16 Machine Descriptions

A machine description has two parts: a file of instruction patterns (.md file) and a C header file of macro definitions.

The .md file for a target machine contains a pattern for each instruction that the target machine supports (or at least each instruction that is worth telling the compiler about). It may also contain comments. A semicolon causes the rest of the line to be a comment, unless the semicolon is inside a quoted string.

See the next chapter for information on the C header file.

• Overview: How the machine description is used.
• Patterns: How to write instruction patterns.
• Example: An explained example of a define_insn pattern.
• RTL Template: The RTL template defines what insns match a pattern.
• Output Template: The output template says how to make assembler code from such an insn.
• Output Statement: For more generality, write C code to output the assembler code.
• Predicates: Controlling what kinds of operands can be used for an insn.
• Constraints: Fine-tuning operand selection.
• Standard Names: Names mark patterns to use for code generation.
• Pattern Ordering: When the order of patterns makes a difference.
• Dependent Patterns: Having one pattern may make you need another.
• Jump Patterns: Special considerations for patterns for jump insns.
• Looping Patterns: How to define patterns for special looping insns.
• Insn Canonicalizations: Canonicalization of Instructions
• Expander Definitions: Generating a sequence of several RTL insns for a standard operation.
• Insn Splitting: Splitting Instructions into Multiple Instructions.
• Including Patterns: Including Patterns in Machine Descriptions.
• Peephole Definitions: Defining machine-specific peephole optimizations.
• Insn Attributes: Specifying the value of attributes for generated insns.
• Conditional Execution: Generating define_insn patterns for predication.
• Define Subst: Generating define_insn and define_expand patterns from other patterns.
• Constant Definitions: Defining symbolic constants that can be used in the md file.
• Iterators: Using iterators to generate patterns from a template.