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160 lines
8.2 KiB
HTML
160 lines
8.2 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Floating Point - GNU Compiler Collection (GCC) Internals</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="GNU Compiler Collection (GCC) Internals">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Target-Macros.html#Target-Macros" title="Target Macros">
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<link rel="prev" href="Debugging-Info.html#Debugging-Info" title="Debugging Info">
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<link rel="next" href="Mode-Switching.html#Mode-Switching" title="Mode Switching">
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Copyright (C) 1988-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being ``Funding Free Software'', the Front-Cover
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(see below). A copy of the license is included in the section entitled
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``GNU Free Documentation License''.
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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</head>
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<body>
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<div class="node">
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<a name="Floating-Point"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Mode-Switching.html#Mode-Switching">Mode Switching</a>,
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Previous: <a rel="previous" accesskey="p" href="Debugging-Info.html#Debugging-Info">Debugging Info</a>,
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Up: <a rel="up" accesskey="u" href="Target-Macros.html#Target-Macros">Target Macros</a>
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<hr>
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</div>
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<h3 class="section">17.22 Cross Compilation and Floating Point</h3>
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<p><a name="index-cross-compilation-and-floating-point-4804"></a><a name="index-floating-point-and-cross-compilation-4805"></a>
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While all modern machines use twos-complement representation for integers,
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there are a variety of representations for floating point numbers. This
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means that in a cross-compiler the representation of floating point numbers
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in the compiled program may be different from that used in the machine
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doing the compilation.
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<p>Because different representation systems may offer different amounts of
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range and precision, all floating point constants must be represented in
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the target machine's format. Therefore, the cross compiler cannot
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safely use the host machine's floating point arithmetic; it must emulate
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the target's arithmetic. To ensure consistency, GCC always uses
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emulation to work with floating point values, even when the host and
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target floating point formats are identical.
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<p>The following macros are provided by <samp><span class="file">real.h</span></samp> for the compiler to
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use. All parts of the compiler which generate or optimize
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floating-point calculations must use these macros. They may evaluate
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their operands more than once, so operands must not have side effects.
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<div class="defun">
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— Macro: <b>REAL_VALUE_TYPE</b><var><a name="index-REAL_005fVALUE_005fTYPE-4806"></a></var><br>
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<blockquote><p>The C data type to be used to hold a floating point value in the target
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machine's format. Typically this is a <code>struct</code> containing an
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array of <code>HOST_WIDE_INT</code>, but all code should treat it as an opaque
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quantity.
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</p></blockquote></div>
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<div class="defun">
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— Macro: int <b>REAL_VALUES_EQUAL</b> (<var>REAL_VALUE_TYPE x, REAL_VALUE_TYPE y</var>)<var><a name="index-REAL_005fVALUES_005fEQUAL-4807"></a></var><br>
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<blockquote><p>Compares for equality the two values, <var>x</var> and <var>y</var>. If the target
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floating point format supports negative zeroes and/or NaNs,
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‘<samp><span class="samp">REAL_VALUES_EQUAL (-0.0, 0.0)</span></samp>’ is true, and
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‘<samp><span class="samp">REAL_VALUES_EQUAL (NaN, NaN)</span></samp>’ is false.
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</p></blockquote></div>
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<div class="defun">
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— Macro: int <b>REAL_VALUES_LESS</b> (<var>REAL_VALUE_TYPE x, REAL_VALUE_TYPE y</var>)<var><a name="index-REAL_005fVALUES_005fLESS-4808"></a></var><br>
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<blockquote><p>Tests whether <var>x</var> is less than <var>y</var>.
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</p></blockquote></div>
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<div class="defun">
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— Macro: HOST_WIDE_INT <b>REAL_VALUE_FIX</b> (<var>REAL_VALUE_TYPE x</var>)<var><a name="index-REAL_005fVALUE_005fFIX-4809"></a></var><br>
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<blockquote><p>Truncates <var>x</var> to a signed integer, rounding toward zero.
