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132 lines
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132 lines
5.8 KiB
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<title>Consequences of using poly_int (GNU Compiler Collection (GCC) Internals)</title>
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<link href="Comparisons-involving-poly_005fint.html#Comparisons-involving-poly_005fint" rel="next" title="Comparisons involving poly_int">
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<a name="Consequences-of-using-poly_005fint"></a>
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<p>
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Next: <a href="Comparisons-involving-poly_005fint.html#Comparisons-involving-poly_005fint" accesskey="n" rel="next">Comparisons involving <code>poly_int</code></a>, Previous: <a href="Overview-of-poly_005fint.html#Overview-of-poly_005fint" accesskey="p" rel="prev">Overview of <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="Consequences-of-using-poly_005fint-1"></a>
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<h3 class="section">10.2 Consequences of using <code>poly_int</code></h3>
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<p>The two main consequences of using polynomial sizes and offsets are that:
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</p>
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<ul>
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<li> there is no total ordering between the values at compile time, and
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</li><li> some operations might yield results that cannot be expressed as a
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<code>poly_int</code>.
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</li></ul>
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<p>For example, if <var>x</var> is a runtime invariant, we cannot tell at
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compile time whether:
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</p>
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<div class="smallexample">
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<pre class="smallexample">3 + 4<var>x</var> <= 1 + 5<var>x</var>
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</pre></div>
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<p>since the condition is false when <var>x</var> <= 1 and true when <var>x</var> >= 2.
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</p>
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<p>Similarly, <code>poly_int</code> cannot represent the result of:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(3 + 4<var>x</var>) * (1 + 5<var>x</var>)
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</pre></div>
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<p>since it cannot (and in practice does not need to) store powers greater
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than one. It also cannot represent the result of:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(3 + 4<var>x</var>) / (1 + 5<var>x</var>)
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</pre></div>
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<p>The following sections describe how we deal with these restrictions.
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</p>
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<a name="index-poly_005fint_002c-use-in-target_002dindependent-code"></a>
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<p>As described earlier, a <code>poly_int<1, <var>T</var>></code> has no indeterminates
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and so degenerates to a compile-time constant of type <var>T</var>. It would
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be possible in that case to do all normal arithmetic on the <var>T</var>,
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and to compare the <var>T</var> using the normal C++ operators. We deliberately
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prevent target-independent code from doing this, since the compiler needs
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to support other <code>poly_int<<var>n</var>, <var>T</var>></code> as well, regardless of
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the current target’s <code>NUM_POLY_INT_COEFFS</code>.
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</p>
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<a name="index-poly_005fint_002c-use-in-target_002dspecific-code"></a>
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<p>However, it would be very artificial to force target-specific code
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to follow these restrictions if the target has no runtime indeterminates.
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There is therefore an implicit conversion from <code>poly_int<1, <var>T</var>></code>
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to <var>T</var> when compiling target-specific translation units.
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</p>
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