1 //===-- llvm/MC/MCAsmBackend.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/ADT/ArrayRef.h"
14 #include "llvm/ADT/Optional.h"
15 #include "llvm/MC/MCDirectives.h"
16 #include "llvm/MC/MCDwarf.h"
17 #include "llvm/MC/MCFixup.h"
18 #include "llvm/Support/DataTypes.h"
19 #include "llvm/Support/ErrorHandling.h"
20 
21 namespace llvm {
22 class MCAsmLayout;
23 class MCAssembler;
24 class MCELFObjectTargetWriter;
25 struct MCFixupKindInfo;
26 class MCFragment;
27 class MCInst;
28 class MCRelaxableFragment;
29 class MCObjectWriter;
30 class MCSection;
31 class MCValue;
32 class raw_pwrite_stream;
33 
34 /// Generic interface to target specific assembler backends.
35 class MCAsmBackend {
36   MCAsmBackend(const MCAsmBackend &) = delete;
37   void operator=(const MCAsmBackend &) = delete;
38 
39 protected: // Can only create subclasses.
40   MCAsmBackend();
41 
42 public:
43   virtual ~MCAsmBackend();
44 
45   /// lifetime management
reset()46   virtual void reset() {}
47 
48   /// Create a new MCObjectWriter instance for use by the assembler backend to
49   /// emit the final object file.
50   virtual MCObjectWriter *createObjectWriter(raw_pwrite_stream &OS) const = 0;
51 
52   /// \name Target Fixup Interfaces
53   /// @{
54 
55   /// Get the number of target specific fixup kinds.
56   virtual unsigned getNumFixupKinds() const = 0;
57 
58   /// Map a relocation name used in .reloc to a fixup kind.
59   virtual Optional<MCFixupKind> getFixupKind(StringRef Name) const;
60 
61   /// Get information on a fixup kind.
62   virtual const MCFixupKindInfo &getFixupKindInfo(MCFixupKind Kind) const;
63 
64   /// Target hook to adjust the literal value of a fixup if necessary.
65   /// IsResolved signals whether the caller believes a relocation is needed; the
66   /// target can modify the value. The default does nothing.
processFixupValue(const MCAssembler & Asm,const MCAsmLayout & Layout,const MCFixup & Fixup,const MCFragment * DF,const MCValue & Target,uint64_t & Value,bool & IsResolved)67   virtual void processFixupValue(const MCAssembler &Asm,
68                                  const MCAsmLayout &Layout,
69                                  const MCFixup &Fixup, const MCFragment *DF,
70                                  const MCValue &Target, uint64_t &Value,
71                                  bool &IsResolved) {}
72 
73   /// Apply the \p Value for given \p Fixup into the provided data fragment, at
74   /// the offset specified by the fixup and following the fixup kind as
75   /// appropriate.
76   virtual void applyFixup(const MCFixup &Fixup, char *Data, unsigned DataSize,
77                           uint64_t Value, bool IsPCRel) const = 0;
78 
79   /// @}
80 
81   /// \name Target Relaxation Interfaces
82   /// @{
83 
84   /// Check whether the given instruction may need relaxation.
85   ///
86   /// \param Inst - The instruction to test.
87   virtual bool mayNeedRelaxation(const MCInst &Inst) const = 0;
88 
89   /// Target specific predicate for whether a given fixup requires the
90   /// associated instruction to be relaxed.
91   virtual bool fixupNeedsRelaxationAdvanced(const MCFixup &Fixup, bool Resolved,
92                                             uint64_t Value,
93                                             const MCRelaxableFragment *DF,
94                                             const MCAsmLayout &Layout) const;
95 
96   /// Simple predicate for targets where !Resolved implies requiring relaxation
97   virtual bool fixupNeedsRelaxation(const MCFixup &Fixup, uint64_t Value,
98                                     const MCRelaxableFragment *DF,
99                                     const MCAsmLayout &Layout) const = 0;
100 
101   /// Relax the instruction in the given fragment to the next wider instruction.
102   ///
103   /// \param Inst The instruction to relax, which may be the same as the
104   /// output.
105   /// \param STI the subtarget information for the associated instruction.
106   /// \param [out] Res On return, the relaxed instruction.
107   virtual void relaxInstruction(const MCInst &Inst, const MCSubtargetInfo &STI,
108                                 MCInst &Res) const = 0;
109 
110   /// @}
111 
112   /// Returns the minimum size of a nop in bytes on this target. The assembler
113   /// will use this to emit excess padding in situations where the padding
114   /// required for simple alignment would be less than the minimum nop size.
115   ///
getMinimumNopSize()116   virtual unsigned getMinimumNopSize() const { return 1; }
117 
118   /// Write an (optimal) nop sequence of Count bytes to the given output. If the
119   /// target cannot generate such a sequence, it should return an error.
120   ///
121   /// \return - True on success.
122   virtual bool writeNopData(uint64_t Count, MCObjectWriter *OW) const = 0;
123 
124   /// Give backend an opportunity to finish layout after relaxation
finishLayout(MCAssembler const & Asm,MCAsmLayout & Layout)125   virtual void finishLayout(MCAssembler const &Asm,
126                             MCAsmLayout &Layout) const {}
127 
128   /// Handle any target-specific assembler flags. By default, do nothing.
handleAssemblerFlag(MCAssemblerFlag Flag)129   virtual void handleAssemblerFlag(MCAssemblerFlag Flag) {}
130 
131   /// \brief Generate the compact unwind encoding for the CFI instructions.
132   virtual uint32_t
generateCompactUnwindEncoding(ArrayRef<MCCFIInstruction>)133       generateCompactUnwindEncoding(ArrayRef<MCCFIInstruction>) const {
134     return 0;
135   }
136 };
137 
138 } // End llvm namespace
139 
140 #endif
141