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/IR/Mangler.h"
16 #include "llvm/MC/MCContext.h"
17 #include "llvm/MC/MCStreamer.h"
18 using namespace llvm;
19
endModule()20 void WasmException::endModule() {
21 // These are symbols used to throw/catch C++ exceptions and C longjmps. These
22 // symbols have to be emitted somewhere once in the module. Check if each of
23 // the symbols has already been created, i.e., we have at least one 'throw' or
24 // 'catch' instruction with the symbol in the module, and emit the symbol only
25 // if so.
26 for (const char *SymName : {"__cpp_exception", "__c_longjmp"}) {
27 SmallString<60> NameStr;
28 Mangler::getNameWithPrefix(NameStr, SymName, Asm->getDataLayout());
29 if (Asm->OutContext.lookupSymbol(NameStr)) {
30 MCSymbol *ExceptionSym = Asm->GetExternalSymbolSymbol(SymName);
31 Asm->OutStreamer->emitLabel(ExceptionSym);
32 }
33 }
34 }
35
markFunctionEnd()36 void WasmException::markFunctionEnd() {
37 // Get rid of any dead landing pads.
38 if (!Asm->MF->getLandingPads().empty()) {
39 auto *NonConstMF = const_cast<MachineFunction *>(Asm->MF);
40 // Wasm does not set BeginLabel and EndLabel information for landing pads,
41 // so we should set the second argument false.
42 NonConstMF->tidyLandingPads(nullptr, /* TidyIfNoBeginLabels */ false);
43 }
44 }
45
endFunction(const MachineFunction * MF)46 void WasmException::endFunction(const MachineFunction *MF) {
47 bool ShouldEmitExceptionTable = false;
48 for (const LandingPadInfo &Info : MF->getLandingPads()) {
49 if (MF->hasWasmLandingPadIndex(Info.LandingPadBlock)) {
50 ShouldEmitExceptionTable = true;
51 break;
52 }
53 }
54 if (!ShouldEmitExceptionTable)
55 return;
56 MCSymbol *LSDALabel = emitExceptionTable();
57 assert(LSDALabel && ".GCC_exception_table has not been emitted!");
58
59 // Wasm requires every data section symbol to have a .size set. So we emit an
60 // end marker and set the size as the difference between the start end the end
61 // marker.
62 MCSymbol *LSDAEndLabel = Asm->createTempSymbol("GCC_except_table_end");
63 Asm->OutStreamer->emitLabel(LSDAEndLabel);
64 MCContext &OutContext = Asm->OutStreamer->getContext();
65 const MCExpr *SizeExp = MCBinaryExpr::createSub(
66 MCSymbolRefExpr::create(LSDAEndLabel, OutContext),
67 MCSymbolRefExpr::create(LSDALabel, OutContext), OutContext);
68 Asm->OutStreamer->emitELFSize(LSDALabel, SizeExp);
69 }
70
71 // Compute the call-site table for wasm EH. Even though we use the same function
72 // name to share the common routines, a call site entry in the table corresponds
73 // to not a call site for possibly-throwing functions but a landing pad. In wasm
74 // EH the VM is responsible for stack unwinding. After an exception occurs and
75 // the stack is unwound, the control flow is transferred to wasm 'catch'
76 // instruction by the VM, after which the personality function is called from
77 // the compiler-generated code. Refer to WasmEHPrepare pass for more
78 // information.
computeCallSiteTable(SmallVectorImpl<CallSiteEntry> & CallSites,SmallVectorImpl<CallSiteRange> & CallSiteRanges,const SmallVectorImpl<const LandingPadInfo * > & LandingPads,const SmallVectorImpl<unsigned> & FirstActions)79 void WasmException::computeCallSiteTable(
80 SmallVectorImpl<CallSiteEntry> &CallSites,
81 SmallVectorImpl<CallSiteRange> &CallSiteRanges,
82 const SmallVectorImpl<const LandingPadInfo *> &LandingPads,
83 const SmallVectorImpl<unsigned> &FirstActions) {
84 MachineFunction &MF = *Asm->MF;
85 for (unsigned I = 0, N = LandingPads.size(); I < N; ++I) {
86 const LandingPadInfo *Info = LandingPads[I];
87 MachineBasicBlock *LPad = Info->LandingPadBlock;
88 // We don't emit LSDA for single catch (...).
89 if (!MF.hasWasmLandingPadIndex(LPad))
90 continue;
91 // Wasm EH must maintain the EH pads in the order assigned to them by the
92 // WasmEHPrepare pass.
93 unsigned LPadIndex = MF.getWasmLandingPadIndex(LPad);
94 CallSiteEntry Site = {nullptr, nullptr, Info, FirstActions[I]};
95 if (CallSites.size() < LPadIndex + 1)
96 CallSites.resize(LPadIndex + 1);
97 CallSites[LPadIndex] = Site;
98 }
99 }
100