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