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</p></blockquote></div>
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<div class="defun">
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— Macro: unsigned HOST_WIDE_INT <b>REAL_VALUE_UNSIGNED_FIX</b> (<var>REAL_VALUE_TYPE x</var>)<var><a name="index-REAL_005fVALUE_005fUNSIGNED_005fFIX-4810"></a></var><br>
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<blockquote><p>Truncates <var>x</var> to an unsigned integer, rounding toward zero. If
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<var>x</var> is negative, returns zero.
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</p></blockquote></div>
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<div class="defun">
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— Macro: REAL_VALUE_TYPE <b>REAL_VALUE_ATOF</b> (<var>const char *string, machine_mode mode</var>)<var><a name="index-REAL_005fVALUE_005fATOF-4811"></a></var><br>
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<blockquote><p>Converts <var>string</var> into a floating point number in the target machine's
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representation for mode <var>mode</var>. This routine can handle both
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decimal and hexadecimal floating point constants, using the syntax
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defined by the C language for both.
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</p></blockquote></div>
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<div class="defun">
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— Macro: int <b>REAL_VALUE_NEGATIVE</b> (<var>REAL_VALUE_TYPE x</var>)<var><a name="index-REAL_005fVALUE_005fNEGATIVE-4812"></a></var><br>
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<blockquote><p>Returns 1 if <var>x</var> is negative (including negative zero), 0 otherwise.
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</p></blockquote></div>
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<div class="defun">
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— Macro: int <b>REAL_VALUE_ISINF</b> (<var>REAL_VALUE_TYPE x</var>)<var><a name="index-REAL_005fVALUE_005fISINF-4813"></a></var><br>
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<blockquote><p>Determines whether <var>x</var> represents infinity (positive or negative).
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</p></blockquote></div>
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<div class="defun">
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— Macro: int <b>REAL_VALUE_ISNAN</b> (<var>REAL_VALUE_TYPE x</var>)<var><a name="index-REAL_005fVALUE_005fISNAN-4814"></a></var><br>
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<blockquote><p>Determines whether <var>x</var> represents a “NaN” (not-a-number).
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</p></blockquote></div>
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<div class="defun">
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— Macro: void <b>REAL_ARITHMETIC</b> (<var>REAL_VALUE_TYPE output, enum tree_code code, REAL_VALUE_TYPE x, REAL_VALUE_TYPE y</var>)<var><a name="index-REAL_005fARITHMETIC-4815"></a></var><br>
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<blockquote><p>Calculates an arithmetic operation on the two floating point values
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<var>x</var> and <var>y</var>, storing the result in <var>output</var> (which must be a
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variable).
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<p>The operation to be performed is specified by <var>code</var>. Only the
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following codes are supported: <code>PLUS_EXPR</code>, <code>MINUS_EXPR</code>,
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<code>MULT_EXPR</code>, <code>RDIV_EXPR</code>, <code>MAX_EXPR</code>, <code>MIN_EXPR</code>.
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<p>If <code>REAL_ARITHMETIC</code> is asked to evaluate division by zero and the
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target's floating point format cannot represent infinity, it will call
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<code>abort</code>. Callers should check for this situation first, using
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<code>MODE_HAS_INFINITIES</code>. See <a href="Storage-Layout.html#Storage-Layout">Storage Layout</a>.
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</p></blockquote></div>
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<div class="defun">
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— Macro: REAL_VALUE_TYPE <b>REAL_VALUE_NEGATE</b> (<var>REAL_VALUE_TYPE x</var>)<var><a name="index-REAL_005fVALUE_005fNEGATE-4816"></a></var><br>
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<blockquote><p>Returns the negative of the floating point value <var>x</var>.
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</p></blockquote></div>
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<div class="defun">
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— Macro: REAL_VALUE_TYPE <b>REAL_VALUE_ABS</b> (<var>REAL_VALUE_TYPE x</var>)<var><a name="index-REAL_005fVALUE_005fABS-4817"></a></var><br>
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<blockquote><p>Returns the absolute value of <var>x</var>.
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</p></blockquote></div>
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</body></html>
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