lld/ELF/Thunks.h

   1 //===- Thunks.h --------------------------------------------------------===//
   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 #ifndef LLD_ELF_THUNKS_H
  10 #define LLD_ELF_THUNKS_H
  11
  12 #include "llvm/ADT/SmallVector.h"
  13 #include "Relocations.h"
  14
  15 namespace lld::elf {
  16 class Defined;
  17 class InputFile;
  18 class Symbol;
  19 class ThunkSection;
  20 // Class to describe an instance of a Thunk.
  21 // A Thunk is a code-sequence inserted by the linker in between a caller and
  22 // the callee. The relocation to the callee is redirected to the Thunk, which
  23 // after executing transfers control to the callee. Typical uses of Thunks
  24 // include transferring control from non-pi to pi and changing state on
  25 // targets like ARM.
  26 //
  27 // Thunks can be created for Defined, Shared and Undefined Symbols.
  28 // Thunks are assigned to synthetic ThunkSections
  29 class Thunk {
  30 public:
  31   Thunk(Symbol &destination, int64_t addend);
  32   virtual ~Thunk();
  33
  34   virtual uint32_t size() = 0;
  35   virtual void writeTo(uint8_t *buf) = 0;
  36
  37   // All Thunks must define at least one symbol, known as the thunk target
  38   // symbol, so that we can redirect relocations to it. The thunk may define
  39   // additional symbols, but these are never targets for relocations.
  40   virtual void addSymbols(ThunkSection &isec) = 0;
  41
  42   void setOffset(uint64_t offset);
  43   Defined *addSymbol(StringRef name, uint8_t type, uint64_t value,
  44                      InputSectionBase &section);
  45
  46   // Some Thunks must be placed immediately before their Target as they elide
  47   // a branch and fall through to the first Symbol in the Target.
  48   virtual InputSection *getTargetInputSection() const { return nullptr; }
  49
  50   // To reuse a Thunk the InputSection and the relocation must be compatible
  51   // with it.
  52   virtual bool isCompatibleWith(const InputSection &,
  53                                 const Relocation &) const {
  54     return true;
  55   }
  56
  57   Defined *getThunkTargetSym() const { return syms[0]; }
  58
  59   Symbol &destination;
  60   int64_t addend;
  61   llvm::SmallVector<Defined *, 3> syms;
  62   uint64_t offset = 0;
  63   // The alignment requirement for this Thunk, defaults to the size of the
  64   // typical code section alignment.
  65   uint32_t alignment = 4;
  66 };
  67
  68 // For a Relocation to symbol S create a Thunk to be added to a synthetic
  69 // ThunkSection.
  70 Thunk *addThunk(const InputSection &isec, Relocation &rel);
  71
  72 void writePPC32PltCallStub(uint8_t *buf, uint64_t gotPltVA,
  73                            const InputFile *file, int64_t addend);
  74 void writePPC64LoadAndBranch(uint8_t *buf, int64_t offset);
  75
  76 } // namespace lld::elf
  77
  78 #endif