161 lines
7.5 KiB
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161 lines
7.5 KiB
HTML
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<title>GNU Compiler Collection (GCC) Internals: Anchored Addresses</title>
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<a name="Anchored-Addresses"></a>
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<div class="header">
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<p>
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Next: <a href="Condition-Code.html#Condition-Code" accesskey="n" rel="next">Condition Code</a>, Previous: <a href="Addressing-Modes.html#Addressing-Modes" accesskey="p" rel="prev">Addressing Modes</a>, Up: <a href="Target-Macros.html#Target-Macros" accesskey="u" rel="up">Target Macros</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="Anchored-Addresses-1"></a>
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<h3 class="section">17.14 Anchored Addresses</h3>
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<a name="index-anchored-addresses"></a>
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<a name="index-_002dfsection_002danchors-1"></a>
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<p>GCC usually addresses every static object as a separate entity.
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For example, if we have:
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</p>
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<div class="smallexample">
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<pre class="smallexample">static int a, b, c;
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int foo (void) { return a + b + c; }
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</pre></div>
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<p>the code for <code>foo</code> will usually calculate three separate symbolic
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addresses: those of <code>a</code>, <code>b</code> and <code>c</code>. On some targets,
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it would be better to calculate just one symbolic address and access
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the three variables relative to it. The equivalent pseudocode would
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be something like:
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</p>
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<div class="smallexample">
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<pre class="smallexample">int foo (void)
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{
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register int *xr = &x;
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return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
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}
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</pre></div>
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<p>(which isn’t valid C). We refer to shared addresses like <code>x</code> as
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“section anchors”. Their use is controlled by <samp>-fsection-anchors</samp>.
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</p>
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<p>The hooks below describe the target properties that GCC needs to know
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in order to make effective use of section anchors. It won’t use
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section anchors at all unless either <code>TARGET_MIN_ANCHOR_OFFSET</code>
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or <code>TARGET_MAX_ANCHOR_OFFSET</code> is set to a nonzero value.
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</p>
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<dl>
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<dt><a name="index-TARGET_005fMIN_005fANCHOR_005fOFFSET"></a>Target Hook: <em>HOST_WIDE_INT</em> <strong>TARGET_MIN_ANCHOR_OFFSET</strong></dt>
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<dd><p>The minimum offset that should be applied to a section anchor.
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On most targets, it should be the smallest offset that can be
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applied to a base register while still giving a legitimate address
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for every mode. The default value is 0.
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</p></dd></dl>
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<dl>
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<dt><a name="index-TARGET_005fMAX_005fANCHOR_005fOFFSET"></a>Target Hook: <em>HOST_WIDE_INT</em> <strong>TARGET_MAX_ANCHOR_OFFSET</strong></dt>
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<dd><p>Like <code>TARGET_MIN_ANCHOR_OFFSET</code>, but the maximum (inclusive)
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offset that should be applied to section anchors. The default
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value is 0.
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</p></dd></dl>
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<dl>
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<dt><a name="index-TARGET_005fASM_005fOUTPUT_005fANCHOR"></a>Target Hook: <em>void</em> <strong>TARGET_ASM_OUTPUT_ANCHOR</strong> <em>(rtx <var>x</var>)</em></dt>
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<dd><p>Write the assembly code to define section anchor <var>x</var>, which is a
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<code>SYMBOL_REF</code> for which ‘<samp>SYMBOL_REF_ANCHOR_P (<var>x</var>)</samp>’ is true.
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The hook is called with the assembly output position set to the beginning
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of <code>SYMBOL_REF_BLOCK (<var>x</var>)</code>.
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</p>
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<p>If <code>ASM_OUTPUT_DEF</code> is available, the hook’s default definition uses
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it to define the symbol as ‘<samp>. + SYMBOL_REF_BLOCK_OFFSET (<var>x</var>)</samp>’.
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If <code>ASM_OUTPUT_DEF</code> is not available, the hook’s default definition
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is <code>NULL</code>, which disables the use of section anchors altogether.
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</p></dd></dl>
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<dl>
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<dt><a name="index-TARGET_005fUSE_005fANCHORS_005fFOR_005fSYMBOL_005fP"></a>Target Hook: <em>bool</em> <strong>TARGET_USE_ANCHORS_FOR_SYMBOL_P</strong> <em>(const_rtx <var>x</var>)</em></dt>
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<dd><p>Return true if GCC should attempt to use anchors to access <code>SYMBOL_REF</code>
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<var>x</var>. You can assume ‘<samp>SYMBOL_REF_HAS_BLOCK_INFO_P (<var>x</var>)</samp>’ and
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‘<samp>!SYMBOL_REF_ANCHOR_P (<var>x</var>)</samp>’.
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</p>
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<p>The default version is correct for most targets, but you might need to
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intercept this hook to handle things like target-specific attributes
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or target-specific sections.
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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="Condition-Code.html#Condition-Code" accesskey="n" rel="next">Condition Code</a>, Previous: <a href="Addressing-Modes.html#Addressing-Modes" accesskey="p" rel="prev">Addressing Modes</a>, Up: <a href="Target-Macros.html#Target-Macros" accesskey="u" rel="up">Target Macros</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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