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| <h4 class="subsection">6.42.1 Defining Global Register Variables</h4> |
| |
| <p><a name="index-global-register-variables-2590"></a><a name="index-registers_002c-global-variables-in-2591"></a> |
| You can define a global register variable in GNU C 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. Choose a |
| register which is normally saved and restored by function calls on your |
| machine, so that library routines will not clobber it. |
| |
| <p>Naturally the register name is cpu-dependent, so you would need to |
| conditionalize your program according to cpu type. The register |
| <code>a5</code> would be a good choice on a 68000 for a variable of pointer |
| type. On machines with register windows, be sure to choose a “global” |
| register that is not affected magically by the function call mechanism. |
| |
| <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>Eventually there may be a way of asking the compiler to choose a register |
| automatically, but first we need to figure out how it should choose and |
| how to enable you to guide the choice. No solution is evident. |
| |
| <p>Defining a global register variable in a certain register reserves that |
| register entirely for this use, at least within the current compilation. |
| The register will not be allocated for any other purpose in the functions |
| in the current compilation. The register will not be saved and restored by |
| these functions. Stores into this register are never deleted even if they |
| would appear to be dead, but references may be deleted or moved or |
| simplified. |
| |
| <p>It is not safe to access the global register variables from signal |
| handlers, or from more than one thread of control, because the system |
| library routines may temporarily use the register for other things (unless |
| you recompile them specially for the task at hand). |
| |
| <p><a name="index-g_t_0040code_007bqsort_007d_002c-and-global-register-variables-2592"></a>It is not safe for one function that uses a global register variable to |
| call another such function <code>foo</code> by way of a third function |
| <code>lose</code> that was compiled without knowledge of this variable (i.e. in a |
| different source file in which the variable wasn't declared). This is |
| because <code>lose</code> might save the register and put some other value there. |
| For example, you can't expect a global register variable to be available in |
| the comparison-function that you pass to <code>qsort</code>, since <code>qsort</code> |
| might have put something else in that register. (If you are prepared to |
| recompile <code>qsort</code> with the same global register variable, you can |
| solve this problem.) |
| |
| <p>If you want to recompile <code>qsort</code> or other source files which do not |
| actually use your global register variable, so that they will not use that |
| register for any other purpose, then it suffices to specify the compiler |
| option <samp><span class="option">-ffixed-</span><var>reg</var></samp>. You need not actually add a global |
| register declaration to their source code. |
| |
| <p>A function which can alter the value of a global register variable cannot |
| safely be called from a function compiled without this variable, because it |
| could clobber the value the caller expects to find there on return. |
| Therefore, the function which is the entry point into the part of the |
| program that uses the global register variable must explicitly save and |
| restore the value which belongs to its caller. |
| |
| <p><a name="index-register-variable-after-_0040code_007blongjmp_007d-2593"></a><a name="index-global-register-after-_0040code_007blongjmp_007d-2594"></a><a name="index-value-after-_0040code_007blongjmp_007d-2595"></a><a name="index-longjmp-2596"></a><a name="index-setjmp-2597"></a>On most machines, <code>longjmp</code> will restore to each global register |
| variable the value it had at the time of the <code>setjmp</code>. On some |
| machines, however, <code>longjmp</code> will not change the value of global |
| register variables. To be portable, the function that called <code>setjmp</code> |
| should make other arrangements to save the values of the global register |
| variables, and to restore them in a <code>longjmp</code>. This way, the same |
| thing will happen regardless of what <code>longjmp</code> does. |
| |
| <p>All global register variable declarations must precede all function |
| definitions. If such a declaration could appear after function |
| definitions, the declaration would be too late to prevent the register from |
| being used for other purposes in the preceding functions. |
| |
| <p>Global register variables may not have initial values, because an |
| executable file has no means to supply initial contents for a register. |
| |
| <p>On the SPARC, there are reports that g3 <small class="dots">...</small> g7 are suitable |
| registers, but certain library functions, such as <code>getwd</code>, as well |
| as the subroutines for division and remainder, modify g3 and g4. g1 and |
| g2 are local temporaries. |
| |
| <p>On the 68000, a2 <small class="dots">...</small> a5 should be suitable, as should d2 <small class="dots">...</small> d7. |
| Of course, it will not do to use more than a few of those. |
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