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<title>Using the GNU Compiler Collection (GCC): Global Register Variables</title>
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<a name="Global-Register-Variables"></a>
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<div class="header">
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<p>
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Next: <a href="Local-Register-Variables.html#Local-Register-Variables" accesskey="n" rel="next">Local 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="Defining-Global-Register-Variables"></a>
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<h4 class="subsubsection">6.44.5.1 Defining Global Register Variables</h4>
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<a name="Global-Reg-Vars"></a><a name="index-global-register-variables"></a>
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<a name="index-registers_002c-global-variables-in"></a>
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<a name="index-registers_002c-global-allocation"></a>
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<p>You can define a global 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 that
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this is the same syntax used for defining local register variables, but for
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a global variable the declaration appears outside 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>Registers are a scarce resource on most systems and allowing the
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compiler to manage their usage usually results in the best code. However,
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under special circumstances it can make sense to reserve some globally.
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For example this may be useful in programs such as programming language
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interpreters that have a couple of global variables that are accessed
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very often.
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</p>
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<p>After defining a global register variable, for the current compilation
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unit:
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</p>
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<ul>
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<li> The register is reserved entirely for this use, and will not be
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allocated for any other purpose.
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</li><li> The register is not saved and restored by any functions.
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</li><li> Stores into this register are never deleted even if they appear to be
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dead, but references may be deleted, moved or simplified.
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</li></ul>
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<p>Note that these points <em>only</em> apply to code that is compiled with the
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definition. The behavior of code that is merely linked in (for example
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code from libraries) is not affected.
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</p>
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<p>If you want to recompile source files that do not actually use your global
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register variable so they do not use the specified register for any other
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purpose, you need not actually add the global register declaration to
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their source code. It suffices to specify the compiler option
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<samp>-ffixed-<var>reg</var></samp> (see <a href="Code-Gen-Options.html#Code-Gen-Options">Code Gen Options</a>) to reserve the
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register.
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</p>
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<a name="Declaring-the-variable"></a>
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<h4 class="subsubheading">Declaring the variable</h4>
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<p>Global register variables can not have initial values, because an
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executable file has no means to supply initial contents for a register.
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</p>
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<p>When selecting a register, choose one that is normally saved and
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restored by function calls on your machine. This ensures that code
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which is unaware of this reservation (such as library routines) will
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restore it before returning.
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</p>
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<p>On machines with register windows, be sure to choose a global
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register that is not affected magically by the function call mechanism.
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</p>
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<a name="Using-the-variable"></a>
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<h4 class="subsubheading">Using the variable</h4>
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<a name="index-qsort_002c-and-global-register-variables"></a>
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<p>When calling routines that are not aware of the reservation, be
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cautious if those routines call back into code which uses them. As an
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example, if you call the system library version of <code>qsort</code>, it may
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clobber your registers during execution, but (if you have selected
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appropriate registers) it will restore them before returning. However
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it will <em>not</em> restore them before calling <code>qsort</code>’s comparison
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function. As a result, global values will not reliably be available to
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the comparison function unless the <code>qsort</code> function itself is rebuilt.
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</p>
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<p>Similarly, it is not safe to access the global register variables from signal
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handlers or from more than one thread of control. Unless you recompile
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them specially for the task at hand, the system library routines may
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temporarily use the register for other things.
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</p>
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<a name="index-register-variable-after-longjmp"></a>
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<a name="index-global-register-after-longjmp"></a>
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<a name="index-value-after-longjmp"></a>
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<a name="index-longjmp"></a>
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<a name="index-setjmp"></a>
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<p>On most machines, <code>longjmp</code> restores to each global register
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variable the value it had at the time of the <code>setjmp</code>. On some
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machines, however, <code>longjmp</code> does not change the value of global
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register variables. To be portable, the function that called <code>setjmp</code>
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should make other arrangements to save the values of the global register
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variables, and to restore them in a <code>longjmp</code>. This way, the same
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thing happens regardless of what <code>longjmp</code> does.
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</p>
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<p>Eventually there may be a way of asking the compiler to choose a register
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automatically, but first we need to figure out how it should choose and
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how to enable you to guide the choice. No solution is evident.
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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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Next: <a href="Local-Register-Variables.html#Local-Register-Variables" accesskey="n" rel="next">Local 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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