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<title>Varargs - GNU Compiler Collection (GCC) Internals</title>
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<div class="node">
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<a name="Varargs"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Trampolines.html#Trampolines">Trampolines</a>,
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Previous: <a rel="previous" accesskey="p" href="Stack-and-Calling.html#Stack-and-Calling">Stack and Calling</a>,
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Up: <a rel="up" accesskey="u" href="Target-Macros.html#Target-Macros">Target Macros</a>
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<hr>
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</div>
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<h3 class="section">17.10 Implementing the Varargs Macros</h3>
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<p><a name="index-varargs-implementation-4327"></a>
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GCC comes with an implementation of <code><varargs.h></code> and
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<code><stdarg.h></code> that work without change on machines that pass arguments
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on the stack. Other machines require their own implementations of
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varargs, and the two machine independent header files must have
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conditionals to include it.
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<p>ISO <code><stdarg.h></code> differs from traditional <code><varargs.h></code> mainly in
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the calling convention for <code>va_start</code>. The traditional
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implementation takes just one argument, which is the variable in which
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to store the argument pointer. The ISO implementation of
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<code>va_start</code> takes an additional second argument. The user is
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supposed to write the last named argument of the function here.
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<p>However, <code>va_start</code> should not use this argument. The way to find
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the end of the named arguments is with the built-in functions described
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below.
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<div class="defun">
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— Macro: <b>__builtin_saveregs</b> ()<var><a name="index-g_t_005f_005fbuiltin_005fsaveregs-4328"></a></var><br>
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<blockquote><p>Use this built-in function to save the argument registers in memory so
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that the varargs mechanism can access them. Both ISO and traditional
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versions of <code>va_start</code> must use <code>__builtin_saveregs</code>, unless
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you use <code>TARGET_SETUP_INCOMING_VARARGS</code> (see below) instead.
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<p>On some machines, <code>__builtin_saveregs</code> is open-coded under the
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control of the target hook <code>TARGET_EXPAND_BUILTIN_SAVEREGS</code>. On
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other machines, it calls a routine written in assembler language,
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found in <samp><span class="file">libgcc2.c</span></samp>.
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<p>Code generated for the call to <code>__builtin_saveregs</code> appears at the
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beginning of the function, as opposed to where the call to
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<code>__builtin_saveregs</code> is written, regardless of what the code is.
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This is because the registers must be saved before the function starts
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to use them for its own purposes.
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<!-- i rewrote the first sentence above to fix an overfull hbox. -mew -->
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<!-- 10feb93 -->
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>__builtin_next_arg</b> (<var>lastarg</var>)<var><a name="index-g_t_005f_005fbuiltin_005fnext_005farg-4329"></a></var><br>
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<blockquote><p>This builtin returns the address of the first anonymous stack
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argument, as type <code>void *</code>. If <code>ARGS_GROW_DOWNWARD</code>, it
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returns the address of the location above the first anonymous stack
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argument. Use it in <code>va_start</code> to initialize the pointer for
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fetching arguments from the stack. Also use it in <code>va_start</code> to
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verify that the second parameter <var>lastarg</var> is the last named argument
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of the current function.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>__builtin_classify_type</b> (<var>object</var>)<var><a name="index-g_t_005f_005fbuiltin_005fclassify_005ftype-4330"></a></var><br>
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<blockquote><p>Since each machine has its own conventions for which data types are
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passed in which kind of register, your implementation of <code>va_arg</code>
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has to embody these conventions. The easiest way to categorize the
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specified data type is to use <code>__builtin_classify_type</code> together
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with <code>sizeof</code> and <code>__alignof__</code>.
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<p><code>__builtin_classify_type</code> ignores the value of <var>object</var>,
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considering only its data type. It returns an integer describing what
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kind of type that is—integer, floating, pointer, structure, and so on.
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<p>The file <samp><span class="file">typeclass.h</span></samp> defines an enumeration that you can use to
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interpret the values of <code>__builtin_classify_type</code>.
