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<title>Using the GNU Compiler Collection (GCC): Hints implementation</title>
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<a name="Hints-implementation"></a>
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Next: <a href="Structures-unions-enumerations-and-bit_002dfields-implementation.html#Structures-unions-enumerations-and-bit_002dfields-implementation" accesskey="n" rel="next">Structures unions enumerations and bit-fields implementation</a>, Previous: <a href="Arrays-and-pointers-implementation.html#Arrays-and-pointers-implementation" accesskey="p" rel="prev">Arrays and pointers implementation</a>, Up: <a href="C-Implementation.html#C-Implementation" accesskey="u" rel="up">C Implementation</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="Hints"></a>
<h3 class="section">4.8 Hints</h3>
<ul>
<li> <cite>The extent to which suggestions made by using the <code>register</code>
storage-class specifier are effective (C90 6.5.1, C99 and C11 6.7.1).</cite>
<p>The <code>register</code> specifier affects code generation only in these ways:
</p>
<ul>
<li> When used as part of the register variable extension, see
<a href="Explicit-Register-Variables.html#Explicit-Register-Variables">Explicit Register Variables</a>.
</li><li> When <samp>-O0</samp> is in use, the compiler allocates distinct stack
memory for all variables that do not have the <code>register</code>
storage-class specifier; if <code>register</code> is specified, the variable
may have a shorter lifespan than the code would indicate and may never
be placed in memory.
</li><li> On some rare x86 targets, <code>setjmp</code> doesn’t save the registers in
all circumstances. In those cases, GCC doesn’t allocate any variables
in registers unless they are marked <code>register</code>.
</li></ul>
</li><li> <cite>The extent to which suggestions made by using the inline function
specifier are effective (C99 and C11 6.7.4).</cite>
<p>GCC will not inline any functions if the <samp>-fno-inline</samp> option is
used or if <samp>-O0</samp> is used. Otherwise, GCC may still be unable to
inline a function for many reasons; the <samp>-Winline</samp> option may be
used to determine if a function has not been inlined and why not.
</p>
</li></ul>
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