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<title>Using the GNU Compiler Collection (GCC): Size of an asm</title>

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<a name="Size-of-an-asm"></a>
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Previous: <a href="Explicit-Register-Variables.html#Explicit-Register-Variables" accesskey="p" rel="prev">Explicit Register Variables</a>, Up: <a href="Using-Assembly-Language-with-C.html#Using-Assembly-Language-with-C" accesskey="u" rel="up">Using Assembly Language with C</a> &nbsp; [<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="Size-of-an-asm-1"></a>
<h4 class="subsection">6.45.6 Size of an <code>asm</code></h4>

<p>Some targets require that GCC track the size of each instruction used
in order to generate correct code.  Because the final length of the
code produced by an <code>asm</code> statement is only known by the
assembler, GCC must make an estimate as to how big it will be.  It
does this by counting the number of instructions in the pattern of the
<code>asm</code> and multiplying that by the length of the longest
instruction supported by that processor.  (When working out the number
of instructions, it assumes that any occurrence of a newline or of
whatever statement separator character is supported by the assembler &mdash;
typically &lsquo;<samp>;</samp>&rsquo; &mdash; indicates the end of an instruction.)
</p>
<p>Normally, GCC&rsquo;s estimate is adequate to ensure that correct
code is generated, but it is possible to confuse the compiler if you use
pseudo instructions or assembler macros that expand into multiple real
instructions, or if you use assembler directives that expand to more
space in the object file than is needed for a single instruction.
If this happens then the assembler may produce a diagnostic saying that
a label is unreachable.
</p>
<a name="index-asm-inline"></a>
<p>This size is also used for inlining decisions.  If you use <code>asm inline</code>
instead of just <code>asm</code>, then for inlining purposes the size of the asm
is taken as the minimum size, ignoring how many instructions GCC thinks it is.
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