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<title>Local Reg Vars - Using the GNU Compiler Collection (GCC)</title>
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<a name="Local-Reg-Vars"></a>
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<p>
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Previous: <a rel="previous" accesskey="p" href="Global-Reg-Vars.html#Global-Reg-Vars">Global Reg Vars</a>,
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Up: <a rel="up" accesskey="u" href="Explicit-Reg-Vars.html#Explicit-Reg-Vars">Explicit Reg Vars</a>
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<h5 class="subsubsection">6.43.5.2 Specifying Registers for Local Variables</h5>
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<p><a name="index-local-variables_002c-specifying-registers-3504"></a><a name="index-specifying-registers-for-local-variables-3505"></a><a name="index-registers-for-local-variables-3506"></a>
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You can define a local register variable with a specified register
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like this:
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<pre class="smallexample"> register int *foo asm ("a5");
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</pre>
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<p class="noindent">Here <code>a5</code> is the name of the register that should be used. Note
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that this is the same syntax used for defining global register
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variables, but for a local variable it appears within a function.
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<p>Naturally the register name is CPU-dependent, but this is not a
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problem, since specific registers are most often useful with explicit
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assembler instructions (see <a href="Extended-Asm.html#Extended-Asm">Extended Asm</a>). Both of these things
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generally require that you conditionalize your program according to
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CPU type.
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<p>In addition, operating systems on one type of CPU may differ in how they
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name the registers; then you need additional conditionals. For
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example, some 68000 operating systems call this register <code>%a5</code>.
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<p>Defining such a register variable does not reserve the register; it
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remains available for other uses in places where flow control determines
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the variable's value is not live.
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<p>This option does not guarantee that GCC generates code that has
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this variable in the register you specify at all times. You may not
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code an explicit reference to this register in the assembler
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instruction template part of an <code>asm</code> statement and assume it
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always refers to this variable.
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However, using the variable as an input or output operand to the <code>asm</code>
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guarantees that the specified register is used for that operand.
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See <a href="Extended-Asm.html#Extended-Asm">Extended Asm</a>, for more information.
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<p>Stores into local register variables may be deleted when they appear to be dead
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according to dataflow analysis. References to local register variables may
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be deleted or moved or simplified.
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<p>As with global register variables, it is recommended that you choose a
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register that is normally saved and restored by function calls on
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your machine, so that library routines will not clobber it.
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<p>Sometimes when writing inline <code>asm</code> code, you need to make an operand be a
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specific register, but there's no matching constraint letter for that
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register. To force the operand into that register, create a local variable
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and specify the register in the variable's declaration. Then use the local
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variable for the asm operand and specify any constraint letter that matches
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the register:
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<pre class="smallexample"> register int *p1 asm ("r0") = ...;
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register int *p2 asm ("r1") = ...;
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register int *result asm ("r0");
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asm ("sysint" : "=r" (result) : "0" (p1), "r" (p2));
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</pre>
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<p><em>Warning:</em> In the above example, be aware that a register (for example r0) can be
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call-clobbered by subsequent code, including function calls and library calls
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for arithmetic operators on other variables (for example the initialization
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of p2). In this case, use temporary variables for expressions between the
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register assignments:
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<pre class="smallexample"> int t1 = ...;
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register int *p1 asm ("r0") = ...;
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register int *p2 asm ("r1") = t1;
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register int *result asm ("r0");
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asm ("sysint" : "=r" (result) : "0" (p1), "r" (p2));
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</pre>
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