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| <h4 class="subsection">9.27.10 Directives to record which NaN encoding is being used</h4> |
| |
| <p><a name="index-MIPS-IEEE-754-NaN-data-encoding-selection-1596"></a><a name="index-g_t_0040code_007b_002enan_007d-directive_002c-MIPS-1597"></a>The IEEE 754 floating-point standard defines two types of not-a-number |
| (NaN) data: “signalling” NaNs and “quiet” NaNs. The original version |
| of the standard did not specify how these two types should be |
| distinguished. Most implementations followed the i387 model, in which |
| the first bit of the significand is set for quiet NaNs and clear for |
| signalling NaNs. However, the original MIPS implementation assigned the |
| opposite meaning to the bit, so that it was set for signalling NaNs and |
| clear for quiet NaNs. |
| |
| <p>The 2008 revision of the standard formally suggested the i387 choice |
| and as from Sep 2012 the current release of the MIPS architecture |
| therefore optionally supports that form. Code that uses one NaN encoding |
| would usually be incompatible with code that uses the other NaN encoding, |
| so MIPS ELF objects have a flag (<code>EF_MIPS_NAN2008</code>) to record which |
| encoding is being used. |
| |
| <p>Assembly files can use the <code>.nan</code> directive to select between the |
| two encodings. ‘<samp><span class="samp">.nan 2008</span></samp>’ says that the assembly file uses the |
| IEEE 754-2008 encoding while ‘<samp><span class="samp">.nan legacy</span></samp>’ says that the file uses |
| the original MIPS encoding. If several <code>.nan</code> directives are given, |
| the final setting is the one that is used. |
| |
| <p>The command-line options <samp><span class="option">-mnan=legacy</span></samp> and <samp><span class="option">-mnan=2008</span></samp> |
| can be used instead of ‘<samp><span class="samp">.nan legacy</span></samp>’ and ‘<samp><span class="samp">.nan 2008</span></samp>’ |
| respectively. However, any <code>.nan</code> directive overrides the |
| command-line setting. |
| |
| <p>‘<samp><span class="samp">.nan legacy</span></samp>’ is the default if no <code>.nan</code> directive or |
| <samp><span class="option">-mnan</span></samp> option is given. |
| |
| <p>Note that <span class="sc">gnu</span> <code>as</code> does not produce NaNs itself and |
| therefore these directives do not affect code generation. They simply |
| control the setting of the <code>EF_MIPS_NAN2008</code> flag. |
| |
| <p>Traditional MIPS assemblers do not support these directives. |
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