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| <a name="Elimination"></a> |
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| Next: <a href="Stack-Arguments.html#Stack-Arguments" accesskey="n" rel="next">Stack Arguments</a>, Previous: <a href="Frame-Registers.html#Frame-Registers" accesskey="p" rel="prev">Frame Registers</a>, Up: <a href="Stack-and-Calling.html#Stack-and-Calling" accesskey="u" rel="up">Stack and Calling</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p> |
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| <a name="Eliminating-Frame-Pointer-and-Arg-Pointer"></a> |
| <h4 class="subsection">17.10.5 Eliminating Frame Pointer and Arg Pointer</h4> |
| |
| <p>This is about eliminating the frame pointer and arg pointer. |
| </p> |
| <dl> |
| <dt><a name="index-TARGET_005fFRAME_005fPOINTER_005fREQUIRED"></a>Target Hook: <em>bool</em> <strong>TARGET_FRAME_POINTER_REQUIRED</strong> <em>(void)</em></dt> |
| <dd><p>This target hook should return <code>true</code> if a function must have and use |
| a frame pointer. This target hook is called in the reload pass. If its return |
| value is <code>true</code> the function will have a frame pointer. |
| </p> |
| <p>This target hook can in principle examine the current function and decide |
| according to the facts, but on most machines the constant <code>false</code> or the |
| constant <code>true</code> suffices. Use <code>false</code> when the machine allows code |
| to be generated with no frame pointer, and doing so saves some time or space. |
| Use <code>true</code> when there is no possible advantage to avoiding a frame |
| pointer. |
| </p> |
| <p>In certain cases, the compiler does not know how to produce valid code |
| without a frame pointer. The compiler recognizes those cases and |
| automatically gives the function a frame pointer regardless of what |
| <code>TARGET_FRAME_POINTER_REQUIRED</code> returns. You don’t need to worry about |
| them. |
| </p> |
| <p>In a function that does not require a frame pointer, the frame pointer |
| register can be allocated for ordinary usage, unless you mark it as a |
| fixed register. See <code>FIXED_REGISTERS</code> for more information. |
| </p> |
| <p>Default return value is <code>false</code>. |
| </p></dd></dl> |
| |
| <a name="index-get_005fframe_005fsize"></a> |
| <dl> |
| <dt><a name="index-INITIAL_005fFRAME_005fPOINTER_005fOFFSET"></a>Macro: <strong>INITIAL_FRAME_POINTER_OFFSET</strong> <em>(<var>depth-var</var>)</em></dt> |
| <dd><p>A C statement to store in the variable <var>depth-var</var> the difference |
| between the frame pointer and the stack pointer values immediately after |
| the function prologue. The value would be computed from information |
| such as the result of <code>get_frame_size ()</code> and the tables of |
| registers <code>regs_ever_live</code> and <code>call_used_regs</code>. |
| </p> |
| <p>If <code>ELIMINABLE_REGS</code> is defined, this macro will be not be used and |
| need not be defined. Otherwise, it must be defined even if |
| <code>TARGET_FRAME_POINTER_REQUIRED</code> always returns true; in that |
| case, you may set <var>depth-var</var> to anything. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-ELIMINABLE_005fREGS"></a>Macro: <strong>ELIMINABLE_REGS</strong></dt> |
| <dd><p>If defined, this macro specifies a table of register pairs used to |
| eliminate unneeded registers that point into the stack frame. If it is not |
| defined, the only elimination attempted by the compiler is to replace |
| references to the frame pointer with references to the stack pointer. |
| </p> |
| <p>The definition of this macro is a list of structure initializations, each |
| of which specifies an original and replacement register. |
| </p> |
| <p>On some machines, the position of the argument pointer is not known until |
| the compilation is completed. In such a case, a separate hard register |
| must be used for the argument pointer. This register can be eliminated by |
| replacing it with either the frame pointer or the argument pointer, |
| depending on whether or not the frame pointer has been eliminated. |
| </p> |
| <p>In this case, you might specify: |
| </p><div class="smallexample"> |
| <pre class="smallexample">#define ELIMINABLE_REGS \ |
| {{ARG_POINTER_REGNUM, STACK_POINTER_REGNUM}, \ |
| {ARG_POINTER_REGNUM, FRAME_POINTER_REGNUM}, \ |
| {FRAME_POINTER_REGNUM, STACK_POINTER_REGNUM}} |
| </pre></div> |
| |
| <p>Note that the elimination of the argument pointer with the stack pointer is |
| specified first since that is the preferred elimination. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-TARGET_005fCAN_005fELIMINATE"></a>Target Hook: <em>bool</em> <strong>TARGET_CAN_ELIMINATE</strong> <em>(const int <var>from_reg</var>, const int <var>to_reg</var>)</em></dt> |
| <dd><p>This target hook should returns <code>true</code> if the compiler is allowed to |
| try to replace register number <var>from_reg</var> with register number |
| <var>to_reg</var>. This target hook need only be defined if <code>ELIMINABLE_REGS</code> |
| is defined, and will usually be <code>true</code>, since most of the cases |
| preventing register elimination are things that the compiler already |
| knows about. |
| </p> |
| <p>Default return value is <code>true</code>. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-INITIAL_005fELIMINATION_005fOFFSET"></a>Macro: <strong>INITIAL_ELIMINATION_OFFSET</strong> <em>(<var>from-reg</var>, <var>to-reg</var>, <var>offset-var</var>)</em></dt> |
| <dd><p>This macro is similar to <code>INITIAL_FRAME_POINTER_OFFSET</code>. It |
| specifies the initial difference between the specified pair of |
| registers. This macro must be defined if <code>ELIMINABLE_REGS</code> is |
| defined. |
| </p></dd></dl> |
| |
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| Next: <a href="Stack-Arguments.html#Stack-Arguments" accesskey="n" rel="next">Stack Arguments</a>, Previous: <a href="Frame-Registers.html#Frame-Registers" accesskey="p" rel="prev">Frame Registers</a>, Up: <a href="Stack-and-Calling.html#Stack-and-Calling" accesskey="u" rel="up">Stack and Calling</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p> |
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