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165 lines
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165 lines
7.5 KiB
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<head>
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<title>Overview of poly_int (GNU Compiler Collection (GCC) Internals)</title>
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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="Consequences-of-using-poly_005fint.html#Consequences-of-using-poly_005fint" rel="next" title="Consequences of using poly_int">
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<a name="Overview-of-poly_005fint"></a>
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<div class="header">
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<p>
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Next: <a href="Consequences-of-using-poly_005fint.html#Consequences-of-using-poly_005fint" accesskey="n" rel="next">Consequences of using <code>poly_int</code></a>, Up: <a href="poly_005fint.html#poly_005fint" accesskey="u" rel="up">poly_int</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="Overview-of-poly_005fint-1"></a>
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<h3 class="section">10.1 Overview of <code>poly_int</code></h3>
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<a name="index-poly_005fint_002c-runtime-value"></a>
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<p>We define indeterminates <var>x1</var>, …, <var>xn</var> whose values are
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only known at runtime and use polynomials of the form:
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</p>
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<div class="smallexample">
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<pre class="smallexample"><var>c0</var> + <var>c1</var> * <var>x1</var> + … + <var>cn</var> * <var>xn</var>
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</pre></div>
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<p>to represent a size or offset whose value might depend on some
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of these indeterminates. The coefficients <var>c0</var>, …, <var>cn</var>
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are always known at compile time, with the <var>c0</var> term being the
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“constant” part that does not depend on any runtime value.
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</p>
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<p>GCC uses the <code>poly_int</code> class to represent these coefficients.
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The class has two template parameters: the first specifies the number of
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coefficients (<var>n</var> + 1) and the second specifies the type of the
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coefficients. For example, ‘<samp>poly_int<2, unsigned short></samp>’ represents
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a polynomial with two coefficients (and thus one indeterminate), with each
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coefficient having type <code>unsigned short</code>. When <var>n</var> is 0,
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the class degenerates to a single compile-time constant <var>c0</var>.
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</p>
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<a name="index-poly_005fint_002c-template-parameters"></a>
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<a name="index-NUM_005fPOLY_005fINT_005fCOEFFS"></a>
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<p>The number of coefficients needed for compilation is a fixed
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property of each target and is specified by the configuration macro
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<code>NUM_POLY_INT_COEFFS</code>. The default value is 1, since most targets
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do not have such runtime invariants. Targets that need a different
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value should <code>#define</code> the macro in their <samp><var>cpu</var>-modes.def</samp>
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file. See <a href="Back-End.html#Back-End">Back End</a>.
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</p>
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<a name="index-poly_005fint_002c-invariant-range"></a>
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<p><code>poly_int</code> makes the simplifying requirement that each indeterminate
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must be a nonnegative integer. An indeterminate value of 0 should usually
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represent the minimum possible runtime value, with <var>c0</var> specifying
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the value in that case.
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</p>
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<p>For example, when targetting the Arm SVE ISA, the single indeterminate
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represents the number of 128-bit blocks in a vector <em>beyond the minimum
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length of 128 bits</em>. Thus the number of 64-bit doublewords in a vector
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is 2 + 2 * <var>x1</var>. If an aggregate has a single SVE vector and 16
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additional bytes, its total size is 32 + 16 * <var>x1</var> bytes.
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</p>
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<p>The header file <samp>poly-int-types.h</samp> provides typedefs for the
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most common forms of <code>poly_int</code>, all having
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<code>NUM_POLY_INT_COEFFS</code> coefficients:
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</p>
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<a name="index-poly_005fint_002c-main-typedefs"></a>
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<dl compact="compact">
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<dt><code>poly_uint16</code></dt>
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<dd><p>a ‘<samp>poly_int</samp>’ with <code>unsigned short</code> coefficients.
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</p>
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</dd>
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<dt><code>poly_int64</code></dt>
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<dd><p>a ‘<samp>poly_int</samp>’ with <code>HOST_WIDE_INT</code> coefficients.
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</p>
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</dd>
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<dt><code>poly_uint64</code></dt>
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<dd><p>a ‘<samp>poly_int</samp>’ with <code>unsigned HOST_WIDE_INT</code> coefficients.
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</p>
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</dd>
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<dt><code>poly_offset_int</code></dt>
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<dd><p>a ‘<samp>poly_int</samp>’ with <code>offset_int</code> coefficients.
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</p>
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</dd>
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<dt><code>poly_wide_int</code></dt>
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<dd><p>a ‘<samp>poly_int</samp>’ with <code>wide_int</code> coefficients.
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</p>
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</dd>
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<dt><code>poly_widest_int</code></dt>
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<dd><p>a ‘<samp>poly_int</samp>’ with <code>widest_int</code> coefficients.
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</p></dd>
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</dl>
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<p>Since the main purpose of <code>poly_int</code> is to represent sizes and
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offsets, the last two typedefs are only rarely used.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Consequences-of-using-poly_005fint.html#Consequences-of-using-poly_005fint" accesskey="n" rel="next">Consequences of using <code>poly_int</code></a>, Up: <a href="poly_005fint.html#poly_005fint" accesskey="u" rel="up">poly_int</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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