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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.

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