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</p></blockquote></div>
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<p>These machine description macros help implement varargs:
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<div class="defun">
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— Target Hook: rtx <b>TARGET_EXPAND_BUILTIN_SAVEREGS</b> (<var>void</var>)<var><a name="index-TARGET_005fEXPAND_005fBUILTIN_005fSAVEREGS-4331"></a></var><br>
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<blockquote><p>If defined, this hook produces the machine-specific code for a call to
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<code>__builtin_saveregs</code>. This code will be moved to the very
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beginning of the function, before any parameter access are made. The
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return value of this function should be an RTX that contains the value
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to use as the return of <code>__builtin_saveregs</code>.
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</p></blockquote></div>
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<div class="defun">
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— Target Hook: void <b>TARGET_SETUP_INCOMING_VARARGS</b> (<var>cumulative_args_t args_so_far, machine_mode mode, tree type, int *pretend_args_size, int second_time</var>)<var><a name="index-TARGET_005fSETUP_005fINCOMING_005fVARARGS-4332"></a></var><br>
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<blockquote><p>This target hook offers an alternative to using
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<code>__builtin_saveregs</code> and defining the hook
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<code>TARGET_EXPAND_BUILTIN_SAVEREGS</code>. Use it to store the anonymous
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register arguments into the stack so that all the arguments appear to
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have been passed consecutively on the stack. Once this is done, you can
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use the standard implementation of varargs that works for machines that
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pass all their arguments on the stack.
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<p>The argument <var>args_so_far</var> points to the <code>CUMULATIVE_ARGS</code> data
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structure, containing the values that are obtained after processing the
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named arguments. The arguments <var>mode</var> and <var>type</var> describe the
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last named argument—its machine mode and its data type as a tree node.
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<p>The target hook should do two things: first, push onto the stack all the
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argument registers <em>not</em> used for the named arguments, and second,
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store the size of the data thus pushed into the <code>int</code>-valued
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variable pointed to by <var>pretend_args_size</var>. The value that you
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store here will serve as additional offset for setting up the stack
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frame.
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<p>Because you must generate code to push the anonymous arguments at
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compile time without knowing their data types,
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<code>TARGET_SETUP_INCOMING_VARARGS</code> is only useful on machines that
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have just a single category of argument register and use it uniformly
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for all data types.
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<p>If the argument <var>second_time</var> is nonzero, it means that the
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arguments of the function are being analyzed for the second time. This
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happens for an inline function, which is not actually compiled until the
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end of the source file. The hook <code>TARGET_SETUP_INCOMING_VARARGS</code> should
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not generate any instructions in this case.
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</p></blockquote></div>
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<div class="defun">
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— Target Hook: bool <b>TARGET_STRICT_ARGUMENT_NAMING</b> (<var>cumulative_args_t ca</var>)<var><a name="index-TARGET_005fSTRICT_005fARGUMENT_005fNAMING-4333"></a></var><br>
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<blockquote><p>Define this hook to return <code>true</code> if the location where a function
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argument is passed depends on whether or not it is a named argument.
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<p>This hook controls how the <var>named</var> argument to <code>TARGET_FUNCTION_ARG</code>
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is set for varargs and stdarg functions. If this hook returns
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<code>true</code>, the <var>named</var> argument is always true for named
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arguments, and false for unnamed arguments. If it returns <code>false</code>,
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but <code>TARGET_PRETEND_OUTGOING_VARARGS_NAMED</code> returns <code>true</code>,
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then all arguments are treated as named. Otherwise, all named arguments
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except the last are treated as named.
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<p>You need not define this hook if it always returns <code>false</code>.
