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RTL Representation

Most of the work of the compiler is done on an intermediate representation called register transfer language. In this language, the instructions to be output are described, pretty much one by one, in an algebraic form that describes what the instruction does.

RTL is inspired by Lisp lists. It has both an internal form, made up of structures that point at other structures, and a textual form that is used in the machine description and in printed debugging dumps. The textual form uses nested parentheses to indicate the pointers in the internal form.

RTL Object Types

RTL uses five kinds of objects: expressions, integers, wide integers, strings and vectors. Expressions are the most important ones. An RTL expression ("RTX", for short) is a C structure, but it is usually referred to with a pointer; a type that is given the typedef name rtx.

An integer is simply an int; their written form uses decimal digits. A wide integer is an integral object whose type is HOST_WIDE_INT (see section The Configuration File); their written form uses decimal digits.

A string is a sequence of characters. In core it is represented as a char * in usual C fashion, and it is written in C syntax as well. However, strings in RTL may never be null. If you write an empty string in a machine description, it is represented in core as a null pointer rather than as a pointer to a null character. In certain contexts, these null pointers instead of strings are valid. Within RTL code, strings are most commonly found inside symbol_ref expressions, but they appear in other contexts in the RTL expressions that make up machine descriptions.

A vector contains an arbitrary number of pointers to expressions. The number of elements in the vector is explicitly present in the vector. The written form of a vector consists of square brackets (`[...]') surrounding the elements, in sequence and with whitespace separating them. Vectors of length zero are not created; null pointers are used instead.

Expressions are classified by expression codes (also called RTX codes). The expression code is a name defined in `rtl.def', which is also (in upper case) a C enumeration constant. The possible expression codes and their meanings are machine-independent. The code of an RTX can be extracted with the macro GET_CODE (x) and altered with PUT_CODE (x, newcode).

The expression code determines how many operands the expression contains, and what kinds of objects they are. In RTL, unlike Lisp, you cannot tell by looking at an operand what kind of object it is. Instead, you must know from its context--from the expression code of the containing expression.