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| <h4 class="subsection">6.42.2 Specifying Registers for Local Variables</h4> |
| |
| <p><a name="index-local-variables_002c-specifying-registers-2598"></a><a name="index-specifying-registers-for-local-variables-2599"></a><a name="index-registers-for-local-variables-2600"></a> |
| You can define a local register variable with a specified register |
| like this: |
| |
| <pre class="smallexample"> register int *foo asm ("a5"); |
| </pre> |
| <p class="noindent">Here <code>a5</code> is the name of the register which should be used. Note |
| that this is the same syntax used for defining global register |
| variables, but for a local variable it would appear within a function. |
| |
| <p>Naturally the register name is cpu-dependent, but this is not a |
| problem, since specific registers are most often useful with explicit |
| assembler instructions (see <a href="Extended-Asm.html#Extended-Asm">Extended Asm</a>). Both of these things |
| generally require that you conditionalize your program according to |
| cpu type. |
| |
| <p>In addition, operating systems on one type of cpu may differ in how they |
| name the registers; then you would need additional conditionals. For |
| example, some 68000 operating systems call this register <code>%a5</code>. |
| |
| <p>Defining such a register variable does not reserve the register; it |
| remains available for other uses in places where flow control determines |
| the variable's value is not live. |
| |
| <p>This option does not guarantee that GCC will generate code that has |
| this variable in the register you specify at all times. You may not |
| code an explicit reference to this register in the <em>assembler |
| instruction template</em> part of an <code>asm</code> statement and assume it will |
| always refer to this variable. However, using the variable as an |
| <code>asm</code> <em>operand</em> guarantees that the specified register is used |
| for the operand. |
| |
| <p>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>As for global register variables, it's recommended that you choose a |
| register which is normally saved and restored by function calls on |
| your machine, so that library routines will not clobber it. A common |
| pitfall is to initialize multiple call-clobbered registers with |
| arbitrary expressions, where a function call or library call for an |
| arithmetic operator will overwrite a register value from a previous |
| assignment, for example <code>r0</code> below: |
| <pre class="smallexample"> register int *p1 asm ("r0") = ...; |
| register int *p2 asm ("r1") = ...; |
| </pre> |
| <p>In those cases, a solution is to use a temporary variable for |
| each arbitrary expression. See <a href="Example-of-asm-with-clobbered-asm-reg.html#Example-of-asm-with-clobbered-asm-reg">Example of asm with clobbered asm reg</a>. |
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