MachO/SectionPriorities.h

//===- SectionPriorities.h --------------------------------------*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#ifndef LLD_MACHO_SECTION_PRIORITIES_H
#define LLD_MACHO_SECTION_PRIORITIES_H

#include "InputSection.h"
#include "llvm/ADT/DenseMap.h"

namespace lld::macho {

using SectionPair = std::pair<const InputSection *, const InputSection *>;

class PriorityBuilder {
public:
  // Reads every input section's call graph profile, and combines them into
  // callGraphProfile. If an order file is present, any edges where one or both
  // of the vertices are specified in the order file are discarded.
  void extractCallGraphProfile();

  // Reads the order file at `path` into config->priorities.
  //
  // An order file has one entry per line, in the following format:
  //
  //   <cpu>:<object file>:<symbol name>
  //
  // <cpu> and <object file> are optional. If not specified, then that entry
  // matches any symbol of that name. Parsing this format is not quite
  // straightforward because the symbol name itself can contain colons, so when
  // encountering a colon, we consider the preceding characters to decide if it
  // can be a valid CPU type or file path.
  //
  // If a symbol is matched by multiple entries, then it takes the
  // lowest-ordered entry (the one nearest to the front of the list.)
  //
  // The file can also have line comments that start with '#'.
  void parseOrderFile(StringRef path);

  // Returns layout priorities for some or all input sections. Sections are laid
  // out in decreasing order; that is, a higher priority section will be closer
  // to the beginning of its output section.
  //
  // If either an order file or a call graph profile are present, this is used
  // as the source of priorities. If both are present, the order file takes
  // precedence, but the call graph profile is still used for symbols that don't
  // appear in the order file. If neither is present, an empty map is returned.
  //
  // Each section gets assigned the priority of the highest-priority symbol it
  // contains.
  llvm::DenseMap<const InputSection *, size_t> buildInputSectionPriorities();

private:
  // The symbol with the highest priority should be ordered first in the output
  // section (modulo input section contiguity constraints). Using priority
  // (highest first) instead of order (lowest first) has the convenient property
  // that the default-constructed zero priority -- for symbols/sections without
  // a user-defined order -- naturally ends up putting them at the end of the
  // output.
  struct SymbolPriorityEntry {
    // The priority given to a matching symbol, regardless of which object file
    // it originated from.
    size_t anyObjectFile = 0;
    // The priority given to a matching symbol from a particular object file.
    llvm::DenseMap<llvm::StringRef, size_t> objectFiles;
  };

  std::optional<size_t> getSymbolPriority(const Defined *sym);
  llvm::DenseMap<llvm::StringRef, SymbolPriorityEntry> priorities;
  llvm::MapVector<SectionPair, uint64_t> callGraphProfile;
};

extern PriorityBuilder priorityBuilder;
} // namespace lld::macho

#endif