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<title>Conditional Execution - GNU Compiler Collection (GCC) Internals</title>
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<a name="Conditional-Execution"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Define-Subst.html#Define-Subst">Define Subst</a>,
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Previous: <a rel="previous" accesskey="p" href="Insn-Attributes.html#Insn-Attributes">Insn Attributes</a>,
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Up: <a rel="up" accesskey="u" href="Machine-Desc.html#Machine-Desc">Machine Desc</a>
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<h3 class="section">16.20 Conditional Execution</h3>
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<p><a name="index-conditional-execution-3819"></a><a name="index-predication-3820"></a>
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A number of architectures provide for some form of conditional
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execution, or predication. The hallmark of this feature is the
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ability to nullify most of the instructions in the instruction set.
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When the instruction set is large and not entirely symmetric, it
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can be quite tedious to describe these forms directly in the
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<samp><span class="file">.md</span></samp> file. An alternative is the <code>define_cond_exec</code> template.
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<p><a name="index-define_005fcond_005fexec-3821"></a>
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<pre class="smallexample"> (define_cond_exec
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[<var>predicate-pattern</var>]
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"<var>condition</var>"
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"<var>output-template</var>"
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"<var>optional-insn-attribues</var>")
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</pre>
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<p><var>predicate-pattern</var> is the condition that must be true for the
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insn to be executed at runtime and should match a relational operator.
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One can use <code>match_operator</code> to match several relational operators
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at once. Any <code>match_operand</code> operands must have no more than one
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alternative.
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<p><var>condition</var> is a C expression that must be true for the generated
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pattern to match.
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<p><a name="index-current_005finsn_005fpredicate-3822"></a><var>output-template</var> is a string similar to the <code>define_insn</code>
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output template (see <a href="Output-Template.html#Output-Template">Output Template</a>), except that the ‘<samp><span class="samp">*</span></samp>’
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and ‘<samp><span class="samp">@</span></samp>’ special cases do not apply. This is only useful if the
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assembly text for the predicate is a simple prefix to the main insn.
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In order to handle the general case, there is a global variable
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<code>current_insn_predicate</code> that will contain the entire predicate
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if the current insn is predicated, and will otherwise be <code>NULL</code>.
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<p><var>optional-insn-attributes</var> is an optional vector of attributes that gets
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appended to the insn attributes of the produced cond_exec rtx. It can
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be used to add some distinguishing attribute to cond_exec rtxs produced
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that way. An example usage would be to use this attribute in conjunction
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with attributes on the main pattern to disable particular alternatives under
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certain conditions.
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<p>When <code>define_cond_exec</code> is used, an implicit reference to
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the <code>predicable</code> instruction attribute is made.
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See <a href="Insn-Attributes.html#Insn-Attributes">Insn Attributes</a>. This attribute must be a boolean (i.e. have
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exactly two elements in its <var>list-of-values</var>), with the possible
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values being <code>no</code> and <code>yes</code>. The default and all uses in
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the insns must be a simple constant, not a complex expressions. It
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may, however, depend on the alternative, by using a comma-separated
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list of values. If that is the case, the port should also define an
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<code>enabled</code> attribute (see <a href="Disable-Insn-Alternatives.html#Disable-Insn-Alternatives">Disable Insn Alternatives</a>), which
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should also allow only <code>no</code> and <code>yes</code> as its values.
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<p>For each <code>define_insn</code> for which the <code>predicable</code>
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attribute is true, a new <code>define_insn</code> pattern will be
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generated that matches a predicated version of the instruction.
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For example,
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<pre class="smallexample"> (define_insn "addsi"
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[(set (match_operand:SI 0 "register_operand" "r")
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(plus:SI (match_operand:SI 1 "register_operand" "r")
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(match_operand:SI 2 "register_operand" "r")))]
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"<var>test1</var>"
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"add %2,%1,%0")
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(define_cond_exec
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[(ne (match_operand:CC 0 "register_operand" "c")
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(const_int 0))]
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"<var>test2</var>"
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"(%0)")
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</pre>
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<p class="noindent">generates a new pattern
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<pre class="smallexample"> (define_insn ""
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[(cond_exec
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(ne (match_operand:CC 3 "register_operand" "c") (const_int 0))
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(set (match_operand:SI 0 "register_operand" "r")
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(plus:SI (match_operand:SI 1 "register_operand" "r")
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(match_operand:SI 2 "register_operand" "r"))))]
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"(<var>test2</var>) && (<var>test1</var>)"
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"(%3) add %2,%1,%0")
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</pre>
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</body></html>
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