lld/ELF/Thunks.h

   1 //===- Thunks.h --------------------------------------------------------===//
   2 //
   3 //                             The LLVM Linker
   4 //
   5 // This file is distributed under the University of Illinois Open Source
   6 // License. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9
  10 #ifndef LLD_ELF_THUNKS_H
  11 #define LLD_ELF_THUNKS_H
  12
  13 #include "Relocations.h"
  14
  15 namespace lld {
  16 namespace elf {
  17 class SymbolBody;
  18 class ThunkSection;
  19 // Class to describe an instance of a Thunk.
  20 // A Thunk is a code-sequence inserted by the linker in between a caller and
  21 // the callee. The relocation to the callee is redirected to the Thunk, which
  22 // after executing transfers control to the callee. Typical uses of Thunks
  23 // include transferring control from non-pi to pi and changing state on
  24 // targets like ARM.
  25 //
  26 // Thunks can be created for DefinedRegular, Shared and Undefined Symbols.
  27 // Thunks are assigned to synthetic ThunkSections
  28 class Thunk {
  29 public:
  30   Thunk(const SymbolBody &Destination);
  31   virtual ~Thunk();
  32
  33   virtual uint32_t size() const { return 0; }
  34   virtual void writeTo(uint8_t *Buf, ThunkSection &IS) const {}
  35
  36   // All Thunks must define at least one symbol ThunkSym so that we can
  37   // redirect relocations to it.
  38   virtual void addSymbols(ThunkSection &IS) {}
  39
  40   // Some Thunks must be placed immediately before their Target as they elide
  41   // a branch and fall through to the first Symbol in the Target.
  42   virtual InputSection *getTargetInputSection() const { return nullptr; }
  43
  44   // To reuse a Thunk the caller as identified by the RelocType must be
  45   // compatible with it.
  46   virtual bool isCompatibleWith(uint32_t RelocType) const { return true; }
  47
  48   // The alignment requirement for this Thunk, defaults to the size of the
  49   // typical code section alignment.
  50   const SymbolBody &Destination;
  51   SymbolBody *ThunkSym;
  52   uint64_t Offset;
  53   uint32_t Alignment = 4;
  54 };
  55
  56 // For a Relocation to symbol S create a Thunk to be added to a synthetic
  57 // ThunkSection. At present there are implementations for ARM and Mips Thunks.
  58 Thunk *addThunk(uint32_t RelocType, SymbolBody &S);
  59
  60 } // namespace elf
  61 } // namespace lld
  62
  63 #endif