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<a name="poly_005fint"></a>
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<hr>
<a name="Sizes-and-offsets-as-runtime-invariants"></a>
<h2 class="chapter">10 Sizes and offsets as runtime invariants</h2>
<a name="index-polynomial-integers"></a>
<a name="index-poly_005fint"></a>

<p>GCC allows the size of a hardware register to be a runtime invariant
rather than a compile-time constant.  This in turn means that various
sizes and offsets must also be runtime invariants rather than
compile-time constants, such as:
</p>
<ul>
<li> the size of a general <code>machine_mode</code> (see <a href="Machine-Modes.html#Machine-Modes">Machine Modes</a>);

</li><li> the size of a spill slot;

</li><li> the offset of something within a stack frame;

</li><li> the number of elements in a vector;

</li><li> the size and offset of a <code>mem</code> rtx (see <a href="Regs-and-Memory.html#Regs-and-Memory">Regs and Memory</a>); and

</li><li> the byte offset in a <code>subreg</code> rtx (see <a href="Regs-and-Memory.html#Regs-and-Memory">Regs and Memory</a>).
</li></ul>

<p>The motivating example is the Arm SVE ISA, whose vector registers can be
any multiple of 128 bits between 128 and 2048 inclusive.  The compiler
normally produces code that works for all SVE register sizes, with the
actual size only being known at runtime.
</p>
<p>GCC&rsquo;s main representation of such runtime invariants is the
<code>poly_int</code> class.  This chapter describes what <code>poly_int</code>
does, lists the available operations, and gives some general
usage guidelines.
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top">&bull; <a href="Overview-of-poly_005fint.html#Overview-of-poly_005fint" accesskey="1">Overview of <code>poly_int</code></a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Consequences-of-using-poly_005fint.html#Consequences-of-using-poly_005fint" accesskey="2">Consequences of using <code>poly_int</code></a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Comparisons-involving-poly_005fint.html#Comparisons-involving-poly_005fint" accesskey="3">Comparisons involving <code>poly_int</code></a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Arithmetic-on-poly_005fints.html#Arithmetic-on-poly_005fints" accesskey="4">Arithmetic on <code>poly_int</code>s</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Alignment-of-poly_005fints.html#Alignment-of-poly_005fints" accesskey="5">Alignment of <code>poly_int</code>s</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Computing-bounds-on-poly_005fints.html#Computing-bounds-on-poly_005fints" accesskey="6">Computing bounds on <code>poly_int</code>s</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Converting-poly_005fints.html#Converting-poly_005fints" accesskey="7">Converting <code>poly_int</code>s</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Miscellaneous-poly_005fint-routines.html#Miscellaneous-poly_005fint-routines" accesskey="8">Miscellaneous <code>poly_int</code> routines</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Guidelines-for-using-poly_005fint.html#Guidelines-for-using-poly_005fint" accesskey="9">Guidelines for using <code>poly_int</code></a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
</table>

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