadIndex + 1);
96     CallSites[LPadIndex] = Site;
97   }
98 }
99