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| <title>Using the GNU Compiler Collection (GCC): Explicit Reg Vars</title> |
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| <a name="Variables-in-Specified-Registers"></a> |
| <h3 class="section">6.44 Variables in Specified Registers</h3> |
| <a name="index-explicit-register-variables"></a> |
| <a name="index-variables-in-specified-registers"></a> |
| <a name="index-specified-registers"></a> |
| <a name="index-registers_002c-global-allocation"></a> |
| |
| <p>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. |
| </p> |
| <ul> |
| <li> Global register variables reserve registers throughout the program. |
| This may be useful in programs such as programming language |
| interpreters that have a couple of global variables that are accessed |
| very often. |
| |
| </li><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 works provided the register you specify fits the constraints |
| specified for that operand in the <code>asm</code>.) |
| </p></li></ul> |
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| <tr><td align="left" valign="top">• <a href="Global-Reg-Vars.html#Global-Reg-Vars" accesskey="1">Global Reg Vars</a>:</td><td> </td><td align="left" valign="top"> |
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| <tr><td align="left" valign="top">• <a href="Local-Reg-Vars.html#Local-Reg-Vars" accesskey="2">Local Reg Vars</a>:</td><td> </td><td align="left" valign="top"> |
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