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<title>GNU Compiler Collection (GCC) Internals: Elimination</title>
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<a name="Elimination"></a>
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<div class="header">
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<p>
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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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</div>
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<hr>
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<a name="Eliminating-Frame-Pointer-and-Arg-Pointer"></a>
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<h4 class="subsection">17.9.5 Eliminating Frame Pointer and Arg Pointer</h4>
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<p>This is about eliminating the frame pointer and arg pointer.
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</p>
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<dl>
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<dt><a name="index-TARGET_005fFRAME_005fPOINTER_005fREQUIRED"></a>Target Hook: <em>bool</em> <strong>TARGET_FRAME_POINTER_REQUIRED</strong> <em>(void)</em></dt>
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<dd><p>This target hook should return <code>true</code> if a function must have and use
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a frame pointer. This target hook is called in the reload pass. If its return
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value is <code>true</code> the function will have a frame pointer.
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</p>
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<p>This target hook can in principle examine the current function and decide
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according to the facts, but on most machines the constant <code>false</code> or the
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constant <code>true</code> suffices. Use <code>false</code> when the machine allows code
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to be generated with no frame pointer, and doing so saves some time or space.
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Use <code>true</code> when there is no possible advantage to avoiding a frame
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pointer.
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</p>
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<p>In certain cases, the compiler does not know how to produce valid code
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without a frame pointer. The compiler recognizes those cases and
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automatically gives the function a frame pointer regardless of what
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<code>TARGET_FRAME_POINTER_REQUIRED</code> returns. You don’t need to worry about
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them.
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</p>
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<p>In a function that does not require a frame pointer, the frame pointer
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register can be allocated for ordinary usage, unless you mark it as a
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fixed register. See <code>FIXED_REGISTERS</code> for more information.
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</p>
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<p>Default return value is <code>false</code>.
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</p></dd></dl>
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<a name="index-get_005fframe_005fsize"></a>
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<dl>
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<dt><a name="index-INITIAL_005fFRAME_005fPOINTER_005fOFFSET"></a>Macro: <strong>INITIAL_FRAME_POINTER_OFFSET</strong> <em>(<var>depth-var</var>)</em></dt>
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<dd><p>A C statement to store in the variable <var>depth-var</var> the difference
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between the frame pointer and the stack pointer values immediately after
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the function prologue. The value would be computed from information
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such as the result of <code>get_frame_size ()</code> and the tables of
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registers <code>regs_ever_live</code> and <code>call_used_regs</code>.
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</p>
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<p>If <code>ELIMINABLE_REGS</code> is defined, this macro will be not be used and
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need not be defined. Otherwise, it must be defined even if
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<code>TARGET_FRAME_POINTER_REQUIRED</code> always returns true; in that
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case, you may set <var>depth-var</var> to anything.
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</p></dd></dl>
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<dl>
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<dt><a name="index-ELIMINABLE_005fREGS"></a>Macro: <strong>ELIMINABLE_REGS</strong></dt>
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<dd><p>If defined, this macro specifies a table of register pairs used to
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eliminate unneeded registers that point into the stack frame. If it is not
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defined, the only elimination attempted by the compiler is to replace
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references to the frame pointer with references to the stack pointer.
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</p>
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<p>The definition of this macro is a list of structure initializations, each
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of which specifies an original and replacement register.
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</p>
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<p>On some machines, the position of the argument pointer is not known until
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the compilation is completed. In such a case, a separate hard register
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must be used for the argument pointer. This register can be eliminated by
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replacing it with either the frame pointer or the argument pointer,
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depending on whether or not the frame pointer has been eliminated.
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</p>
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<p>In this case, you might specify:
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</p><div class="smallexample">
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<pre class="smallexample">#define ELIMINABLE_REGS \
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{{ARG_POINTER_REGNUM, STACK_POINTER_REGNUM}, \
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{ARG_POINTER_REGNUM, FRAME_POINTER_REGNUM}, \
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{FRAME_POINTER_REGNUM, STACK_POINTER_REGNUM}}
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</pre></div>
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<p>Note that the elimination of the argument pointer with the stack pointer is
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specified first since that is the preferred elimination.
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</p></dd></dl>
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<dl>
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<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>
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<dd><p>This target hook should returns <code>true</code> if the compiler is allowed to
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try to replace register number <var>from_reg</var> with register number
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<var>to_reg</var>. This target hook need only be defined if <code>ELIMINABLE_REGS</code>
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is defined, and will usually be <code>true</code>, since most of the cases
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preventing register elimination are things that the compiler already
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knows about.
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</p>
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<p>Default return value is <code>true</code>.
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</p></dd></dl>
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<dl>
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<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>
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<dd><p>This macro is similar to <code>INITIAL_FRAME_POINTER_OFFSET</code>. It
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specifies the initial difference between the specified pair of
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registers. This macro must be defined if <code>ELIMINABLE_REGS</code> is
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defined.
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</p></dd></dl>
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<hr>
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<div class="header">
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<p>
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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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