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1 //===-- llvm/MC/MCAsmBack.h - MC Asm Backend --------------------*- 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_MCASMBACKEND_H
11 #define LLVM_MC_MCASMBACKEND_H
12 
13 #include "llvm/MC/MCDirectives.h"
14 #include "llvm/MC/MCFixup.h"
15 #include "llvm/Support/DataTypes.h"
16 #include "llvm/Support/ErrorHandling.h"
17 
18 namespace llvm {
19 class MCAsmLayout;
20 class MCAssembler;
21 class MCELFObjectTargetWriter;
22 struct MCFixupKindInfo;
23 class MCFragment;
24 class MCInst;
25 class MCRelaxableFragment;
26 class MCObjectWriter;
27 class MCSection;
28 class MCValue;
29 class raw_ostream;
30 
31 /// MCAsmBackend - Generic interface to target specific assembler backends.
32 class MCAsmBackend {
33   MCAsmBackend(const MCAsmBackend &) LLVM_DELETED_FUNCTION;
34   void operator=(const MCAsmBackend &) LLVM_DELETED_FUNCTION;
35 protected: // Can only create subclasses.
36   MCAsmBackend();
37 
38   unsigned HasReliableSymbolDifference : 1;
39   unsigned HasDataInCodeSupport : 1;
40 
41 public:
42   virtual ~MCAsmBackend();
43 
44   /// lifetime management
reset()45   virtual void reset() { }
46 
47   /// createObjectWriter - Create a new MCObjectWriter instance for use by the
48   /// assembler backend to emit the final object file.
49   virtual MCObjectWriter *createObjectWriter(raw_ostream &OS) const = 0;
50 
51   /// createELFObjectTargetWriter - Create a new ELFObjectTargetWriter to enable
52   /// non-standard ELFObjectWriters.
createELFObjectTargetWriter()53   virtual  MCELFObjectTargetWriter *createELFObjectTargetWriter() const {
54     llvm_unreachable("createELFObjectTargetWriter is not supported by asm "
55                      "backend");
56   }
57 
58   /// hasReliableSymbolDifference - Check whether this target implements
59   /// accurate relocations for differences between symbols. If not, differences
60   /// between symbols will always be relocatable expressions and any references
61   /// to temporary symbols will be assumed to be in the same atom, unless they
62   /// reside in a different section.
63   ///
64   /// This should always be true (since it results in fewer relocations with no
65   /// loss of functionality), but is currently supported as a way to maintain
66   /// exact object compatibility with Darwin 'as' (on non-x86_64). It should
67   /// eventually should be eliminated.
hasReliableSymbolDifference()68   bool hasReliableSymbolDifference() const {
69     return HasReliableSymbolDifference;
70   }
71 
72   /// hasDataInCodeSupport - Check whether this target implements data-in-code
73   /// markers. If not, data region directives will be ignored.
hasDataInCodeSupport()74   bool hasDataInCodeSupport() const {
75     return HasDataInCodeSupport;
76   }
77 
78   /// doesSectionRequireSymbols - Check whether the given section requires that
79   /// all symbols (even temporaries) have symbol table entries.
doesSectionRequireSymbols(const MCSection & Section)80   virtual bool doesSectionRequireSymbols(const MCSection &Section) const {
81     return false;
82   }
83 
84   /// isSectionAtomizable - Check whether the given section can be split into
85   /// atoms.
86   ///
87   /// \see MCAssembler::isSymbolLinkerVisible().
isSectionAtomizable(const MCSection & Section)88   virtual bool isSectionAtomizable(const MCSection &Section) const {
89     return true;
90   }
91 
92   /// @name Target Fixup Interfaces
93   /// @{
94 
95   /// getNumFixupKinds - Get the number of target specific fixup kinds.
96   virtual unsigned getNumFixupKinds() const = 0;
97 
98   /// getFixupKindInfo - Get information on a fixup kind.
99   virtual const MCFixupKindInfo &getFixupKindInfo(MCFixupKind Kind) const;
100 
101   /// processFixupValue - Target hook to adjust the literal value of a fixup
102   /// if necessary. IsResolved signals whether the caller believes a relocation
103   /// is needed; the target can modify the value. The default does nothing.
processFixupValue(const MCAssembler & Asm,const MCAsmLayout & Layout,const MCFixup & Fixup,const MCFragment * DF,MCValue & Target,uint64_t & Value,bool & IsResolved)104   virtual void processFixupValue(const MCAssembler &Asm,
105                                  const MCAsmLayout &Layout,
106                                  const MCFixup &Fixup, const MCFragment *DF,
107                                  MCValue &Target, uint64_t &Value,
108                                  bool &IsResolved) {}
109 
110   /// @}
111 
112   /// applyFixup - Apply the \p Value for given \p Fixup into the provided
113   /// data fragment, at the offset specified by the fixup and following the
114   /// fixup kind as appropriate.
115   virtual void applyFixup(const MCFixup &Fixup, char *Data, unsigned DataSize,
116                           uint64_t Value) const = 0;
117 
118   /// @}
119 
120   /// @name Target Relaxation Interfaces
121   /// @{
122 
123   /// mayNeedRelaxation - Check whether the given instruction may need
124   /// relaxation.
125   ///
126   /// \param Inst - The instruction to test.
127   virtual bool mayNeedRelaxation(const MCInst &Inst) const = 0;
128 
129   /// fixupNeedsRelaxation - Target specific predicate for whether a given
130   /// fixup requires the associated instruction to be relaxed.
131   virtual bool fixupNeedsRelaxation(const MCFixup &Fixup,
132                                     uint64_t Value,
133                                     const MCRelaxableFragment *DF,
134                                     const MCAsmLayout &Layout) const = 0;
135 
136   /// RelaxInstruction - Relax the instruction in the given fragment to the next
137   /// wider instruction.
138   ///
139   /// \param Inst The instruction to relax, which may be the same as the
140   /// output.
141   /// \param [out] Res On return, the relaxed instruction.
142   virtual void relaxInstruction(const MCInst &Inst, MCInst &Res) const = 0;
143 
144   /// @}
145 
146   /// getMinimumNopSize - Returns the minimum size of a nop in bytes on this
147   /// target. The assembler will use this to emit excess padding in situations
148   /// where the padding required for simple alignment would be less than the
149   /// minimum nop size.
150   ///
getMinimumNopSize()151   virtual unsigned getMinimumNopSize() const { return 1; }
152 
153   /// writeNopData - Write an (optimal) nop sequence of Count bytes to the given
154   /// output. If the target cannot generate such a sequence, it should return an
155   /// error.
156   ///
157   /// \return - True on success.
158   virtual bool writeNopData(uint64_t Count, MCObjectWriter *OW) const = 0;
159 
160   /// handleAssemblerFlag - Handle any target-specific assembler flags.
161   /// By default, do nothing.
handleAssemblerFlag(MCAssemblerFlag Flag)162   virtual void handleAssemblerFlag(MCAssemblerFlag Flag) {}
163 };
164 
165 } // End llvm namespace
166 
167 #endif
168