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<a name="Mode-Switching"></a>
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<a name="Mode-Switching-Instructions"></a>
<h3 class="section">17.23 Mode Switching Instructions</h3>
<a name="index-mode-switching"></a>
<p>The following macros control mode switching optimizations:
</p>
<dl>
<dt><a name="index-OPTIMIZE_005fMODE_005fSWITCHING"></a>Macro: <strong>OPTIMIZE_MODE_SWITCHING</strong> <em>(<var>entity</var>)</em></dt>
<dd><p>Define this macro if the port needs extra instructions inserted for mode
switching in an optimizing compilation.
</p>
<p>For an example, the SH4 can perform both single and double precision
floating point operations, but to perform a single precision operation,
the FPSCR PR bit has to be cleared, while for a double precision
operation, this bit has to be set.  Changing the PR bit requires a general
purpose register as a scratch register, hence these FPSCR sets have to
be inserted before reload, i.e. you cannot put this into instruction emitting
or <code>TARGET_MACHINE_DEPENDENT_REORG</code>.
</p>
<p>You can have multiple entities that are mode-switched, and select at run time
which entities actually need it.  <code>OPTIMIZE_MODE_SWITCHING</code> should
return nonzero for any <var>entity</var> that needs mode-switching.
If you define this macro, you also have to define
<code>NUM_MODES_FOR_MODE_SWITCHING</code>, <code>TARGET_MODE_NEEDED</code>,
<code>TARGET_MODE_PRIORITY</code> and <code>TARGET_MODE_EMIT</code>.
<code>TARGET_MODE_AFTER</code>, <code>TARGET_MODE_ENTRY</code>, and <code>TARGET_MODE_EXIT</code>
are optional.
</p></dd></dl>

<dl>
<dt><a name="index-NUM_005fMODES_005fFOR_005fMODE_005fSWITCHING"></a>Macro: <strong>NUM_MODES_FOR_MODE_SWITCHING</strong></dt>
<dd><p>If you define <code>OPTIMIZE_MODE_SWITCHING</code>, you have to define this as
initializer for an array of integers.  Each initializer element
N refers to an entity that needs mode switching, and specifies the number
of different modes that might need to be set for this entity.
The position of the initializer in the initializer&mdash;starting counting at
zero&mdash;determines the integer that is used to refer to the mode-switched
entity in question.
In macros that take mode arguments / yield a mode result, modes are
represented as numbers 0 &hellip; N - 1.  N is used to specify that no mode
switch is needed / supplied.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fMODE_005fEMIT"></a>Target Hook: <em>void</em> <strong>TARGET_MODE_EMIT</strong> <em>(int <var>entity</var>, int <var>mode</var>, int <var>prev_mode</var>, HARD_REG_SET <var>regs_live</var>)</em></dt>
<dd><p>Generate one or more insns to set <var>entity</var> to <var>mode</var>. <var>hard_reg_live</var> is the set of hard registers live at the point where the insn(s) are to be inserted. <var>prev_moxde</var> indicates the mode to switch from. Sets of a lower numbered entity will be emitted before sets of a higher numbered entity to a mode of the same or lower priority.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fMODE_005fNEEDED"></a>Target Hook: <em>int</em> <strong>TARGET_MODE_NEEDED</strong> <em>(int <var>entity</var>, rtx_insn *<var>insn</var>)</em></dt>
<dd><p><var>entity</var> is an integer specifying a mode-switched entity.  If <code>OPTIMIZE_MODE_SWITCHING</code> is defined, you must define this macro to return an integer value not larger than the corresponding element in <code>NUM_MODES_FOR_MODE_SWITCHING</code>, to denote the mode that <var>entity</var> must be switched into prior to the execution of <var>insn</var>.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fMODE_005fAFTER"></a>Target Hook: <em>int</em> <strong>TARGET_MODE_AFTER</strong> <em>(int <var>entity</var>, int <var>mode</var>, rtx_insn *<var>insn</var>)</em></dt>
<dd><p><var>entity</var> is an integer specifying a mode-switched entity.  If this macro is defined, it is evaluated for every <var>insn</var> during mode switching.  It determines the mode that an insn results in (if different from the incoming mode).
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fMODE_005fENTRY"></a>Target Hook: <em>int</em> <strong>TARGET_MODE_ENTRY</strong> <em>(int <var>entity</var>)</em></dt>
<dd><p>If this macro is defined, it is evaluated for every <var>entity</var> that needs mode switching.  It should evaluate to an integer, which is a mode that <var>entity</var> is assumed to be switched to at function entry.  If <code>TARGET_MODE_ENTRY</code> is defined then <code>TARGET_MODE_EXIT</code> must be defined.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fMODE_005fEXIT"></a>Target Hook: <em>int</em> <strong>TARGET_MODE_EXIT</strong> <em>(int <var>entity</var>)</em></dt>
<dd><p>If this macro is defined, it is evaluated for every <var>entity</var> that needs mode switching.  It should evaluate to an integer, which is a mode that <var>entity</var> is assumed to be switched to at function exit.  If <code>TARGET_MODE_EXIT</code> is defined then <code>TARGET_MODE_ENTRY</code> must be defined.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fMODE_005fPRIORITY"></a>Target Hook: <em>int</em> <strong>TARGET_MODE_PRIORITY</strong> <em>(int <var>entity</var>, int <var>n</var>)</em></dt>
<dd><p>This macro specifies the order in which modes for <var>entity</var> are processed. 0 is the highest priority, <code>NUM_MODES_FOR_MODE_SWITCHING[<var>entity</var>] - 1</code> the lowest.  The value of the macro should be an integer designating a mode for <var>entity</var>.  For any fixed <var>entity</var>, <code>mode_priority</code> (<var>entity</var>, <var>n</var>) shall be a bijection in 0 &hellip; <code>num_modes_for_mode_switching[<var>entity</var>] - 1</code>.
</p></dd></dl>

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