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150 lines
7.3 KiB
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150 lines
7.3 KiB
HTML
4 years ago
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<html lang="en">
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<title>Elimination - GNU Compiler Collection (GCC) Internals</title>
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<meta name="description" content="GNU Compiler Collection (GCC) Internals">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Stack-and-Calling.html#Stack-and-Calling" title="Stack and Calling">
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<link rel="prev" href="Frame-Registers.html#Frame-Registers" title="Frame Registers">
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<link rel="next" href="Stack-Arguments.html#Stack-Arguments" title="Stack Arguments">
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Copyright (C) 1988-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being ``Funding Free Software'', the Front-Cover
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(see below). A copy of the license is included in the section entitled
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``GNU Free Documentation License''.
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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</head>
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<body>
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<div class="node">
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<a name="Elimination"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Stack-Arguments.html#Stack-Arguments">Stack Arguments</a>,
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Previous: <a rel="previous" accesskey="p" href="Frame-Registers.html#Frame-Registers">Frame Registers</a>,
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Up: <a rel="up" accesskey="u" href="Stack-and-Calling.html#Stack-and-Calling">Stack and Calling</a>
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<hr>
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</div>
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<h4 class="subsection">17.9.5 Eliminating Frame Pointer and Arg Pointer</h4>
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<!-- prevent bad page break with this line -->
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<p>This is about eliminating the frame pointer and arg pointer.
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<div class="defun">
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— Target Hook: bool <b>TARGET_FRAME_POINTER_REQUIRED</b> (<var>void</var>)<var><a name="index-TARGET_005fFRAME_005fPOINTER_005fREQUIRED-4207"></a></var><br>
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<blockquote><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>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>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>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>Default return value is <code>false</code>.
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</p></blockquote></div>
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<p><a name="index-get_005fframe_005fsize-4208"></a>
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<div class="defun">
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— Macro: <b>INITIAL_FRAME_POINTER_OFFSET</b> (<var>depth-var</var>)<var><a name="index-INITIAL_005fFRAME_005fPOINTER_005fOFFSET-4209"></a></var><br>
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<blockquote><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>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></blockquote></div>
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<div class="defun">
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— Macro: <b>ELIMINABLE_REGS</b><var><a name="index-ELIMINABLE_005fREGS-4210"></a></var><br>
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<blockquote><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>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>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>In this case, you might specify:
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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>
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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></blockquote></div>
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<div class="defun">
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— Target Hook: bool <b>TARGET_CAN_ELIMINATE</b> (<var>const int from_reg, const int to_reg</var>)<var><a name="index-TARGET_005fCAN_005fELIMINATE-4211"></a></var><br>
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<blockquote><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>Default return value is <code>true</code>.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>INITIAL_ELIMINATION_OFFSET</b> (<var>from-reg, to-reg, offset-var</var>)<var><a name="index-INITIAL_005fELIMINATION_005fOFFSET-4212"></a></var><br>
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<blockquote><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></blockquote></div>
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</body></html>
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