LLVM  10.0.0svn
WasmException.cpp
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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 
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 
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 
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.
79  const SmallVectorImpl<const LandingPadInfo *> &LandingPads,
80  const SmallVectorImpl<unsigned> &FirstActions) {
81  MachineFunction &MF = *Asm->MF;
82  for (unsigned I = 0, N = LandingPads.size(); I < N; ++I) {
83  const LandingPadInfo *Info = LandingPads[I];
84  MachineBasicBlock *LPad = Info->LandingPadBlock;
85  // We don't emit LSDA for single catch (...).
86  if (!MF.hasWasmLandingPadIndex(LPad))
87  continue;
88  // Wasm EH must maintain the EH pads in the order assigned to them by the
89  // WasmEHPrepare pass.
90  unsigned LPadIndex = MF.getWasmLandingPadIndex(LPad);
91  CallSiteEntry Site = {nullptr, nullptr, Info, FirstActions[I]};
92  if (CallSites.size() < LPadIndex + 1)
93  CallSites.resize(LPadIndex + 1);
94  CallSites[LPadIndex] = Site;
95  }
96 }
Structure describing an entry in the call-site table.
Definition: EHStreamer.h:61
std::unique_ptr< MCStreamer > OutStreamer
This is the MCStreamer object for the file we are generating.
Definition: AsmPrinter.h:93
MCSymbol * GetExternalSymbolSymbol(StringRef Sym) const
Return the MCSymbol for the specified ExternalSymbol.
static const MCSymbolRefExpr * create(const MCSymbol *Symbol, MCContext &Ctx)
Definition: MCExpr.h:327
MCSymbol * lookupSymbol(const Twine &Name) const
Get the symbol for Name, or null.
Definition: MCContext.cpp:264
This class represents lattice values for constants.
Definition: AllocatorList.h:23
MCSymbol - Instances of this class represent a symbol name in the MC file, and MCSymbols are created ...
Definition: MCSymbol.h:41
MCContext & OutContext
This is the context for the output file that we are streaming.
Definition: AsmPrinter.h:88
MachineFunction * MF
The current machine function.
Definition: AsmPrinter.h:96
void endFunction(const MachineFunction *MF) override
Gather post-function debug information.
This class consists of common code factored out of the SmallVector class to reduce code duplication b...
Definition: APFloat.h:41
Base class for the full range of assembler expressions which are needed for parsing.
Definition: MCExpr.h:35
void computeCallSiteTable(SmallVectorImpl< CallSiteEntry > &CallSites, const SmallVectorImpl< const LandingPadInfo *> &LandingPads, const SmallVectorImpl< unsigned > &FirstActions) override
Compute the call-site table.
Context object for machine code objects.
Definition: MCContext.h:65
This structure is used to retain landing pad info for the current function.
static const MCBinaryExpr * createSub(const MCExpr *LHS, const MCExpr *RHS, MCContext &Ctx)
Definition: MCExpr.h:550
void tidyLandingPads(DenseMap< MCSymbol *, uintptr_t > *LPMap=nullptr, bool TidyIfNoBeginLabels=true)
Remap landing pad labels and remove any deleted landing pads.
SmallString - A SmallString is just a SmallVector with methods and accessors that make it work better...
Definition: SmallString.h:25
Analysis containing CSE Info
Definition: CSEInfo.cpp:20
size_t size() const
Definition: SmallVector.h:52
AsmPrinter * Asm
Target of directive emission.
Definition: EHStreamer.h:33
const std::vector< LandingPadInfo > & getLandingPads() const
Return a reference to the landing pad info for the current function.
unsigned getWasmLandingPadIndex(const MachineBasicBlock *LPad) const
Get the index in wasm EH for a given landing pad.
virtual void markFunctionEnd() override
MachineBasicBlock * LandingPadBlock
#define I(x, y, z)
Definition: MD5.cpp:58
#define N
bool hasWasmLandingPadIndex(const MachineBasicBlock *LPad) const
Returns true if the landing pad has an associate index in wasm EH.
void endModule() override
Emit all sections that should come after the content.
assert(ImpDefSCC.getReg()==AMDGPU::SCC &&ImpDefSCC.isDef())
MCSymbol * emitExceptionTable()
Emit landing pads and actions.
Definition: EHStreamer.cpp:349
void getNameWithPrefix(raw_ostream &OS, const GlobalValue *GV, bool CannotUsePrivateLabel) const
Print the appropriate prefix and the specified global variable&#39;s name.
Definition: Mangler.cpp:111
const DataLayout & getDataLayout() const
Return information about data layout.
Definition: AsmPrinter.cpp:219
MCSymbol * createTempSymbol(const Twine &Name) const
void resize(size_type N)
Definition: SmallVector.h:344