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<h3 class="section">16.8 Operand Constraints</h3>
<p><a name="index-operand-constraints-3304"></a><a name="index-constraints-3305"></a>
Each <code>match_operand</code> in an instruction pattern can specify
constraints for the operands allowed. The constraints allow you to
fine-tune matching within the set of operands allowed by the
predicate.
<p>Constraints can say whether
an operand may be in a register, and which kinds of register; whether the
operand can be a memory reference, and which kinds of address; whether the
operand may be an immediate constant, and which possible values it may
have. Constraints can also require two operands to match.
Side-effects aren't allowed in operands of inline <code>asm</code>, unless
‘<samp><span class="samp"><</span></samp>’ or ‘<samp><span class="samp">></span></samp>’ constraints are used, because there is no guarantee
that the side-effects will happen exactly once in an instruction that can update
the addressing register.
<ul class="menu">
<li><a accesskey="1" href="Simple-Constraints.html#Simple-Constraints">Simple Constraints</a>: Basic use of constraints.
<li><a accesskey="2" href="Multi_002dAlternative.html#Multi_002dAlternative">Multi-Alternative</a>: When an insn has two alternative constraint-patterns.
<li><a accesskey="3" href="Class-Preferences.html#Class-Preferences">Class Preferences</a>: Constraints guide which hard register to put things in.
<li><a accesskey="4" href="Modifiers.html#Modifiers">Modifiers</a>: More precise control over effects of constraints.
<li><a accesskey="5" href="Machine-Constraints.html#Machine-Constraints">Machine Constraints</a>: Existing constraints for some particular machines.
<li><a accesskey="6" href="Disable-Insn-Alternatives.html#Disable-Insn-Alternatives">Disable Insn Alternatives</a>: Disable insn alternatives using attributes.
<li><a accesskey="7" href="Define-Constraints.html#Define-Constraints">Define Constraints</a>: How to define machine-specific constraints.
<li><a accesskey="8" href="C-Constraint-Interface.html#C-Constraint-Interface">C Constraint Interface</a>: How to test constraints from C code.
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