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<h4 class="subsection">9.27.4 Controlling the use of small data accesses</h4>
<!-- This section deliberately glosses over the possibility of using -G -->
<!-- in SVR4-style PIC, as could be done on IRIX. We don't support that. -->
<p><a name="index-small-data_002c-MIPS-1494"></a><a name="index-g_t_0040code_007bgp_007d-register_002c-MIPS-1495"></a>It often takes several instructions to load the address of a symbol.
For example, when ‘<samp><span class="samp">addr</span></samp>’ is a 32-bit symbol, the non-PIC expansion
of ‘<samp><span class="samp">dla $4,addr</span></samp>’ is usually:
<pre class="smallexample"> lui $4,%hi(addr)
daddiu $4,$4,%lo(addr)
</pre>
<p>The sequence is much longer when ‘<samp><span class="samp">addr</span></samp>’ is a 64-bit symbol.
See <a href="MIPS-Symbol-Sizes.html#MIPS-Symbol-Sizes">Directives to override the size of symbols</a>.
<p>In order to cut down on this overhead, most embedded MIPS systems
set aside a 64-kilobyte “small data” area and guarantee that all
data of size <var>n</var> and smaller will be placed in that area.
The limit <var>n</var> is passed to both the assembler and the linker
using the command-line option <samp><span class="option">-G </span><var>n</var></samp>, see <a href="MIPS-Options.html#MIPS-Options">Assembler options</a>. Note that the same value of <var>n</var> must be used
when linking and when assembling all input files to the link; any
inconsistency could cause a relocation overflow error.
<p>The size of an object in the <code>.bss</code> section is set by the
<code>.comm</code> or <code>.lcomm</code> directive that defines it. The size of
an external object may be set with the <code>.extern</code> directive. For
example, ‘<samp><span class="samp">.extern sym,4</span></samp>’ declares that the object at <code>sym</code>
is 4 bytes in length, while leaving <code>sym</code> otherwise undefined.
<p>When no <samp><span class="option">-G</span></samp> option is given, the default limit is 8 bytes.
The option <samp><span class="option">-G 0</span></samp> prevents any data from being automatically
classified as small.
<p>It is also possible to mark specific objects as small by putting them
in the special sections <code>.sdata</code> and <code>.sbss</code>, which are
“small” counterparts of <code>.data</code> and <code>.bss</code> respectively.
The toolchain will treat such data as small regardless of the
<samp><span class="option">-G</span></samp> setting.
<p>On startup, systems that support a small data area are expected to
initialize register <code>$28</code>, also known as <code>$gp</code>, in such a
way that small data can be accessed using a 16-bit offset from that
register. For example, when ‘<samp><span class="samp">addr</span></samp>’ is small data,
the ‘<samp><span class="samp">dla $4,addr</span></samp>’ instruction above is equivalent to:
<pre class="smallexample"> daddiu $4,$28,%gp_rel(addr)
</pre>
<p>Small data is not supported for SVR4-style PIC.
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