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<a name="Hints-implementation"></a>
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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&rsquo;t save the registers in
all circumstances.  In those cases, GCC doesn&rsquo;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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