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<title>Delay Slots - GNU Compiler Collection (GCC) Internals</title>
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<a name="Delay-Slots"></a>
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Next: <a rel="next" accesskey="n" href="Processor-pipeline-description.html#Processor-pipeline-description">Processor pipeline description</a>,
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Previous: <a rel="previous" accesskey="p" href="Mnemonic-Attribute.html#Mnemonic-Attribute">Mnemonic Attribute</a>,
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<h4 class="subsection">16.19.8 Delay Slot Scheduling</h4>
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<p><a name="index-delay-slots_002c-defining-3778"></a>
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The insn attribute mechanism can be used to specify the requirements for
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delay slots, if any, on a target machine. An instruction is said to
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require a <dfn>delay slot</dfn> if some instructions that are physically
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after the instruction are executed as if they were located before it.
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Classic examples are branch and call instructions, which often execute
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the following instruction before the branch or call is performed.
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<p>On some machines, conditional branch instructions can optionally
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<dfn>annul</dfn> instructions in the delay slot. This means that the
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instruction will not be executed for certain branch outcomes. Both
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instructions that annul if the branch is true and instructions that
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annul if the branch is false are supported.
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<p>Delay slot scheduling differs from instruction scheduling in that
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determining whether an instruction needs a delay slot is dependent only
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on the type of instruction being generated, not on data flow between the
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instructions. See the next section for a discussion of data-dependent
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instruction scheduling.
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<p><a name="index-define_005fdelay-3779"></a>The requirement of an insn needing one or more delay slots is indicated
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via the <code>define_delay</code> expression. It has the following form:
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<pre class="smallexample"> (define_delay <var>test</var>
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[<var>delay-1</var> <var>annul-true-1</var> <var>annul-false-1</var>
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<var>delay-2</var> <var>annul-true-2</var> <var>annul-false-2</var>
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...])
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</pre>
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<p><var>test</var> is an attribute test that indicates whether this
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<code>define_delay</code> applies to a particular insn. If so, the number of
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required delay slots is determined by the length of the vector specified
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as the second argument. An insn placed in delay slot <var>n</var> must
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satisfy attribute test <var>delay-n</var>. <var>annul-true-n</var> is an
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attribute test that specifies which insns may be annulled if the branch
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is true. Similarly, <var>annul-false-n</var> specifies which insns in the
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delay slot may be annulled if the branch is false. If annulling is not
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supported for that delay slot, <code>(nil)</code> should be coded.
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<p>For example, in the common case where branch and call insns require
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a single delay slot, which may contain any insn other than a branch or
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call, the following would be placed in the <samp><span class="file">md</span></samp> file:
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<pre class="smallexample"> (define_delay (eq_attr "type" "branch,call")
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[(eq_attr "type" "!branch,call") (nil) (nil)])
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</pre>
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<p>Multiple <code>define_delay</code> expressions may be specified. In this
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case, each such expression specifies different delay slot requirements
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and there must be no insn for which tests in two <code>define_delay</code>
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expressions are both true.
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<p>For example, if we have a machine that requires one delay slot for branches
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but two for calls, no delay slot can contain a branch or call insn,
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and any valid insn in the delay slot for the branch can be annulled if the
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branch is true, we might represent this as follows:
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<pre class="smallexample"> (define_delay (eq_attr "type" "branch")
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[(eq_attr "type" "!branch,call")
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(eq_attr "type" "!branch,call")
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(nil)])
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(define_delay (eq_attr "type" "call")
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[(eq_attr "type" "!branch,call") (nil) (nil)
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(eq_attr "type" "!branch,call") (nil) (nil)])
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</pre>
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