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| <a name="Varargs"></a> |
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| <a name="Implementing-the-Varargs-Macros"></a> |
| <h3 class="section">17.11 Implementing the Varargs Macros</h3> |
| <a name="index-varargs-implementation"></a> |
| |
| <p>GCC comes with an implementation of <code><varargs.h></code> and |
| <code><stdarg.h></code> that work without change on machines that pass arguments |
| on the stack. Other machines require their own implementations of |
| varargs, and the two machine independent header files must have |
| conditionals to include it. |
| </p> |
| <p>ISO <code><stdarg.h></code> differs from traditional <code><varargs.h></code> mainly in |
| the calling convention for <code>va_start</code>. The traditional |
| implementation takes just one argument, which is the variable in which |
| to store the argument pointer. The ISO implementation of |
| <code>va_start</code> takes an additional second argument. The user is |
| supposed to write the last named argument of the function here. |
| </p> |
| <p>However, <code>va_start</code> should not use this argument. The way to find |
| the end of the named arguments is with the built-in functions described |
| below. |
| </p> |
| <dl> |
| <dt><a name="index-_005f_005fbuiltin_005fsaveregs"></a>Macro: <strong>__builtin_saveregs</strong> <em>()</em></dt> |
| <dd><p>Use this built-in function to save the argument registers in memory so |
| that the varargs mechanism can access them. Both ISO and traditional |
| versions of <code>va_start</code> must use <code>__builtin_saveregs</code>, unless |
| you use <code>TARGET_SETUP_INCOMING_VARARGS</code> (see below) instead. |
| </p> |
| <p>On some machines, <code>__builtin_saveregs</code> is open-coded under the |
| control of the target hook <code>TARGET_EXPAND_BUILTIN_SAVEREGS</code>. On |
| other machines, it calls a routine written in assembler language, |
| found in <samp>libgcc2.c</samp>. |
| </p> |
| <p>Code generated for the call to <code>__builtin_saveregs</code> appears at the |
| beginning of the function, as opposed to where the call to |
| <code>__builtin_saveregs</code> is written, regardless of what the code is. |
| This is because the registers must be saved before the function starts |
| to use them for its own purposes. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-_005f_005fbuiltin_005fnext_005farg"></a>Macro: <strong>__builtin_next_arg</strong> <em>(<var>lastarg</var>)</em></dt> |
| <dd><p>This builtin returns the address of the first anonymous stack |
| argument, as type <code>void *</code>. If <code>ARGS_GROW_DOWNWARD</code>, it |
| returns the address of the location above the first anonymous stack |
| argument. Use it in <code>va_start</code> to initialize the pointer for |
| fetching arguments from the stack. Also use it in <code>va_start</code> to |
| verify that the second parameter <var>lastarg</var> is the last named argument |
| of the current function. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-_005f_005fbuiltin_005fclassify_005ftype"></a>Macro: <strong>__builtin_classify_type</strong> <em>(<var>object</var>)</em></dt> |
| <dd><p>Since each machine has its own conventions for which data types are |
| passed in which kind of register, your implementation of <code>va_arg</code> |
| has to embody these conventions. The easiest way to categorize the |
| specified data type is to use <code>__builtin_classify_type</code> together |
| with <code>sizeof</code> and <code>__alignof__</code>. |
| </p> |
| <p><code>__builtin_classify_type</code> ignores the value of <var>object</var>, |
| considering only its data type. It returns an integer describing what |
| kind of type that is—integer, floating, pointer, structure, and so on. |
| </p> |
| <p>The file <samp>typeclass.h</samp> defines an enumeration that you can use to |
| interpret the values of <code>__builtin_classify_type</code>. |
| </p></dd></dl> |
| |
| <p>These machine description macros help implement varargs: |
| </p> |
| <dl> |
| <dt><a name="index-TARGET_005fEXPAND_005fBUILTIN_005fSAVEREGS"></a>Target Hook: <em>rtx</em> <strong>TARGET_EXPAND_BUILTIN_SAVEREGS</strong> <em>(void)</em></dt> |
| <dd><p>If defined, this hook produces the machine-specific code for a call to |
