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diff --git a/libs/endian/doc/fp_concerns.html b/libs/endian/doc/fp_concerns.html new file mode 100644 index 000000000..5c5f67c81 --- /dev/null +++ b/libs/endian/doc/fp_concerns.html @@ -0,0 +1,97 @@ +<html> + +<head> +<meta http-equiv="Content-Language" content="en-us"> +<meta name="GENERATOR" content="Microsoft FrontPage 5.0"> +<meta name="ProgId" content="FrontPage.Editor.Document"> +<meta http-equiv="Content-Type" content="text/html; charset=windows-1252"> +<title>Floating Point Concerns</title> +<link href="styles.css" rel="stylesheet"> +</head> + +<body> + +<h1>Floating Point Concerns</h1> + +<p>Consider this simple implementation of <code>endian_reverse</code>:</p> + +<blockquote> + <pre>template <class T> +inline T endian_reverse(T x) BOOST_NOEXCEPT +{ + std::reverse(reinterpret_cast<unsigned char*>(&x), + reinterpret_cast<unsigned char*>(&x) + sizeof(T)); + return x; +}</pre> +</blockquote> +<p><b>Under what conditions with this code fail?</b></p> +<p dir="ltr">It will fail if an object of type <code>T</code> has one or more +bit patterns that cause a failure. Failures usually occur when an invalid +or otherwise special bit pattern is loaded into or saved from a hardware +register.</p> +<p dir="ltr">The problem could in theory occur with both integers and floating +point numbers, but the <a href="http://en.wikipedia.org/wiki/Two's_complement"> +two's complement integers</a> ubiquitous in modern computer architectures do not +have any invalid or otherwise special bit patterns that cause failure when +byte-wise reversed.</p> +<p dir="ltr">But floating point numbers are a different story. Even if we limit +discussion to IEEE 754 (aka ISO/IEC/IEEE 60559) binary representations of 4 and +8 byte sizes, several problems are easy to demonstrate:</p> +<ul> + <li dir="ltr"> + <p dir="ltr">...</li> +</ul> +<h2 dir="ltr">Safe interfaces and possible reference implementations</h2> +<h3 dir="ltr">In-place interface</h3> +<blockquote> + <pre dir="ltr">template <class T> +inline void endian_reverse_inplace(T& x) +{ + std::reverse(reinterpret_cast<unsigned char*>(&x), + reinterpret_cast<unsigned char*>(&x) + sizeof(T)); +}</pre> +</blockquote> +<p dir="ltr">This is the same as the current (i.e integer) customization point +interface, so there is no need for any change.</p> +<p dir="ltr"><b>Warning:</b> Even thought <code>x</code> may have had a valid +value on the originating platform, after calling this function the value of +<code>x</code> may differ or be invalid on this platform.</p> +<h3 dir="ltr">Copy interface</h3> +<blockquote> + <pre dir="ltr">template <class T> +inline void endian_reverse_copy(const T& from, T& to) +{ + std::reverse_copy(reinterpret_cast<const unsigned char*>(&from), + reinterpret_cast<const unsigned char*>(&from) + sizeof(T), + reinterpret_cast<unsigned char*>(&to)); +}</pre> +</blockquote> +<p><b>Warning:</b> Even thought <code>from</code> may have been a valid value on +the originating platform, after calling this function the value of <code>to</code> +may differ or be invalid on this platform.</p> +<h3>Return-by-value interface</h3> +<blockquote> + <pre>template <class T> +inline T endian_reverse_to_native(<span style="background-color: #FFFF00">const</span><span style="background-color: #FFFF00"> T&</span> x) BOOST_NOEXCEPT +{ + T tmp; + std::reverse_copy(reinterpret_cast<const unsigned char*>(&x), + reinterpret_cast<const unsigned char*>(&x) + sizeof(T), + reinterpret_cast<unsigned char*>(&tmp)); + return tmp; +}</pre> +</blockquote> +<p><b>Warning:</b> Even thought <code>x</code> may have had a valid value on the +originating platform, the value of returned by this function may differ or be +invalid on this platform.</p> +<h2>Acknowledgements</h2> +<hr> +<p>Last revised: <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->27 March, 2015<!--webbot bot="Timestamp" endspan i-checksum="28924" --></p> +<p>© Copyright Beman Dawes, 2015</p> +<p>Distributed under the Boost Software License, Version 1.0. See <a href="http://www.boost.org/LICENSE_1_0.txt">www.boost.org/ LICENSE_1_0.txt</a></p> + +<p> </p> + +</body> + +</html>
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