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| 58 | <span id="Invalid-Optimizations"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Exception-Flags.html" accesskey="n" rel="next">Exception Flags</a>, Previous: <a href="Special-Float-Values.html" accesskey="p" rel="prev">Special Float Values</a>, Up: <a href="Floating-Point-in-Depth.html" accesskey="u" rel="up">Floating Point in Depth</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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| 61 | </div>
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| 62 | <hr>
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| 63 | <span id="Invalid-Optimizations-1"></span><h3 class="section">28.4 Invalid Optimizations</h3>
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| 64 | <span id="index-invalid-optimizations-in-floating_002dpoint-arithmetic"></span>
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| 65 | <span id="index-floating_002dpoint-arithmetic-invalid-optimizations"></span>
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| 66 |
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| 67 | <p>Signed zeros, Infinity, and NaN invalidate some optimizations by
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| 68 | programmers and compilers that might otherwise have seemed obvious:
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| 69 | </p>
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| 70 | <ul>
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| 71 | <li> <code>x + 0</code> and <code>x - 0</code> are not the same as <code>x</code> when
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| 72 | <code>x</code> is zero, because the result depends on the rounding rule.
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| 73 | See <a href="Rounding.html">Rounding</a>, for more about rounding rules.
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| 74 |
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| 75 | </li><li> <code>x * 0.0</code> is not the same as <code>0.0</code> when <code>x</code> is
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| 76 | Infinity, a NaN, or negative zero.
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| 77 |
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| 78 | </li><li> <code>x / x</code> is not the same as <code>1.0</code> when <code>x</code> is Infinity,
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| 79 | a NaN, or zero.
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| 80 |
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| 81 | </li><li> <code>(x - y)</code> is not the same as <code>-(y - x)</code> because when the
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| 82 | operands are finite and equal, one evaluates to <code>+0</code> and the
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| 83 | other to <code>-0</code>.
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| 84 |
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| 85 | </li><li> <code>x - x</code> is not the same as <code>0.0</code> when <var>x</var> is Infinity or
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| 86 | a NaN.
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| 87 |
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| 88 | </li><li> <code>x == x</code> and <code>x != x</code> are not equivalent to <code>1</code> and
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| 89 | <code>0</code> when <var>x</var> is a NaN.
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| 90 |
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| 91 | </li><li> <code>x < y</code> and <code>isless (x, y)</code> are not equivalent, because the
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| 92 | first sets a sticky exception flag (see <a href="Exception-Flags.html">Exception Flags</a>) when an
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| 93 | operand is a NaN, whereas the second does not affect that flag. The
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| 94 | same holds for the other <code>isxxx</code> functions that are companions to
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| 95 | relational operators. See <a href="https://www.gnu.org/software/libc/manual/html_node/FP-Comparison-Functions.html#FP-Comparison-Functions">FP Comparison Functions</a> in <cite>The
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| 96 | GNU C Library Reference Manual</cite>.
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| 97 |
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| 98 | </li></ul>
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| 99 |
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| 100 | <p>The <samp>-funsafe-math-optimizations</samp> option enables
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| 101 | these optimizations.
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| 102 | </p>
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| 103 |
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