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<title>Using the GNU Compiler Collection (GCC): Local Register Variables</title>
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<a name="Local-Register-Variables"></a>
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<p>
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Previous: <a href="Global-Register-Variables.html#Global-Register-Variables" accesskey="p" rel="prev">Global Register Variables</a>, Up: <a href="Explicit-Register-Variables.html#Explicit-Register-Variables" accesskey="u" rel="up">Explicit Register Variables</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
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<a name="Specifying-Registers-for-Local-Variables"></a>
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<h4 class="subsubsection">6.44.5.2 Specifying Registers for Local Variables</h4>
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<a name="Local-Reg-Vars"></a><a name="index-local-variables_002c-specifying-registers"></a>
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<a name="index-specifying-registers-for-local-variables"></a>
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<a name="index-registers-for-local-variables"></a>
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<p>You can define a local register variable and associate it with a specified
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register like this:
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</p>
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<div class="smallexample">
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<pre class="smallexample">register int *foo asm ("r12");
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</pre></div>
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<p>Here <code>r12</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 variables,
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but for a local variable the declaration appears within a function. The
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<code>register</code> keyword is required, and cannot be combined with
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<code>static</code>. The register name must be a valid register name for the
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target platform.
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</p>
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<p>As with global register variables, it is recommended that you choose
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a register that is normally saved and restored by function calls on your
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machine, so that calls to library routines will not clobber it.
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</p>
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<p>The only supported use for this feature is to specify registers
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for input and output operands when calling Extended <code>asm</code>
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(see <a href="Extended-Asm.html#Extended-Asm">Extended Asm</a>). This may be necessary if the constraints for a
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particular machine don’t provide sufficient control to select the desired
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register. To force an operand into a register, create a local variable
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and specify the register name after the variable’s declaration. Then use
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the local variable for the <code>asm</code> operand and specify any constraint
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letter that matches the register:
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</p>
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<div class="smallexample">
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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></div>
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<p><em>Warning:</em> In the above example, be aware that a register (for example
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<code>r0</code>) can be call-clobbered by subsequent code, including function
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calls and library calls for arithmetic operators on other variables (for
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example the initialization of <code>p2</code>). In this case, use temporary
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variables for expressions between the register assignments:
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</p>
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<div class="smallexample">
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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></div>
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<p>Defining a register variable does not reserve the register. Other than
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when invoking the Extended <code>asm</code>, the contents of the specified
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register are not guaranteed. For this reason, the following uses
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are explicitly <em>not</em> supported. If they appear to work, it is only
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happenstance, and may stop working as intended due to (seemingly)
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unrelated changes in surrounding code, or even minor changes in the
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optimization of a future version of gcc:
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</p>
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<ul>
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<li> Passing parameters to or from Basic <code>asm</code>
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</li><li> Passing parameters to or from Extended <code>asm</code> without using input
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or output operands.
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</li><li> Passing parameters to or from routines written in assembler (or
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other languages) using non-standard calling conventions.
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</li></ul>
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<p>Some developers use Local Register Variables in an attempt to improve
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gcc’s allocation of registers, especially in large functions. In this
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case the register name is essentially a hint to the register allocator.
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While in some instances this can generate better code, improvements are
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subject to the whims of the allocator/optimizers. Since there are no
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guarantees that your improvements won’t be lost, this usage of Local
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Register Variables is discouraged.
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</p>
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<p>On the MIPS platform, there is related use for local register variables
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with slightly different characteristics (see <a href="http://gcc.gnu.org/onlinedocs/gccint/MIPS-Coprocessors.html#MIPS-Coprocessors">Defining coprocessor specifics for MIPS targets</a> in <cite>GNU Compiler Collection (GCC) Internals</cite>).
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</p>
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<hr>
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<div class="header">
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<p>
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Previous: <a href="Global-Register-Variables.html#Global-Register-Variables" accesskey="p" rel="prev">Global Register Variables</a>, Up: <a href="Explicit-Register-Variables.html#Explicit-Register-Variables" accesskey="u" rel="up">Explicit Register Variables</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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