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//===- Thunks.h --------------------------------------------------------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
#include "Relocations.h"
namespace lld {
namespace elf {
class Defined;
class Symbol;
class ThunkSection;
// Class to describe an instance of a Thunk.
// A Thunk is a code-sequence inserted by the linker in between a caller and
// the callee. The relocation to the callee is redirected to the Thunk, which
// after executing transfers control to the callee. Typical uses of Thunks
// include transferring control from non-pi to pi and changing state on
// targets like ARM.
// Thunks can be created for Defined, Shared and Undefined Symbols.
// Thunks are assigned to synthetic ThunkSections
class Thunk {
Thunk(Symbol &destination);
virtual ~Thunk();
virtual uint32_t size() = 0;
virtual void writeTo(uint8_t *buf) = 0;
// All Thunks must define at least one symbol, known as the thunk target
// symbol, so that we can redirect relocations to it. The thunk may define
// additional symbols, but these are never targets for relocations.
virtual void addSymbols(ThunkSection &isec) = 0;
void setOffset(uint64_t offset);
Defined *addSymbol(StringRef name, uint8_t type, uint64_t value,
InputSectionBase &section);
// Some Thunks must be placed immediately before their Target as they elide
// a branch and fall through to the first Symbol in the Target.
virtual InputSection *getTargetInputSection() const { return nullptr; }
// To reuse a Thunk the InputSection and the relocation must be compatible
// with it.
virtual bool isCompatibleWith(const InputSection &,
const Relocation &) const {
return true;
Defined *getThunkTargetSym() const { return syms[0]; }
// The alignment requirement for this Thunk, defaults to the size of the
// typical code section alignment.
Symbol &destination;
llvm::SmallVector<Defined *, 3> syms;
uint64_t offset = 0;
uint32_t alignment = 4;
// For a Relocation to symbol S create a Thunk to be added to a synthetic
// ThunkSection.
Thunk *addThunk(const InputSection &isec, Relocation &rel);
} // namespace elf
} // namespace lld