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