A TD file is a definition file used by TableGen, a tool included with the LLVM compiler used to write LLVM backends. It contains classes (class) and definitions (def) written in the TableGen language which specify how LLVM translates compiled source code from a program into code that the target device understands.

TD file open in Apple Xcode 10 LLVM is used to compile developer code, such as Objective-C or C++ code, to code that can be run on a target machine. In order to do this, LLVM first takes developer code and compiles it into its own internal representation (IR), which is an intermediate language that enables the LLVM compiler to be flexible when translating compiled code into code for different target environments. The LLVM backend translates the IR to the target code, but the LLVM backend requires rules for how to correctly translate the IR to the target code. These rules are defined and stored in the TD file using TableGen, with each TD file storing rules for separate target environments.