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145 lines
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145 lines
8.7 KiB
HTML
4 years ago
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<html lang="en">
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<title>Comparisons - GNU Compiler Collection (GCC) Internals</title>
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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="RTL.html#RTL" title="RTL">
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<link rel="prev" href="Arithmetic.html#Arithmetic" title="Arithmetic">
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<link rel="next" href="Bit_002dFields.html#Bit_002dFields" title="Bit-Fields">
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<!--
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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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</head>
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<body>
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<div class="node">
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<a name="Comparisons"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Bit_002dFields.html#Bit_002dFields">Bit-Fields</a>,
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Previous: <a rel="previous" accesskey="p" href="Arithmetic.html#Arithmetic">Arithmetic</a>,
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Up: <a rel="up" accesskey="u" href="RTL.html#RTL">RTL</a>
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<hr>
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</div>
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<h3 class="section">13.10 Comparison Operations</h3>
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<p><a name="index-RTL-comparison-operations-2976"></a>
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Comparison operators test a relation on two operands and are considered
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to represent a machine-dependent nonzero value described by, but not
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necessarily equal to, <code>STORE_FLAG_VALUE</code> (see <a href="Misc.html#Misc">Misc</a>)
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if the relation holds, or zero if it does not, for comparison operators
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whose results have a `MODE_INT' mode,
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<code>FLOAT_STORE_FLAG_VALUE</code> (see <a href="Misc.html#Misc">Misc</a>) if the relation holds, or
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zero if it does not, for comparison operators that return floating-point
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values, and a vector of either <code>VECTOR_STORE_FLAG_VALUE</code> (see <a href="Misc.html#Misc">Misc</a>)
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if the relation holds, or of zeros if it does not, for comparison operators
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that return vector results.
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The mode of the comparison operation is independent of the mode
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of the data being compared. If the comparison operation is being tested
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(e.g., the first operand of an <code>if_then_else</code>), the mode must be
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<code>VOIDmode</code>.
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<p><a name="index-condition-codes-2977"></a>There are two ways that comparison operations may be used. The
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comparison operators may be used to compare the condition codes
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<code>(cc0)</code> against zero, as in <code>(eq (cc0) (const_int 0))</code>. Such
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a construct actually refers to the result of the preceding instruction
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in which the condition codes were set. The instruction setting the
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condition code must be adjacent to the instruction using the condition
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code; only <code>note</code> insns may separate them.
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<p>Alternatively, a comparison operation may directly compare two data
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objects. The mode of the comparison is determined by the operands; they
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must both be valid for a common machine mode. A comparison with both
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operands constant would be invalid as the machine mode could not be
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deduced from it, but such a comparison should never exist in RTL due to
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constant folding.
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<p>In the example above, if <code>(cc0)</code> were last set to
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<code>(compare </code><var>x</var> <var>y</var><code>)</code>, the comparison operation is
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identical to <code>(eq </code><var>x</var> <var>y</var><code>)</code>. Usually only one style
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of comparisons is supported on a particular machine, but the combine
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pass will try to merge the operations to produce the <code>eq</code> shown
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in case it exists in the context of the particular insn involved.
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<p>Inequality comparisons come in two flavors, signed and unsigned. Thus,
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there are distinct expression codes <code>gt</code> and <code>gtu</code> for signed and
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unsigned greater-than. These can produce different results for the same
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pair of integer values: for example, 1 is signed greater-than −1 but not
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unsigned greater-than, because −1 when regarded as unsigned is actually
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<code>0xffffffff</code> which is greater than 1.
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<p>The signed comparisons are also used for floating point values. Floating
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point comparisons are distinguished by the machine modes of the operands.
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<a name="index-eq-2978"></a>
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<a name="index-equal-2979"></a>
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<dl><dt><code>(eq:</code><var>m</var> <var>x</var> <var>y</var><code>)</code><dd><code>STORE_FLAG_VALUE</code> if the values represented by <var>x</var> and <var>y</var>
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are equal, otherwise 0.
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<p><a name="index-ne-2980"></a><a name="index-not-equal-2981"></a><br><dt><code>(ne:</code><var>m</var> <var>x</var> <var>y</var><code>)</code><dd><code>STORE_FLAG_VALUE</code> if the values represented by <var>x</var> and <var>y</var>
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are not equal, otherwise 0.
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<p><a name="index-gt-2982"></a><a name="index-greater-than-2983"></a><br><dt><code>(gt:</code><var>m</var> <var>x</var> <var>y</var><code>)</code><dd><code>STORE_FLAG_VALUE</code> if the <var>x</var> is greater than <var>y</var>. If they
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are fixed-point, the comparison is done in a signed sense.
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<p><a name="index-gtu-2984"></a><a name="index-greater-than-2985"></a><a name="index-unsigned-greater-than-2986"></a><br><dt><code>(gtu:</code><var>m</var> <var>x</var> <var>y</var><code>)</code><dd>Like <code>gt</code> but does unsigned comparison, on fixed-point numbers only.
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<p><a name="index-lt-2987"></a><a name="index-less-than-2988"></a><a name="index-ltu-2989"></a><a name="index-unsigned-less-than-2990"></a><br><dt><code>(lt:</code><var>m</var> <var>x</var> <var>y</var><code>)</code><dt><code>(ltu:</code><var>m</var> <var>x</var> <var>y</var><code>)</code><dd>Like <code>gt</code> and <code>gtu</code> but test for “less than”.
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<p><a name="index-ge-2991"></a><a name="index-greater-than-2992"></a><a name="index-geu-2993"></a><a name="index-unsigned-greater-than-2994"></a><br><dt><code>(ge:</code><var>m</var> <var>x</var> <var>y</var><code>)</code><dt><code>(geu:</code><var>m</var> <var>x</var> <var>y</var><code>)</code><dd>Like <code>gt</code> and <code>gtu</code> but test for “greater than or equal”.
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<p><a name="index-le-2995"></a><a name="index-less-than-or-equal-2996"></a><a name="index-leu-2997"></a><a name="index-unsigned-less-than-2998"></a><br><dt><code>(le:</code><var>m</var> <var>x</var> <var>y</var><code>)</code><dt><code>(leu:</code><var>m</var> <var>x</var> <var>y</var><code>)</code><dd>Like <code>gt</code> and <code>gtu</code> but test for “less than or equal”.
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<p><a name="index-if_005fthen_005felse-2999"></a><br><dt><code>(if_then_else </code><var>cond</var> <var>then</var> <var>else</var><code>)</code><dd>This is not a comparison operation but is listed here because it is
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always used in conjunction with a comparison operation. To be
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precise, <var>cond</var> is a comparison expression. This expression
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represents a choice, according to <var>cond</var>, between the value
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represented by <var>then</var> and the one represented by <var>else</var>.
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<p>On most machines, <code>if_then_else</code> expressions are valid only
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to express conditional jumps.
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<p><a name="index-cond-3000"></a><br><dt><code>(cond [</code><var>test1</var> <var>value1</var> <var>test2</var> <var>value2</var><code> ...] </code><var>default</var><code>)</code><dd>Similar to <code>if_then_else</code>, but more general. Each of <var>test1</var>,
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<var>test2</var>, <small class="dots">...</small> is performed in turn. The result of this expression is
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the <var>value</var> corresponding to the first nonzero test, or <var>default</var> if
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none of the tests are nonzero expressions.
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<p>This is currently not valid for instruction patterns and is supported only
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for insn attributes. See <a href="Insn-Attributes.html#Insn-Attributes">Insn Attributes</a>.
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</dl>
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</body></html>
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