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<head>
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<title>Comparisons (GNU Compiler Collection (GCC) Internals)</title>
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="RTL.html#RTL" rel="up" title="RTL">
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<link href="Bit_002dFields.html#Bit_002dFields" rel="next" title="Bit-Fields">
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<link href="Arithmetic.html#Arithmetic" rel="prev" title="Arithmetic">
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</head>
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<body lang="en">
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<a name="Comparisons"></a>
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<div class="header">
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<p>
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Next: <a href="Bit_002dFields.html#Bit_002dFields" accesskey="n" rel="next">Bit-Fields</a>, Previous: <a href="Arithmetic.html#Arithmetic" accesskey="p" rel="prev">Arithmetic</a>, Up: <a href="RTL.html#RTL" accesskey="u" rel="up">RTL</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Comparison-Operations"></a>
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<h3 class="section">14.10 Comparison Operations</h3>
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<a name="index-RTL-comparison-operations"></a>
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<p>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>
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<a name="index-condition-codes"></a>
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<p>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>
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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>
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<p>In the example above, if <code>(cc0)</code> were last set to
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<code>(compare <var>x</var> <var>y</var>)</code>, the comparison operation is
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identical to <code>(eq <var>x</var> <var>y</var>)</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>
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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>
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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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</p>
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<dl compact="compact">
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<dd><a name="index-eq"></a>
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<a name="index-equal"></a>
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</dd>
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<dt><code>(eq:<var>m</var> <var>x</var> <var>y</var>)</code></dt>
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<dd><p><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>
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<a name="index-ne"></a>
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<a name="index-not-equal"></a>
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</dd>
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<dt><code>(ne:<var>m</var> <var>x</var> <var>y</var>)</code></dt>
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<dd><p><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>
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<a name="index-gt"></a>
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<a name="index-greater-than"></a>
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</dd>
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<dt><code>(gt:<var>m</var> <var>x</var> <var>y</var>)</code></dt>
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<dd><p><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>
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<a name="index-gtu"></a>
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<a name="index-greater-than-1"></a>
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<a name="index-unsigned-greater-than"></a>
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</dd>
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<dt><code>(gtu:<var>m</var> <var>x</var> <var>y</var>)</code></dt>
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<dd><p>Like <code>gt</code> but does unsigned comparison, on fixed-point numbers only.
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</p>
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<a name="index-lt"></a>
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<a name="index-less-than"></a>
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<a name="index-ltu"></a>
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<a name="index-unsigned-less-than"></a>
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</dd>
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<dt><code>(lt:<var>m</var> <var>x</var> <var>y</var>)</code></dt>
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<dt><code>(ltu:<var>m</var> <var>x</var> <var>y</var>)</code></dt>
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<dd><p>Like <code>gt</code> and <code>gtu</code> but test for “less than”.
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</p>
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<a name="index-ge"></a>
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<a name="index-greater-than-2"></a>
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<a name="index-geu"></a>
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<a name="index-unsigned-greater-than-1"></a>
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</dd>
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<dt><code>(ge:<var>m</var> <var>x</var> <var>y</var>)</code></dt>
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<dt><code>(geu:<var>m</var> <var>x</var> <var>y</var>)</code></dt>
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<dd><p>Like <code>gt</code> and <code>gtu</code> but test for “greater than or equal”.
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</p>
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<a name="index-le"></a>
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<a name="index-less-than-or-equal"></a>
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<a name="index-leu"></a>
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<a name="index-unsigned-less-than-1"></a>
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</dd>
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<dt><code>(le:<var>m</var> <var>x</var> <var>y</var>)</code></dt>
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<dt><code>(leu:<var>m</var> <var>x</var> <var>y</var>)</code></dt>
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<dd><p>Like <code>gt</code> and <code>gtu</code> but test for “less than or equal”.
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</p>
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<a name="index-if_005fthen_005felse"></a>
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</dd>
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<dt><code>(if_then_else <var>cond</var> <var>then</var> <var>else</var>)</code></dt>
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<dd><p>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>
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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>
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<a name="index-cond"></a>
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</dd>
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<dt><code>(cond [<var>test1</var> <var>value1</var> <var>test2</var> <var>value2</var> …] <var>default</var>)</code></dt>
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<dd><p>Similar to <code>if_then_else</code>, but more general. Each of <var>test1</var>,
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<var>test2</var>, … 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>
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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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</p></dd>
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</dl>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Bit_002dFields.html#Bit_002dFields" accesskey="n" rel="next">Bit-Fields</a>, Previous: <a href="Arithmetic.html#Arithmetic" accesskey="p" rel="prev">Arithmetic</a>, Up: <a href="RTL.html#RTL" accesskey="u" rel="up">RTL</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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