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 20 namespace llvm { 21 22 class MCAsmLayout; 23 class MCAssembler; 24 class MCCFIInstruction; 25 struct MCFixupKindInfo; 26 class MCInst; 27 class MCObjectStreamer; 28 class MCObjectTargetWriter; 29 class MCObjectWriter; 30 class MCSubtargetInfo; 31 class MCValue; 32 class raw_pwrite_stream; 33 class StringRef; 34 35 /// Generic interface to target specific assembler backends. 36 class MCAsmBackend { 37 protected: // Can only create subclasses. 38 MCAsmBackend(support::endianness Endian); 39 40 public: 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 = 0; 148 149 /// Target specific predicate for whether a given fixup requires the 150 /// associated instruction to be relaxed. 151 virtual bool fixupNeedsRelaxationAdvanced(const MCFixup &Fixup, bool Resolved, 152 uint64_t Value, 153 const MCRelaxableFragment *DF, 154 const MCAsmLayout &Layout, 155 const bool WasForced) const; 156 157 /// Simple predicate for targets where !Resolved implies requiring relaxation 158 virtual bool fixupNeedsRelaxation(const MCFixup &Fixup, uint64_t Value, 159 const MCRelaxableFragment *DF, 160 const MCAsmLayout &Layout) const = 0; 161 162 /// Relax the instruction in the given fragment to the next wider instruction. 163 /// 164 /// \param [out] Inst The instruction to relax, which is also the relaxed 165 /// instruction. 166 /// \param STI the subtarget information for the associated instruction. 167 virtual void relaxInstruction(MCInst &Inst, 168 const MCSubtargetInfo &STI) const {}; 169 170 /// @} 171 172 /// Returns the minimum size of a nop in bytes on this target. The assembler 173 /// will use this to emit excess padding in situations where the padding 174 /// required for simple alignment would be less than the minimum nop size. 175 /// 176 virtual unsigned getMinimumNopSize() const { return 1; } 177 178 /// Write an (optimal) nop sequence of Count bytes to the given output. If the 179 /// target cannot generate such a sequence, it should return an error. 180 /// 181 /// \return - True on success. 182 virtual bool writeNopData(raw_ostream &OS, uint64_t Count) const = 0; 183 184 /// Give backend an opportunity to finish layout after relaxation 185 virtual void finishLayout(MCAssembler const &Asm, 186 MCAsmLayout &Layout) const {} 187 188 /// Handle any target-specific assembler flags. By default, do nothing. 189 virtual void handleAssemblerFlag(MCAssemblerFlag Flag) {} 190 191 /// Generate the compact unwind encoding for the CFI instructions. 192 virtual uint32_t 193 generateCompactUnwindEncoding(ArrayRef<MCCFIInstruction>) const { 194 return 0; 195 } 196 197 /// Check whether a given symbol has been flagged with MICROMIPS flag. 198 virtual bool isMicroMips(const MCSymbol *Sym) const { 199 return false; 200 } 201 }; 202 203 } // end namespace llvm 204 205 #endif // LLVM_MC_MCASMBACKEND_H 206