MCFixup.h revision 1f6efa3996dd1929fbc129203ce5009b620e6969 
1//===-- llvm/MC/MCFixup.h - Instruction Relocation and Patching -*- C++ -*-===//
2//
3//                     The LLVM Compiler Infrastructure
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 LLVM_MC_MCFIXUP_H
11#define LLVM_MC_MCFIXUP_H
12
13#include "llvm/Support/DataTypes.h"
14#include <cassert>
15
16namespace llvm {
17class MCExpr;
18
19/// MCFixupKind - Extensible enumeration to represent the type of a fixup.
20enum MCFixupKind {
21  FK_Data_1 = 0, ///< A one-byte fixup.
22  FK_Data_2,     ///< A two-byte fixup.
23  FK_Data_4,     ///< A four-byte fixup.
24  FK_Data_8,     ///< A eight-byte fixup.
25  FK_PCRel_1,    ///< A one-byte pc relative fixup.
26  FK_PCRel_2,    ///< A two-byte pc relative fixup.
27  FK_PCRel_4,    ///< A four-byte pc relative fixup.
28
29  FirstTargetFixupKind = 128,
30
31  // Limit range of target fixups, in case we want to pack more efficiently
32  // later.
33  MaxTargetFixupKind = (1 << 8)
34};
35
36/// MCFixup - Encode information on a single operation to perform on a byte
37/// sequence (e.g., an encoded instruction) which requires assemble- or run-
38/// time patching.
39///
40/// Fixups are used any time the target instruction encoder needs to represent
41/// some value in an instruction which is not yet concrete. The encoder will
42/// encode the instruction assuming the value is 0, and emit a fixup which
43/// communicates to the assembler backend how it should rewrite the encoded
44/// value.
45///
46/// During the process of relaxation, the assembler will apply fixups as
47/// symbolic values become concrete. When relaxation is complete, any remaining
48/// fixups become relocations in the object file (or errors, if the fixup cannot
49/// be encoded on the target).
50class MCFixup {
51  /// The value to put into the fixup location. The exact interpretation of the
52  /// expression is target dependent, usually it will be one of the operands to
53  /// an instruction or an assembler directive.
54  const MCExpr *Value;
55
56  /// The byte index of start of the relocation inside the encoded instruction.
57  uint32_t Offset;
58
59  /// The target dependent kind of fixup item this is. The kind is used to
60  /// determine how the operand value should be encoded into the instruction.
61  unsigned Kind;
62
63public:
64  static MCFixup Create(uint32_t Offset, const MCExpr *Value,
65                        MCFixupKind Kind) {
66    assert(unsigned(Kind) < MaxTargetFixupKind && "Kind out of range!");
67    MCFixup FI;
68    FI.Value = Value;
69    FI.Offset = Offset;
70    FI.Kind = unsigned(Kind);
71    return FI;
72  }
73
74  MCFixupKind getKind() const { return MCFixupKind(Kind); }
75
76  uint32_t getOffset() const { return Offset; }
77  void setOffset(uint32_t Value) { Offset = Value; }
78
79  const MCExpr *getValue() const { return Value; }
80
81  /// getKindForSize - Return the generic fixup kind for a value with the given
82  /// size. It is an error to pass an unsupported size.
83  static MCFixupKind getKindForSize(unsigned Size, bool isPCRel) {
84    switch (Size) {
85    default: assert(0 && "Invalid generic fixup size!");
86    case 1: return isPCRel ? FK_PCRel_1 : FK_Data_1;
87    case 2: return isPCRel ? FK_PCRel_2 : FK_Data_2;
88    case 4: return isPCRel ? FK_PCRel_4 : FK_Data_4;
89    case 8:
90      assert(!isPCRel && "8 byte pc relative fixup is not supported.");
91      return FK_Data_8;
92    }
93  }
94};
95
96} // End llvm namespace
97
98#endif
99