1//===- llvm/MC/MCAsmBackend.h - MC Asm Backend ------------------*- C++ -*-===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8
9#ifndef LLVM_MC_MCASMBACKEND_H
10#define LLVM_MC_MCASMBACKEND_H
11
12#include "llvm/ADT/ArrayRef.h"
13#include "llvm/ADT/Optional.h"
14#include "llvm/MC/MCDirectives.h"
15#include "llvm/MC/MCFixup.h"
16#include "llvm/MC/MCFragment.h"
17#include "llvm/Support/Endian.h"
18#include <cstdint>
19
20namespace llvm {
21
22class MCAsmLayout;
23class MCAssembler;
24class MCCFIInstruction;
25struct MCFixupKindInfo;
26class MCInst;
27class MCObjectStreamer;
28class MCObjectTargetWriter;
29class MCObjectWriter;
30class MCSubtargetInfo;
31class MCValue;
32class raw_pwrite_stream;
33class StringRef;
34
35/// Generic interface to target specific assembler backends.
36class MCAsmBackend {
37protected: // Can only create subclasses.
38  MCAsmBackend(support::endianness Endian);
39
40public:
41  MCAsmBackend(const MCAsmBackend &) = delete;
42  MCAsmBackend &operator=(const MCAsmBackend &) = delete;
43  virtual ~MCAsmBackend();
44
45  const support::endianness Endian;
46
47  /// Return true if this target might automatically pad instructions and thus
48  /// need to emit padding enable/disable directives around sensative code.
49  virtual bool allowAutoPadding() const { return false; }
50  /// Return true if this target allows an unrelaxable instruction to be
51  /// emitted into RelaxableFragment and then we can increase its size in a
52  /// tricky way for optimization.
53  virtual bool allowEnhancedRelaxation() const { return false; }
54
55  /// Give the target a chance to manipulate state related to instruction
56  /// alignment (e.g. padding for optimization), instruction relaxablility, etc.
57  /// before and after actually emitting the instruction.
58  virtual void emitInstructionBegin(MCObjectStreamer &OS, const MCInst &Inst) {}
59  virtual void emitInstructionEnd(MCObjectStreamer &OS, const MCInst &Inst) {}
60
61  /// lifetime management
62  virtual void reset() {}
63
64  /// Create a new MCObjectWriter instance for use by the assembler backend to
65  /// emit the final object file.
66  std::unique_ptr<MCObjectWriter>
67  createObjectWriter(raw_pwrite_stream &OS) const;
68
69  /// Create an MCObjectWriter that writes two object files: a .o file which is
70  /// linked into the final program and a .dwo file which is used by debuggers.
71  /// This function is only supported with ELF targets.
72  std::unique_ptr<MCObjectWriter>
73  createDwoObjectWriter(raw_pwrite_stream &OS, raw_pwrite_stream &DwoOS) const;
74
75  virtual std::unique_ptr<MCObjectTargetWriter>
76  createObjectTargetWriter() const = 0;
77
78  /// \name Target Fixup Interfaces
79  /// @{
80
81  /// Get the number of target specific fixup kinds.
82  virtual unsigned getNumFixupKinds() const = 0;
83
84  /// Map a relocation name used in .reloc to a fixup kind.
85  virtual Optional<MCFixupKind> getFixupKind(StringRef Name) const;
86
87  /// Get information on a fixup kind.
88  virtual const MCFixupKindInfo &getFixupKindInfo(MCFixupKind Kind) const;
89
90  /// Hook to check if a relocation is needed for some target specific reason.
91  virtual bool shouldForceRelocation(const MCAssembler &Asm,
92                                     const MCFixup &Fixup,
93                                     const MCValue &Target) {
94    return false;
95  }
96
97  /// Hook to check if extra nop bytes must be inserted for alignment directive.
98  /// For some targets this may be necessary in order to support linker
99  /// relaxation. The number of bytes to insert are returned in Size.
