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| <h4 class="subsection">9.24.4 Directives to override the size of symbols</h4> |
| |
| <p><a name="index-g_t_0040code_007b_002eset-sym32_007d-1271"></a><a name="index-g_t_0040code_007b_002eset-nosym32_007d-1272"></a>The n64 ABI allows symbols to have any 64-bit value. Although this |
| provides a great deal of flexibility, it means that some macros have |
| much longer expansions than their 32-bit counterparts. For example, |
| the non-PIC expansion of ‘<samp><span class="samp">dla $4,sym</span></samp>’ is usually: |
| |
| <pre class="smallexample"> lui $4,%highest(sym) |
| lui $1,%hi(sym) |
| daddiu $4,$4,%higher(sym) |
| daddiu $1,$1,%lo(sym) |
| dsll32 $4,$4,0 |
| daddu $4,$4,$1 |
| </pre> |
| <p>whereas the 32-bit expansion is simply: |
| |
| <pre class="smallexample"> lui $4,%hi(sym) |
| daddiu $4,$4,%lo(sym) |
| </pre> |
| <p>n64 code is sometimes constructed in such a way that all symbolic |
| constants are known to have 32-bit values, and in such cases, it's |
| preferable to use the 32-bit expansion instead of the 64-bit |
| expansion. |
| |
| <p>You can use the <code>.set sym32</code> directive to tell the assembler |
| that, from this point on, all expressions of the form |
| ‘<samp><var>symbol</var></samp>’ or ‘<samp><var>symbol</var><span class="samp"> + </span><var>offset</var></samp>’ |
| have 32-bit values. For example: |
| |
| <pre class="smallexample"> .set sym32 |
| dla $4,sym |
| lw $4,sym+16 |
| sw $4,sym+0x8000($4) |
| </pre> |
| <p>will cause the assembler to treat ‘<samp><span class="samp">sym</span></samp>’, <code>sym+16</code> and |
| <code>sym+0x8000</code> as 32-bit values. The handling of non-symbolic |
| addresses is not affected. |
| |
| <p>The directive <code>.set nosym32</code> ends a <code>.set sym32</code> block and |
| reverts to the normal behavior. It is also possible to change the |
| symbol size using the command-line options <samp><span class="option">-msym32</span></samp> and |
| <samp><span class="option">-mno-sym32</span></samp>. |
| |
| <p>These options and directives are always accepted, but at present, |
| they have no effect for anything other than n64. |
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