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| <h3 class="section">6.42 Variables in Specified Registers</h3> |
| |
| <p><a name="index-explicit-register-variables-2586"></a><a name="index-variables-in-specified-registers-2587"></a><a name="index-specified-registers-2588"></a><a name="index-registers_002c-global-allocation-2589"></a> |
| GNU C allows you to put a few global variables into specified hardware |
| registers. You can also specify the register in which an ordinary |
| register variable should be allocated. |
| |
| <ul> |
| <li>Global register variables reserve registers throughout the program. |
| This may be useful in programs such as programming language |
| interpreters which have a couple of global variables that are accessed |
| very often. |
| |
| <li>Local register variables in specific registers do not reserve the |
| registers, except at the point where they are used as input or output |
| operands in an <code>asm</code> statement and the <code>asm</code> statement itself is |
| not deleted. The compiler's data flow analysis is capable of determining |
| where the specified registers contain live values, and where they are |
| available for other uses. Stores into local register variables may be deleted |
| when they appear to be dead according to dataflow analysis. References |
| to local register variables may be deleted or moved or simplified. |
| |
| <p>These local variables are sometimes convenient for use with the extended |
| <code>asm</code> feature (see <a href="Extended-Asm.html#Extended-Asm">Extended Asm</a>), if you want to write one |
| output of the assembler instruction directly into a particular register. |
| (This will work provided the register you specify fits the constraints |
| specified for that operand in the <code>asm</code>.) |
| </ul> |
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| <li><a accesskey="1" href="Global-Reg-Vars.html#Global-Reg-Vars">Global Reg Vars</a> |
| <li><a accesskey="2" href="Local-Reg-Vars.html#Local-Reg-Vars">Local Reg Vars</a> |
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