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<h2 class="chapter">16 Machine Descriptions</h2>
<p><a name="index-machine-descriptions-3241"></a>
A machine description has two parts: a file of instruction patterns
(<samp><span class="file">.md</span></samp> file) and a C header file of macro definitions.
<p>The <samp><span class="file">.md</span></samp> 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.
<p>See the next chapter for information on the C header file.
<ul class="menu">
<li><a accesskey="1" href="Overview.html#Overview">Overview</a>: How the machine description is used.
<li><a accesskey="2" href="Patterns.html#Patterns">Patterns</a>: How to write instruction patterns.
<li><a accesskey="3" href="Example.html#Example">Example</a>: An explained example of a <code>define_insn</code> pattern.
<li><a accesskey="4" href="RTL-Template.html#RTL-Template">RTL Template</a>: The RTL template defines what insns match a pattern.
<li><a accesskey="5" href="Output-Template.html#Output-Template">Output Template</a>: The output template says how to make assembler code
from such an insn.
<li><a accesskey="6" href="Output-Statement.html#Output-Statement">Output Statement</a>: For more generality, write C code to output
the assembler code.
<li><a accesskey="7" href="Predicates.html#Predicates">Predicates</a>: Controlling what kinds of operands can be used
for an insn.
<li><a accesskey="8" href="Constraints.html#Constraints">Constraints</a>: Fine-tuning operand selection.
<li><a accesskey="9" href="Standard-Names.html#Standard-Names">Standard Names</a>: Names mark patterns to use for code generation.
<li><a href="Pattern-Ordering.html#Pattern-Ordering">Pattern Ordering</a>: When the order of patterns makes a difference.
<li><a href="Dependent-Patterns.html#Dependent-Patterns">Dependent Patterns</a>: Having one pattern may make you need another.
<li><a href="Jump-Patterns.html#Jump-Patterns">Jump Patterns</a>: Special considerations for patterns for jump insns.
<li><a href="Looping-Patterns.html#Looping-Patterns">Looping Patterns</a>: How to define patterns for special looping insns.
<li><a href="Insn-Canonicalizations.html#Insn-Canonicalizations">Insn Canonicalizations</a>: Canonicalization of Instructions
<li><a href="Expander-Definitions.html#Expander-Definitions">Expander Definitions</a>: Generating a sequence of several RTL insns
for a standard operation.
<li><a href="Insn-Splitting.html#Insn-Splitting">Insn Splitting</a>: Splitting Instructions into Multiple Instructions.
<li><a href="Including-Patterns.html#Including-Patterns">Including Patterns</a>: Including Patterns in Machine Descriptions.
<li><a href="Peephole-Definitions.html#Peephole-Definitions">Peephole Definitions</a>: Defining machine-specific peephole optimizations.
<li><a href="Insn-Attributes.html#Insn-Attributes">Insn Attributes</a>: Specifying the value of attributes for generated insns.
<li><a href="Conditional-Execution.html#Conditional-Execution">Conditional Execution</a>: Generating <code>define_insn</code> patterns for
predication.
<li><a href="Define-Subst.html#Define-Subst">Define Subst</a>: Generating <code>define_insn</code> and <code>define_expand</code>
patterns from other patterns.
<li><a href="Constant-Definitions.html#Constant-Definitions">Constant Definitions</a>: Defining symbolic constants that can be used in the
md file.
<li><a href="Iterators.html#Iterators">Iterators</a>: Using iterators to generate patterns from a template.
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