Instruction selection

In computer science, instruction selection is the stage of a compiler backend that transforms its middle-level intermediate representation (IR) into a low-level IR. In a typical compiler, instruction selection precedes both instruction scheduling and register allocation; hence its output IR has an infinite set of pseudo-registers (often known as temporaries) and may still be – and typically is – subject to peephole optimization. Otherwise, it closely resembles the target machine code, bytecode, or assembly language.

For example, for the following sequence of middle-level IR code

t1 = a
t2 = b
t3 = t1 + t2
a = t3
b = t1

a good instruction sequence for the x86 architecture is

MOV EAX, a
XCHG EAX, b
ADD a, EAX

For a comprehensive survey on instruction selection, see.