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<h3 class="section">17.14 Anchored Addresses</h3>
<p><a name="index-anchored-addresses-4421"></a><a name="index-g_t_0040option_007b_002dfsection_002danchors_007d-4422"></a>
GCC usually addresses every static object as a separate entity.
For example, if we have:
<pre class="smallexample"> static int a, b, c;
int foo (void) { return a + b + c; }
</pre>
<p>the code for <code>foo</code> will usually calculate three separate symbolic
addresses: those of <code>a</code>, <code>b</code> and <code>c</code>. On some targets,
it would be better to calculate just one symbolic address and access
the three variables relative to it. The equivalent pseudocode would
be something like:
<pre class="smallexample"> int foo (void)
{
register int *xr = &x;
return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
}
</pre>
<p>(which isn't valid C). We refer to shared addresses like <code>x</code> as
“section anchors”. Their use is controlled by <samp><span class="option">-fsection-anchors</span></samp>.
<p>The hooks below describe the target properties that GCC needs to know
in order to make effective use of section anchors. It won't use
section anchors at all unless either <code>TARGET_MIN_ANCHOR_OFFSET</code>
or <code>TARGET_MAX_ANCHOR_OFFSET</code> is set to a nonzero value.
<div class="defun">
— Target Hook: HOST_WIDE_INT <b>TARGET_MIN_ANCHOR_OFFSET</b><var><a name="index-TARGET_005fMIN_005fANCHOR_005fOFFSET-4423"></a></var><br>
<blockquote><p>The minimum offset that should be applied to a section anchor.
On most targets, it should be the smallest offset that can be
applied to a base register while still giving a legitimate address
for every mode. The default value is 0.
</p></blockquote></div>
<div class="defun">
— Target Hook: HOST_WIDE_INT <b>TARGET_MAX_ANCHOR_OFFSET</b><var><a name="index-TARGET_005fMAX_005fANCHOR_005fOFFSET-4424"></a></var><br>
<blockquote><p>Like <code>TARGET_MIN_ANCHOR_OFFSET</code>, but the maximum (inclusive)
offset that should be applied to section anchors. The default
value is 0.
</p></blockquote></div>
<div class="defun">
— Target Hook: void <b>TARGET_ASM_OUTPUT_ANCHOR</b> (<var>rtx x</var>)<var><a name="index-TARGET_005fASM_005fOUTPUT_005fANCHOR-4425"></a></var><br>
<blockquote><p>Write the assembly code to define section anchor <var>x</var>, which is a
<code>SYMBOL_REF</code> for which ‘<samp><span class="samp">SYMBOL_REF_ANCHOR_P (</span><var>x</var><span class="samp">)</span></samp>’ is true.
The hook is called with the assembly output position set to the beginning
of <code>SYMBOL_REF_BLOCK (</code><var>x</var><code>)</code>.
<p>If <code>ASM_OUTPUT_DEF</code> is available, the hook's default definition uses
it to define the symbol as ‘<samp><span class="samp">. + SYMBOL_REF_BLOCK_OFFSET (</span><var>x</var><span class="samp">)</span></samp>’.
If <code>ASM_OUTPUT_DEF</code> is not available, the hook's default definition
is <code>NULL</code>, which disables the use of section anchors altogether.
</p></blockquote></div>
<div class="defun">
— Target Hook: bool <b>TARGET_USE_ANCHORS_FOR_SYMBOL_P</b> (<var>const_rtx x</var>)<var><a name="index-TARGET_005fUSE_005fANCHORS_005fFOR_005fSYMBOL_005fP-4426"></a></var><br>
<blockquote><p>Return true if GCC should attempt to use anchors to access <code>SYMBOL_REF</code>
<var>x</var>. You can assume ‘<samp><span class="samp">SYMBOL_REF_HAS_BLOCK_INFO_P (</span><var>x</var><span class="samp">)</span></samp>’ and
‘<samp><span class="samp">!SYMBOL_REF_ANCHOR_P (</span><var>x</var><span class="samp">)</span></samp>’.
<p>The default version is correct for most targets, but you might need to
intercept this hook to handle things like target-specific attributes
or target-specific sections.
</p></blockquote></div>
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