MCFixup.h revision 84bfc2f090639f933df06cc675c4385511516bef 
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  FK_PCRel_8,    ///< A eight-byte pc relative fixup.
29  FK_GPRel_1,    ///< A one-byte gp relative fixup.
30  FK_GPRel_2,    ///< A two-byte gp relative fixup.
31  FK_GPRel_4,    ///< A four-byte gp relative fixup.
32  FK_GPRel_8,    ///< A eight-byte gp relative fixup.
33
34  FirstTargetFixupKind = 128,
35
36  // Limit range of target fixups, in case we want to pack more efficiently
37  // later.
38  MaxTargetFixupKind = (1 << 8)
39};
40
41/// MCFixup - Encode information on a single operation to perform on a byte
42/// sequence (e.g., an encoded instruction) which requires assemble- or run-
43/// time patching.
44///
45/// Fixups are used any time the target instruction encoder needs to represent
46/// some value in an instruction which is not yet concrete. The encoder will
47/// encode the instruction assuming the value is 0, and emit a fixup which
48/// communicates to the assembler backend how it should rewrite the encoded
49/// value.
50///
51/// During the process of relaxation, the assembler will apply fixups as
52/// symbolic values become concrete. When relaxation is complete, any remaining
53/// fixups become relocations in the object file (or errors, if the fixup cannot
54/// be encoded on the target).
55class MCFixup {
56  /// The value to put into the fixup location. The exact interpretation of the
57  /// expression is target dependent, usually it will be one of the operands to
58  /// an instruction or an assembler directive.
59  const MCExpr *Value;
60
61  /// The byte index of start of the relocation inside the encoded instruction.
62  uint32_t Offset;
63
64  /// The target dependent kind of fixup item this is. The kind is used to
65  /// determine how the operand value should be encoded into the instruction.
66  unsigned Kind;
67
68public:
69  static MCFixup Create(uint32_t Offset, const MCExpr *Value,
70                        MCFixupKind Kind) {
71    assert(unsigned(Kind) < MaxTargetFixupKind && "Kind out of range!");
72    MCFixup FI;
73    FI.Value = Value;
74    FI.Offset = Offset;
75    FI.Kind = unsigned(Kind);
76    return FI;
77  }
78
79  MCFixupKind getKind() const { return MCFixupKind(Kind); }
80
81  uint32_t getOffset() const { return Offset; }
82  void setOffset(uint32_t Value) { Offset = Value; }
83
84  const MCExpr *getValue() const { return Value; }
85
86  /// getKindForSize - Return the generic fixup kind for a value with the given
87  /// size. It is an error to pass an unsupported size.
88  static MCFixupKind getKindForSize(unsigned Size, bool isPCRel) {
89    switch (Size) {
90    default: assert(0 && "Invalid generic fixup size!");
91    case 1: return isPCRel ? FK_PCRel_1 : FK_Data_1;
92    case 2: return isPCRel ? FK_PCRel_2 : FK_Data_2;
93    case 4: return isPCRel ? FK_PCRel_4 : FK_Data_4;
94    case 8: return isPCRel ? FK_PCRel_8 : FK_Data_8;
95    }
96  }
97};
98
99} // End llvm namespace
100
101#endif
102