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<title>MIPS NaN Encodings - Using as</title>
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<a name="MIPS-NaN-Encodings"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="MIPS-Option-Stack.html#MIPS-Option-Stack">MIPS Option Stack</a>,
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Previous: <a rel="previous" accesskey="p" href="MIPS-FP-ABIs.html#MIPS-FP-ABIs">MIPS FP ABIs</a>,
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Up: <a rel="up" accesskey="u" href="MIPS_002dDependent.html#MIPS_002dDependent">MIPS-Dependent</a>
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<h4 class="subsection">9.27.10 Directives to record which NaN encoding is being used</h4>
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<p><a name="index-MIPS-IEEE-754-NaN-data-encoding-selection-1512"></a><a name="index-g_t_0040code_007b_002enan_007d-directive_002c-MIPS-1513"></a>The IEEE 754 floating-point standard defines two types of not-a-number
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(NaN) data: “signalling” NaNs and “quiet” NaNs. The original version
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of the standard did not specify how these two types should be
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distinguished. Most implementations followed the i387 model, in which
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the first bit of the significand is set for quiet NaNs and clear for
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signalling NaNs. However, the original MIPS implementation assigned the
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opposite meaning to the bit, so that it was set for signalling NaNs and
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clear for quiet NaNs.
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<p>The 2008 revision of the standard formally suggested the i387 choice
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and as from Sep 2012 the current release of the MIPS architecture
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therefore optionally supports that form. Code that uses one NaN encoding
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would usually be incompatible with code that uses the other NaN encoding,
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so MIPS ELF objects have a flag (<code>EF_MIPS_NAN2008</code>) to record which
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encoding is being used.
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<p>Assembly files can use the <code>.nan</code> directive to select between the
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two encodings. ‘<samp><span class="samp">.nan 2008</span></samp>’ says that the assembly file uses the
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IEEE 754-2008 encoding while ‘<samp><span class="samp">.nan legacy</span></samp>’ says that the file uses
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the original MIPS encoding. If several <code>.nan</code> directives are given,
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the final setting is the one that is used.
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<p>The command-line options <samp><span class="option">-mnan=legacy</span></samp> and <samp><span class="option">-mnan=2008</span></samp>
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can be used instead of ‘<samp><span class="samp">.nan legacy</span></samp>’ and ‘<samp><span class="samp">.nan 2008</span></samp>’
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respectively. However, any <code>.nan</code> directive overrides the
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command-line setting.
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<p>‘<samp><span class="samp">.nan legacy</span></samp>’ is the default if no <code>.nan</code> directive or
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<samp><span class="option">-mnan</span></samp> option is given.
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<p>Note that <span class="sc">gnu</span> <code>as</code> does not produce NaNs itself and
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therefore these directives do not affect code generation. They simply
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control the setting of the <code>EF_MIPS_NAN2008</code> flag.
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<p>Traditional MIPS assemblers do not support these directives.
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