100  virtual bool shouldInsertExtraNopBytesForCodeAlign(const MCAlignFragment &AF,
101                                                     unsigned &Size) {
102    return false;
103  }
104
105  /// Hook which indicates if the target requires a fixup to be generated when
106  /// handling an align directive in an executable section
107  virtual bool shouldInsertFixupForCodeAlign(MCAssembler &Asm,
108                                             const MCAsmLayout &Layout,
109                                             MCAlignFragment &AF) {
110    return false;
111  }
112
113  virtual bool evaluateTargetFixup(const MCAssembler &Asm,
114                                   const MCAsmLayout &Layout,
115                                   const MCFixup &Fixup, const MCFragment *DF,
116                                   const MCValue &Target, uint64_t &Value,
117                                   bool &WasForced) {
118    llvm_unreachable("Need to implement hook if target has custom fixups");
119  }
120
121  /// Apply the \p Value for given \p Fixup into the provided data fragment, at
122  /// the offset specified by the fixup and following the fixup kind as
123  /// appropriate. Errors (such as an out of range fixup value) should be
124  /// reported via \p Ctx.
125  /// The  \p STI is present only for fragments of type MCRelaxableFragment and
126  /// MCDataFragment with hasInstructions() == true.
127  virtual void applyFixup(const MCAssembler &Asm, const MCFixup &Fixup,
128                          const MCValue &Target, MutableArrayRef<char> Data,
129                          uint64_t Value, bool IsResolved,
130                          const MCSubtargetInfo *STI) const = 0;
131
132  /// Check whether the given target requires emitting differences of two
133  /// symbols as a set of relocations.
134  virtual bool requiresDiffExpressionRelocations() const { return false; }
135
136  /// @}
137
138  /// \name Target Relaxation Interfaces
139  /// @{
140
141  /// Check whether the given instruction may need relaxation.
142  ///
143  /// \param Inst - The instruction to test.
144  /// \param STI - The MCSubtargetInfo in effect when the instruction was
145  /// encoded.
146  virtual bool mayNeedRelaxation(const MCInst &Inst,
147                                 const MCSubtargetInfo &STI) const {
148    return false;
149  }
150
151  /// Target specific predicate for whether a given fixup requires the
152  /// associated instruction to be relaxed.
153  virtual bool fixupNeedsRelaxationAdvanced(const MCFixup &Fixup, bool Resolved,
154                                            uint64_t Value,
155                                            const MCRelaxableFragment *DF,
156                                            const MCAsmLayout &Layout,
157                                            const bool WasForced) const;
158
159  /// Simple predicate for targets where !Resolved implies requiring relaxation
160  virtual bool fixupNeedsRelaxation(const MCFixup &Fixup, uint64_t Value,
161                                    const MCRelaxableFragment *DF,
162                                    const MCAsmLayout &Layout) const = 0;
163
164  /// Relax the instruction in the given fragment to the next wider instruction.
165  ///
166  /// \param [out] Inst The instruction to relax, which is also the relaxed
167  /// instruction.
168  /// \param STI the subtarget information for the associated instruction.
169  virtual void relaxInstruction(MCInst &Inst,
170                                const MCSubtargetInfo &STI) const {};
171
172  /// @}
173
174  /// Returns the minimum size of a nop in bytes on this target. The assembler
175  /// will use this to emit excess padding in situations where the padding
176  /// required for simple alignment would be less than the minimum nop size.
177  ///
178  virtual unsigned getMinimumNopSize() const { return 1; }
179
180  /// Returns the maximum size of a nop in bytes on this target.
181  ///
182  virtual unsigned getMaximumNopSize() const { return 0; }
183
184  /// Write an (optimal) nop sequence of Count bytes to the given output. If the
185  /// target cannot generate such a sequence, it should return an error.
186  ///
187  /// \return - True on success.
188  virtual bool writeNopData(raw_ostream &OS, uint64_t Count) const = 0;
189
190  /// Give backend an opportunity to finish layout after relaxation
191  virtual void finishLayout(MCAssembler const &Asm,
192                            MCAsmLayout &Layout) const {}
193
194  /// Handle any target-specific assembler flags. By default, do nothing.
195  virtual void handleAssemblerFlag(MCAssemblerFlag Flag) {}
196
197  /// Generate the compact unwind encoding for the CFI instructions.
198  virtual uint32_t
199      generateCompactUnwindEncoding(ArrayRef<MCCFIInstruction>) const {
200    return 0;
201  }
202
203  /// Check whether a given symbol has been flagged with MICROMIPS flag.
204  virtual bool isMicroMips(const MCSymbol *Sym) const {
205    return false;
206  }
207};
208
209} // end namespace llvm
210
211#endif // LLVM_MC_MCASMBACKEND_H
212