lld/MachO/SectionPriorities.h

   1 //===- SectionPriorities.h --------------------------------------*- C++ -*-===//
   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_MACHO_SECTION_PRIORITIES_H
  10 #define LLD_MACHO_SECTION_PRIORITIES_H
  11
  12 #include "InputSection.h"
  13 #include "llvm/ADT/DenseMap.h"
  14
  15 namespace lld::macho {
  16
  17 using SectionPair = std::pair<const InputSection *, const InputSection *>;
  18
  19 class PriorityBuilder {
  20 public:
  21   // Reads every input section's call graph profile, and combines them into
  22   // callGraphProfile. If an order file is present, any edges where one or both
  23   // of the vertices are specified in the order file are discarded.
  24   void extractCallGraphProfile();
  25
  26   // Reads the order file at `path` into config->priorities.
  27   //
  28   // An order file has one entry per line, in the following format:
  29   //
  30   //   <cpu>:<object file>:<symbol name>
  31   //
  32   // <cpu> and <object file> are optional. If not specified, then that entry
  33   // matches any symbol of that name. Parsing this format is not quite
  34   // straightforward because the symbol name itself can contain colons, so when
  35   // encountering a colon, we consider the preceding characters to decide if it
  36   // can be a valid CPU type or file path.
  37   //
  38   // If a symbol is matched by multiple entries, then it takes the
  39   // lowest-ordered entry (the one nearest to the front of the list.)
  40   //
  41   // The file can also have line comments that start with '#'.
  42   void parseOrderFile(StringRef path);
  43
  44   // Returns layout priorities for some or all input sections. Sections are laid
  45   // out in decreasing order; that is, a higher priority section will be closer
  46   // to the beginning of its output section.
  47   //
  48   // If either an order file or a call graph profile are present, this is used
  49   // as the source of priorities. If both are present, the order file takes
  50   // precedence, but the call graph profile is still used for symbols that don't
  51   // appear in the order file. If neither is present, an empty map is returned.
  52   //
  53   // Each section gets assigned the priority of the highest-priority symbol it
  54   // contains.
  55   llvm::DenseMap<const InputSection *, size_t> buildInputSectionPriorities();
  56
  57 private:
  58   // The symbol with the highest priority should be ordered first in the output
  59   // section (modulo input section contiguity constraints). Using priority
  60   // (highest first) instead of order (lowest first) has the convenient property
  61   // that the default-constructed zero priority -- for symbols/sections without
  62   // a user-defined order -- naturally ends up putting them at the end of the
  63   // output.
  64   struct SymbolPriorityEntry {
  65     // The priority given to a matching symbol, regardless of which object file
  66     // it originated from.
  67     size_t anyObjectFile = 0;
  68     // The priority given to a matching symbol from a particular object file.
  69     llvm::DenseMap<llvm::StringRef, size_t> objectFiles;
  70   };
  71
  72   llvm::Optional<size_t> getSymbolPriority(const Defined *sym);
  73   llvm::DenseMap<llvm::StringRef, SymbolPriorityEntry> priorities;
  74   llvm::MapVector<SectionPair, uint64_t> callGraphProfile;
  75 };
  76
  77 extern PriorityBuilder priorityBuilder;
  78 } // namespace lld::macho
  79
  80 #endif