子目标支持用于告知代码生成过程特定芯片组的指令集差异，若需要子目标支持，你应为目标架构实现一个目标特定的 XXXSubtarget 类。该类需处理命令行选项-mcpu=和-mattr=。
TableGen 会利用 Target.td 和RISCVFeatures.td 文件中的定义，在RISCVGenSubtargetInfo.inc 中生成代码。如下所示，Target.td 中定义了 SubtargetFeature 接口。SubtargetFeature 接口的前四个字符串参数分别为：特性名称、由该特性设置的 XXXSubtarget 字段、XXXSubtarget 字段的值以及特性描述。（第五个参数是该特性隐含的其他特性列表，默认值为空数组。）
若字段值为字符串 “true” 或 “false”，则该字段被视为布尔值，且仅能有一个 SubtargetFeature 引用它。否则，该字段被视为整数类型，其值可为枚举常量名称。若多个特性使用同一个整数字段，则该字段的值会被设为所有启用的、共享该字段的特性对应值中的最大值。
//===----------------------------------------------------------------------===// // SubtargetFeature - A characteristic of the chip set. // class SubtargetFeature<string n, string f, string v, string d, list<SubtargetFeature> i = []> { // Name - Feature name. Used by command line (-mattr=) to determine the // appropriate target chip. // string Name = n; // FieldName - Field in XXXSubtarget to be set by feature. // string FieldName = f; // Value - Value the XXXSubtarget field to be set to by feature. // // A value of "true" or "false" implies the field is a bool. Otherwise, // it is assumed to be an integer. the integer value may be the name of an // enum constant. If multiple features use the same integer field, the // field will be set to the maximum value of all enabled features that // share the field. // string Value = v; // Desc - Feature description. Used by command line (-mattr=) to display help // information. // string Desc = d; // Implies - Features that this feature implies are present. If one of those // features isn't set, then this one shouldn't be set either. // list<SubtargetFeature> Implies = i; }在RISCVFeaturestd文件中，有大量基于SubtargetFeature定义的特征：
// Feature32Bit exists to mark CPUs that support RV32 to distinguish them from // tuning CPU names. def Feature32Bit : SubtargetFeature<"32bit", "IsRV32", "true", "Implements RV32">; def Feature64Bit : SubtargetFeature<"64bit", "IsRV64", "true", "Implements RV64">; def IsRV64 : Predicate<"Subtarget->is64Bit()">, AssemblerPredicate<(all_of Feature64Bit), "RV64I Base Instruction Set">; def IsRV32 : Predicate<"!Subtarget->is64Bit()">, AssemblerPredicate<(all_of (not Feature64Bit)), "RV32I Base Instruction Set">; defvar RV32 = DefaultMode; def RV64 : HwMode<[IsRV64]>; def FeatureRelax : SubtargetFeature<"relax", "EnableLinkerRelax", "true", "Enable Linker relaxation.">; def FeatureExactAssembly : SubtargetFeature<"exact-asm", "EnableExactAssembly", "true", "Enable Exact Assembly (Disables Compression and Relaxation)">; foreach i = {1-31} in def FeatureReserveX#i : SubtargetFeature<"reserve-x"#i, "UserReservedRegister[RISCV::X"#i#"]", "true", "Reserve X"#i>; def FeatureSaveRestore : SubtargetFeature<"save-restore", "EnableSaveRestore", "true", "Enable save/restore.">; def FeatureNoTrailingSeqCstFence : SubtargetFeature<"no-trailing-seq-cst-fence", "EnableTrailingSeqCstFence", "false", "Disable trailing fence for seq-cst store.">; def FeatureUnalignedScalarMem : SubtargetFeature<"unaligned-scalar-mem", "EnableUnalignedScalarMem", "true", "Has reasonably performant unaligned scalar " "loads and stores">;第一个特征Feature32Bit定义了32位子处理器架构的特征：
def Feature32Bit // 1. 自定义特性名 : SubtargetFeature< // 2. 继承LLVM标准基类 "32bit", // 3. 参数1：命令行/编译标识名 "IsRV32", // 4. 参数2：C++侧的布尔成员变量名 "true", // 5. 参数3：该变量的赋值/启用值 "Implements RV32"// 6. 参数4：特性的描述字符串 >;而第二个特征Feature64Bit定义了64位RISC-V架构特性：
def Feature64Bit : SubtargetFeature<"64bit", "IsRV64", "true", "Implements RV64">;从Target.td和RISCVFeatures.td文件生成的 RISCVGenSubtargetinfo.inc 会指定用于标识特性的枚举值、表示 CPU 特性和 CPU 子类型的常量数组，以及解析用于设置指定子目标选项的特性字符串的 ParseSubtargetFeatures 方法。生成的 RISCVGenSubtargetinfo.inc 文件应被包含在 RiscvSubtarget.cpp 中。XXXSubtarget 方法的目标特定实现应遵循以下伪代码：
XXXSubtarget::XXXSubtarget(const Module &M, const std::string &FS) { // Set the default features // Determine default and user specified characteristics of the CPU // Call ParseSubtargetFeatures(FS, CPU) to parse the features string // Perform any additional operations }例如，RISCV的实现如下：
RISCVSubtarget::RISCVSubtarget(const Triple &TT, StringRef CPU, StringRef TuneCPU, StringRef FS, StringRef ABIName, unsigned RVVVectorBitsMin, unsigned RVVVectorBitsMax, const TargetMachine &TM) : RISCVGenSubtargetInfo(TT, CPU, TuneCPU, FS), RVVVectorBitsMin(RVVVectorBitsMin), RVVVectorBitsMax(RVVVectorBitsMax), FrameLowering( initializeSubtargetDependencies(TT, CPU, TuneCPU, FS, ABIName)), InstrInfo(*this), TLInfo(TM, *this) { TSInfo = std::make_unique<RISCVSelectionDAGInfo>(); }
