What are *.td files in the LLVM / Clang source?

Question

I'm just getting to know the LLVM / Clang source code structure and I find it hard to figure out what *.td files actually are?

Answer 1

*.td files contain TableGen code. TableGen is an interpreted languague to describe "records of domain-specific information". One of its main purposes is generating parts of the llvm / clang C/C++ source code. However it is used for various other purposes as well, see
TableGen Program Reference: http://llvm.org/docs/TableGen/index.html
TableGen Language Reference: http://llvm.org/docs/TableGen/LangRef.html
TableGen Backends: http://llvm.org/docs/TableGen/BackEnds.html