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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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| <a name="Comparison-Operations"></a> |
| <h3 class="section">13.10 Comparison Operations</h3> |
| <a name="index-RTL-comparison-operations"></a> |
| |
| <p>Comparison operators test a relation on two operands and are considered |
| to represent a machine-dependent nonzero value described by, but not |
| necessarily equal to, <code>STORE_FLAG_VALUE</code> (see <a href="Misc.html#Misc">Misc</a>) |
| if the relation holds, or zero if it does not, for comparison operators |
| whose results have a ‘MODE_INT’ mode, |
| <code>FLOAT_STORE_FLAG_VALUE</code> (see <a href="Misc.html#Misc">Misc</a>) if the relation holds, or |
| zero if it does not, for comparison operators that return floating-point |
| values, and a vector of either <code>VECTOR_STORE_FLAG_VALUE</code> (see <a href="Misc.html#Misc">Misc</a>) |
| if the relation holds, or of zeros if it does not, for comparison operators |
| that return vector results. |
| The mode of the comparison operation is independent of the mode |
| of the data being compared. If the comparison operation is being tested |
| (e.g., the first operand of an <code>if_then_else</code>), the mode must be |
| <code>VOIDmode</code>. |
| </p> |
| <a name="index-condition-codes"></a> |
| <p>There are two ways that comparison operations may be used. The |
| comparison operators may be used to compare the condition codes |
| <code>(cc0)</code> against zero, as in <code>(eq (cc0) (const_int 0))</code>. Such |
| a construct actually refers to the result of the preceding instruction |
| in which the condition codes were set. The instruction setting the |
| condition code must be adjacent to the instruction using the condition |
| code; only <code>note</code> insns may separate them. |
| </p> |
| <p>Alternatively, a comparison operation may directly compare two data |
| objects. The mode of the comparison is determined by the operands; they |
| must both be valid for a common machine mode. A comparison with both |
| operands constant would be invalid as the machine mode could not be |
| deduced from it, but such a comparison should never exist in RTL due to |
| constant folding. |
| </p> |
| <p>In the example above, if <code>(cc0)</code> were last set to |
| <code>(compare <var>x</var> <var>y</var>)</code>, the comparison operation is |
| identical to <code>(eq <var>x</var> <var>y</var>)</code>. Usually only one style |
| of comparisons is supported on a particular machine, but the combine |
| pass will try to merge the operations to produce the <code>eq</code> shown |
| in case it exists in the context of the particular insn involved. |
| </p> |
| <p>Inequality comparisons come in two flavors, signed and unsigned. Thus, |
| there are distinct expression codes <code>gt</code> and <code>gtu</code> for signed and |
| unsigned greater-than. These can produce different results for the same |
| pair of integer values: for example, 1 is signed greater-than -1 but not |
| unsigned greater-than, because -1 when regarded as unsigned is actually |
| <code>0xffffffff</code> which is greater than 1. |
| </p> |
| <p>The signed comparisons are also used for floating point values. Floating |
| point comparisons are distinguished by the machine modes of the operands. |
| </p> |
| <dl compact="compact"> |
| <dd><a name="index-eq"></a> |
| <a name="index-equal"></a> |
| </dd> |
| <dt><code>(eq:<var>m</var> <var>x</var> <var>y</var>)</code></dt> |
| <dd><p><code>STORE_FLAG_VALUE</code> if the values represented by <var>x</var> and <var>y</var> |
| are equal, otherwise 0. |
| </p> |
| <a name="index-ne"></a> |
| <a name="index-not-equal"></a> |
| </dd> |
| <dt><code>(ne:<var>m</var> <var>x</var> <var>y</var>)</code></dt> |
| <dd><p><code>STORE_FLAG_VALUE</code> if the values represented by <var>x</var> and <var>y</var> |
| are not equal, otherwise 0. |
| </p> |
| <a name="index-gt"></a> |
| <a name="index-greater-than"></a> |
| </dd> |
| <dt><code>(gt:<var>m</var> <var>x</var> <var>y</var>)</code></dt> |
| <dd><p><code>STORE_FLAG_VALUE</code> if the <var>x</var> is greater than <var>y</var>. If they |
| are fixed-point, the comparison is done in a signed sense. |
| </p> |
| <a name="index-gtu"></a> |
| <a name="index-greater-than-1"></a> |
| <a name="index-unsigned-greater-than"></a> |
| </dd> |
| <dt><code>(gtu:<var>m</var> <var>x</var> <var>y</var>)</code></dt> |
| <dd><p>Like <code>gt</code> but does unsigned comparison, on fixed-point numbers only. |
| </p> |
| <a name="index-lt"></a> |
| <a name="index-less-than"></a> |
| <a name="index-ltu"></a> |
| <a name="index-unsigned-less-than"></a> |
| </dd> |
| <dt><code>(lt:<var>m</var> <var>x</var> <var>y</var>)</code></dt> |
| <dt><code>(ltu:<var>m</var> <var>x</var> <var>y</var>)</code></dt> |
| <dd><p>Like <code>gt</code> and <code>gtu</code> but test for “less than”. |
| </p> |
| <a name="index-ge"></a> |
| <a name="index-greater-than-2"></a> |
| <a name="index-geu"></a> |
| <a name="index-unsigned-greater-than-1"></a> |
| </dd> |
| <dt><code>(ge:<var>m</var> <var>x</var> <var>y</var>)</code></dt> |
| <dt><code>(geu:<var>m</var> <var>x</var> <var>y</var>)</code></dt> |
| <dd><p>Like <code>gt</code> and <code>gtu</code> but test for “greater than or equal”. |
| </p> |
| <a name="index-le"></a> |
| <a name="index-less-than-or-equal"></a> |
| <a name="index-leu"></a> |
| <a name="index-unsigned-less-than-1"></a> |
| </dd> |
| <dt><code>(le:<var>m</var> <var>x</var> <var>y</var>)</code></dt> |
| <dt><code>(leu:<var>m</var> <var>x</var> <var>y</var>)</code></dt> |
| <dd><p>Like <code>gt</code> and <code>gtu</code> but test for “less than or equal”. |
| </p> |
| <a name="index-if_005fthen_005felse"></a> |
| </dd> |
| <dt><code>(if_then_else <var>cond</var> <var>then</var> <var>else</var>)</code></dt> |
| <dd><p>This is not a comparison operation but is listed here because it is |
| always used in conjunction with a comparison operation. To be |
| precise, <var>cond</var> is a comparison expression. This expression |
| represents a choice, according to <var>cond</var>, between the value |
| represented by <var>then</var> and the one represented by <var>else</var>. |
| </p> |
| <p>On most machines, <code>if_then_else</code> expressions are valid only |
| to express conditional jumps. |
| </p> |
| <a name="index-cond"></a> |
| </dd> |
| <dt><code>(cond [<var>test1</var> <var>value1</var> <var>test2</var> <var>value2</var> …] <var>default</var>)</code></dt> |
| <dd><p>Similar to <code>if_then_else</code>, but more general. Each of <var>test1</var>, |
| <var>test2</var>, … is performed in turn. The result of this expression is |
| the <var>value</var> corresponding to the first nonzero test, or <var>default</var> if |
| none of the tests are nonzero expressions. |
| </p> |
| <p>This is currently not valid for instruction patterns and is supported only |
| for insn attributes. See <a href="Insn-Attributes.html#Insn-Attributes">Insn Attributes</a>. |
| </p></dd> |
| </dl> |
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