19. 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.

19.1 RTL Object Types		Expressions vs vectors vs strings vs integers.
19.2 RTL Classes and Formats		Categories of RTL expression objects, and their structure.
19.3 Access to Operands		Macros to access expression operands or vector elts.
19.4 Flags in an RTL Expression		Other flags in an RTL expression.
19.5 Machine Modes		Describing the size and format of a datum.
19.6 Constant Expression Types		Expressions with constant values.
19.7 Registers and Memory		Expressions representing register contents or memory.
19.8 RTL Expressions for Arithmetic		Expressions representing arithmetic on other expressions.
19.9 Comparison Operations		Expressions representing comparison of expressions.
19.10 Bit-Fields		Expressions representing bit-fields in memory or reg.
19.11 Vector Operations		Expressions involving vector datatypes.
19.12 Conversions		Extending, truncating, floating or fixing.
19.13 Declarations		Declaring volatility, constancy, etc.
19.14 Side Effect Expressions		Expressions for storing in registers, etc.
19.15 Embedded Side-Effects on Addresses		Embedded side-effects for autoincrement addressing.
19.16 Assembler Instructions as Expressions		Representing asm with operands.
19.17 Insns		Expression types for entire insns.
19.18 RTL Representation of Function-Call Insns		RTL representation of function call insns.
19.19 Structure Sharing Assumptions		Some expressions are unique; others *must* be copied.
19.20 Reading RTL		Reading textual RTL from a file.