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 {
16 namespace macho {
17 
18 using SectionPair = std::pair<const InputSection *, const InputSection *>;
19 
20 class PriorityBuilder {
21 public:
22   // Reads every input section's call graph profile, and combines them into
23   // callGraphProfile. If an order file is present, any edges where one or both
24   // of the vertices are specified in the order file are discarded.
25   void extractCallGraphProfile();
26 
27   // Reads the order file at `path` into config->priorities.
28   //
29   // An order file has one entry per line, in the following format:
30   //
31   //   <cpu>:<object file>:<symbol name>
32   //
33   // <cpu> and <object file> are optional. If not specified, then that entry
34   // matches any symbol of that name. Parsing this format is not quite
35   // straightforward because the symbol name itself can contain colons, so when
36   // encountering a colon, we consider the preceding characters to decide if it
37   // can be a valid CPU type or file path.
38   //
39   // If a symbol is matched by multiple entries, then it takes the
40   // lowest-ordered entry (the one nearest to the front of the list.)
41   //
42   // The file can also have line comments that start with '#'.
43   void parseOrderFile(StringRef path);
44 
45   // Returns layout priorities for some or all input sections. Sections are laid
46   // out in decreasing order; that is, a higher priority section will be closer
47   // to the beginning of its output section.
48   //
49   // If either an order file or a call graph profile are present, this is used
50   // as the source of priorities. If both are present, the order file takes
51   // precedence, but the call graph profile is still used for symbols that don't
52   // appear in the order file. If neither is present, an empty map is returned.
53   //
54   // Each section gets assigned the priority of the highest-priority symbol it
55   // contains.
56   llvm::DenseMap<const InputSection *, size_t> buildInputSectionPriorities();
57 
58 private:
59   // The symbol with the highest priority should be ordered first in the output
60   // section (modulo input section contiguity constraints). Using priority
61   // (highest first) instead of order (lowest first) has the convenient property
62   // that the default-constructed zero priority -- for symbols/sections without
63   // a user-defined order -- naturally ends up putting them at the end of the
64   // output.
65   struct SymbolPriorityEntry {
66     // The priority given to a matching symbol, regardless of which object file
67     // it originated from.
68     size_t anyObjectFile = 0;
69     // The priority given to a matching symbol from a particular object file.
70     llvm::DenseMap<llvm::StringRef, size_t> objectFiles;
71   };
72 
73   llvm::Optional<size_t> getSymbolPriority(const Defined *sym);
74   llvm::DenseMap<llvm::StringRef, SymbolPriorityEntry> priorities;
75   llvm::MapVector<SectionPair, uint64_t> callGraphProfile;
76 };
77 
78 extern PriorityBuilder priorityBuilder;
79 } // namespace macho
80 } // namespace lld
81 
82 #endif
83