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| <h3 class="section">4.8 Hints</h3> |
| |
| <ul> |
| <li><cite>The extent to which suggestions made by using the </cite><code>register</code><cite> |
| storage-class specifier are effective (C90 6.5.1, C99 6.7.1).</cite> |
| |
| <p>The <code>register</code> specifier affects code generation only in these ways: |
| |
| <ul> |
| <li>When used as part of the register variable extension, see |
| <a href="Explicit-Reg-Vars.html#Explicit-Reg-Vars">Explicit Reg Vars</a>. |
| |
| <li>When <samp><span class="option">-O0</span></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>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>. |
| |
| </ul> |
| |
| <li><cite>The extent to which suggestions made by using the inline function |
| specifier are effective (C99 6.7.4).</cite> |
| |
| <p>GCC will not inline any functions if the <samp><span class="option">-fno-inline</span></samp> option is |
| used or if <samp><span class="option">-O0</span></samp> is used. Otherwise, GCC may still be unable to |
| inline a function for many reasons; the <samp><span class="option">-Winline</span></samp> option may be |
| used to determine if a function has not been inlined and why not. |
| |
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