| <code>__builtin_saveregs</code>. This code will be moved to the very |
| beginning of the function, before any parameter access are made. The |
| return value of this function should be an RTX that contains the value |
| to use as the return of <code>__builtin_saveregs</code>. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-TARGET_005fSETUP_005fINCOMING_005fVARARGS"></a>Target Hook: <em>void</em> <strong>TARGET_SETUP_INCOMING_VARARGS</strong> <em>(cumulative_args_t <var>args_so_far</var>, enum machine_mode <var>mode</var>, tree <var>type</var>, int *<var>pretend_args_size</var>, int <var>second_time</var>)</em></dt> |
| <dd><p>This target hook offers an alternative to using |
| <code>__builtin_saveregs</code> and defining the hook |
| <code>TARGET_EXPAND_BUILTIN_SAVEREGS</code>. Use it to store the anonymous |
| register arguments into the stack so that all the arguments appear to |
| have been passed consecutively on the stack. Once this is done, you can |
| use the standard implementation of varargs that works for machines that |
| pass all their arguments on the stack. |
| </p> |
| <p>The argument <var>args_so_far</var> points to the <code>CUMULATIVE_ARGS</code> data |
| structure, containing the values that are obtained after processing the |
| named arguments. The arguments <var>mode</var> and <var>type</var> describe the |
| last named argument—its machine mode and its data type as a tree node. |
| </p> |
| <p>The target hook should do two things: first, push onto the stack all the |
| argument registers <em>not</em> used for the named arguments, and second, |
| store the size of the data thus pushed into the <code>int</code>-valued |
| variable pointed to by <var>pretend_args_size</var>. The value that you |
| store here will serve as additional offset for setting up the stack |
| frame. |
| </p> |
| <p>Because you must generate code to push the anonymous arguments at |
| compile time without knowing their data types, |
| <code>TARGET_SETUP_INCOMING_VARARGS</code> is only useful on machines that |
| have just a single category of argument register and use it uniformly |
| for all data types. |
| </p> |
| <p>If the argument <var>second_time</var> is nonzero, it means that the |
| arguments of the function are being analyzed for the second time. This |
| happens for an inline function, which is not actually compiled until the |
| end of the source file. The hook <code>TARGET_SETUP_INCOMING_VARARGS</code> should |
| not generate any instructions in this case. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-TARGET_005fSTRICT_005fARGUMENT_005fNAMING"></a>Target Hook: <em>bool</em> <strong>TARGET_STRICT_ARGUMENT_NAMING</strong> <em>(cumulative_args_t <var>ca</var>)</em></dt> |
| <dd><p>Define this hook to return <code>true</code> if the location where a function |
| argument is passed depends on whether or not it is a named argument. |
| </p> |
| <p>This hook controls how the <var>named</var> argument to <code>TARGET_FUNCTION_ARG</code> |
| is set for varargs and stdarg functions. If this hook returns |
| <code>true</code>, the <var>named</var> argument is always true for named |
| arguments, and false for unnamed arguments. If it returns <code>false</code>, |
| but <code>TARGET_PRETEND_OUTGOING_VARARGS_NAMED</code> returns <code>true</code>, |
| then all arguments are treated as named. Otherwise, all named arguments |
| except the last are treated as named. |
| </p> |
| <p>You need not define this hook if it always returns <code>false</code>. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-TARGET_005fPRETEND_005fOUTGOING_005fVARARGS_005fNAMED"></a>Target Hook: <em>bool</em> <strong>TARGET_PRETEND_OUTGOING_VARARGS_NAMED</strong> <em>(cumulative_args_t <var>ca</var>)</em></dt> |
| <dd><p>If you need to conditionally change ABIs so that one works with |
| <code>TARGET_SETUP_INCOMING_VARARGS</code>, but the other works like neither |
| <code>TARGET_SETUP_INCOMING_VARARGS</code> nor <code>TARGET_STRICT_ARGUMENT_NAMING</code> was |
| defined, then define this hook to return <code>true</code> if |
| <code>TARGET_SETUP_INCOMING_VARARGS</code> is used, <code>false</code> otherwise. |
| Otherwise, you should not define this hook. |
| </p></dd></dl> |
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