16.1 Overview of How the Machine Description is Used

There are three main conversions that happen in the compiler:

1. The front end reads the source code and builds a parse tree.
2. The parse tree is used to generate an RTL insn list based on named instruction patterns.
3. The insn list is matched against the RTL templates to produce assembler code.

For the generate pass, only the names of the insns matter, from either a named define_insn or a define_expand. The compiler will choose the pattern with the right name and apply the operands according to the documentation later in this chapter, without regard for the RTL template or operand constraints. Note that the names the compiler looks for are hard-coded in the compiler—it will ignore unnamed patterns and patterns with names it doesn’t know about, but if you don’t provide a named pattern it needs, it will abort.

If a define_insn is used, the template given is inserted into the insn list. If a define_expand is used, one of three things happens, based on the condition logic. The condition logic may manually create new insns for the insn list, say via emit_insn(), and invoke DONE. For certain named patterns, it may invoke FAIL to tell the compiler to use an alternate way of performing that task. If it invokes neither DONE nor FAIL, the template given in the pattern is inserted, as if the define_expand were a define_insn.

Once the insn list is generated, various optimization passes convert, replace, and rearrange the insns in the insn list. This is where the define_split and define_peephole patterns get used, for example.

Finally, the insn list’s RTL is matched up with the RTL templates in the define_insn patterns, and those patterns are used to emit the final assembly code. For this purpose, each named define_insn acts like it’s unnamed, since the names are ignored.