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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-1578"></a><a name="index-g_t_0040code_007bgp_007d-register_002c-MIPS-1579"></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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