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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/MC/MCExpr.h"
14 #include "llvm/Support/DataTypes.h"
15 #include "llvm/Support/ErrorHandling.h"
16 #include "llvm/Support/SMLoc.h"
17 #include <cassert>
18 
19 namespace llvm {
20 class MCExpr;
21 
22 /// \brief Extensible enumeration to represent the type of a fixup.
23 enum MCFixupKind {
24   FK_Data_1 = 0, ///< A one-byte fixup.
25   FK_Data_2,     ///< A two-byte fixup.
26   FK_Data_4,     ///< A four-byte fixup.
27   FK_Data_8,     ///< A eight-byte fixup.
28   FK_PCRel_1,    ///< A one-byte pc relative fixup.
29   FK_PCRel_2,    ///< A two-byte pc relative fixup.
30   FK_PCRel_4,    ///< A four-byte pc relative fixup.
31   FK_PCRel_8,    ///< A eight-byte pc relative fixup.
32   FK_GPRel_1,    ///< A one-byte gp relative fixup.
33   FK_GPRel_2,    ///< A two-byte gp relative fixup.
34   FK_GPRel_4,    ///< A four-byte gp relative fixup.
35   FK_GPRel_8,    ///< A eight-byte gp relative fixup.
36   FK_SecRel_1,   ///< A one-byte section relative fixup.
37   FK_SecRel_2,   ///< A two-byte section relative fixup.
38   FK_SecRel_4,   ///< A four-byte section relative fixup.
39   FK_SecRel_8,   ///< A eight-byte section relative fixup.
40 
41   FirstTargetFixupKind = 128,
42 
43   // Limit range of target fixups, in case we want to pack more efficiently
44   // later.
45   MaxTargetFixupKind = (1 << 8)
46 };
47 
48 /// \brief Encode information on a single operation to perform on a byte
49 /// sequence (e.g., an encoded instruction) which requires assemble- or run-
50 /// time patching.
51 ///
52 /// Fixups are used any time the target instruction encoder needs to represent
53 /// some value in an instruction which is not yet concrete. The encoder will
54 /// encode the instruction assuming the value is 0, and emit a fixup which
55 /// communicates to the assembler backend how it should rewrite the encoded
56 /// value.
57 ///
58 /// During the process of relaxation, the assembler will apply fixups as
59 /// symbolic values become concrete. When relaxation is complete, any remaining
60 /// fixups become relocations in the object file (or errors, if the fixup cannot
61 /// be encoded on the target).
62 class MCFixup {
63   /// The value to put into the fixup location. The exact interpretation of the
64   /// expression is target dependent, usually it will be one of the operands to
65   /// an instruction or an assembler directive.
66   const MCExpr *Value;
67 
68   /// The byte index of start of the relocation inside the encoded instruction.
69   uint32_t Offset;
70 
71   /// The target dependent kind of fixup item this is. The kind is used to
72   /// determine how the operand value should be encoded into the instruction.
73   unsigned Kind;
74 
75   /// The source location which gave rise to the fixup, if any.
76   SMLoc Loc;
77 public:
78   static MCFixup create(uint32_t Offset, const MCExpr *Value,
79                         MCFixupKind Kind, SMLoc Loc = SMLoc()) {
80     assert(unsigned(Kind) < MaxTargetFixupKind && "Kind out of range!");
81     MCFixup FI;
82     FI.Value = Value;
83     FI.Offset = Offset;
84     FI.Kind = unsigned(Kind);
85     FI.Loc = Loc;
86     return FI;
87   }
88 
getKind()89   MCFixupKind getKind() const { return MCFixupKind(Kind); }
90 
getOffset()91   uint32_t getOffset() const { return Offset; }
setOffset(uint32_t Value)92   void setOffset(uint32_t Value) { Offset = Value; }
93 
getValue()94   const MCExpr *getValue() const { return Value; }
95 
96   /// \brief Return the generic fixup kind for a value with the given size. It
97   /// is an error to pass an unsupported size.
getKindForSize(unsigned Size,bool isPCRel)98   static MCFixupKind getKindForSize(unsigned Size, bool isPCRel) {
99     switch (Size) {
100     default: llvm_unreachable("Invalid generic fixup size!");
101     case 1: return isPCRel ? FK_PCRel_1 : FK_Data_1;
102     case 2: return isPCRel ? FK_PCRel_2 : FK_Data_2;
103     case 4: return isPCRel ? FK_PCRel_4 : FK_Data_4;
104     case 8: return isPCRel ? FK_PCRel_8 : FK_Data_8;
105     }
106   }
107 
getLoc()108   SMLoc getLoc() const { return Loc; }
109 };
110 
111 } // End llvm namespace
112 
113 #endif
114