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| <title>GNU Compiler Collection (GCC) Internals: Mode Switching</title> |
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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 can’t 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—starting counting at |
| zero—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 … 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 … <code>num_modes_for_mode_switching[<var>entity</var>] - 1</code>. |
| </p></dd></dl> |
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