How instruction selection works
The task of an LLVM backend is to create machine instructions from the LLVM IR. This process is called instruction selection or lowering. Motivated by the idea to automate this task as much as possible, the LLVM developers invented the TableGen language to capture all the details of a target description. We first look at this language before diving into the instruction selection algorithms.
Specifying the target description in the TableGen language
A machine instruction has a lot of properties: a mnemonic used by the assembler and disassembler, a bit pattern to represent the instruction in memory, input and output operands, and so on. The LLVM developers decided to capture all this information in a single place, the target description. A new language, the TableGen language, was invented for this purpose. The idea was to use a code generator to create various source fragments from the target description, which could then be used in different tools...
