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