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</p></blockquote></div>
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<div class="defun">
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— Target Hook: void <b>TARGET_CALL_ARGS</b> (<var>rtx, tree</var>)<var><a name="index-TARGET_005fCALL_005fARGS-4334"></a></var><br>
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<blockquote><p>While generating RTL for a function call, this target hook is invoked once
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for each argument passed to the function, either a register returned by
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<code>TARGET_FUNCTION_ARG</code> or a memory location. It is called just
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before the point where argument registers are stored. The type of the
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function to be called is also passed as the second argument; it is
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<code>NULL_TREE</code> for libcalls. The <code>TARGET_END_CALL_ARGS</code> hook is
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invoked just after the code to copy the return reg has been emitted.
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This functionality can be used to perform special setup of call argument
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registers if a target needs it.
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For functions without arguments, the hook is called once with <code>pc_rtx</code>
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passed instead of an argument register.
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Most ports do not need to implement anything for this hook.
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</p></blockquote></div>
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<div class="defun">
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— Target Hook: void <b>TARGET_END_CALL_ARGS</b> (<var>void</var>)<var><a name="index-TARGET_005fEND_005fCALL_005fARGS-4335"></a></var><br>
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<blockquote><p>This target hook is invoked while generating RTL for a function call,
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just after the point where the return reg is copied into a pseudo. It
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signals that all the call argument and return registers for the just
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emitted call are now no longer in use.
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Most ports do not need to implement anything for this hook.
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</p></blockquote></div>
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<div class="defun">
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— Target Hook: bool <b>TARGET_PRETEND_OUTGOING_VARARGS_NAMED</b> (<var>cumulative_args_t ca</var>)<var><a name="index-TARGET_005fPRETEND_005fOUTGOING_005fVARARGS_005fNAMED-4336"></a></var><br>
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<blockquote><p>If you need to conditionally change ABIs so that one works with
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<code>TARGET_SETUP_INCOMING_VARARGS</code>, but the other works like neither
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<code>TARGET_SETUP_INCOMING_VARARGS</code> nor <code>TARGET_STRICT_ARGUMENT_NAMING</code> was
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defined, then define this hook to return <code>true</code> if
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<code>TARGET_SETUP_INCOMING_VARARGS</code> is used, <code>false</code> otherwise.
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Otherwise, you should not define this hook.
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</p></blockquote></div>
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<div class="defun">
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— Target Hook: rtx <b>TARGET_LOAD_BOUNDS_FOR_ARG</b> (<var>rtx slot, rtx arg, rtx slot_no</var>)<var><a name="index-TARGET_005fLOAD_005fBOUNDS_005fFOR_005fARG-4337"></a></var><br>
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<blockquote><p>This hook is used by expand pass to emit insn to load bounds of
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<var>arg</var> passed in <var>slot</var>. Expand pass uses this hook in case
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bounds of <var>arg</var> are not passed in register. If <var>slot</var> is a
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memory, then bounds are loaded as for regular pointer loaded from
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memory. If <var>slot</var> is not a memory then <var>slot_no</var> is an integer
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constant holding number of the target dependent special slot which
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should be used to obtain bounds. Hook returns RTX holding loaded bounds.
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</p></blockquote></div>
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<div class="defun">
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— Target Hook: void <b>TARGET_STORE_BOUNDS_FOR_ARG</b> (<var>rtx arg, rtx slot, rtx bounds, rtx slot_no</var>)<var><a name="index-TARGET_005fSTORE_005fBOUNDS_005fFOR_005fARG-4338"></a></var><br>
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<blockquote><p>This hook is used by expand pass to emit insns to store <var>bounds</var> of
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<var>arg</var> passed in <var>slot</var>. Expand pass uses this hook in case
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<var>bounds</var> of <var>arg</var> are not passed in register. If <var>slot</var> is a
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memory, then <var>bounds</var> are stored as for regular pointer stored in
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memory. If <var>slot</var> is not a memory then <var>slot_no</var> is an integer
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constant holding number of the target dependent special slot which
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should be used to store <var>bounds</var>.
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</p></blockquote></div>
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<div class="defun">
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— Target Hook: rtx <b>TARGET_LOAD_RETURNED_BOUNDS</b> (<var>rtx slot</var>)<var><a name="index-TARGET_005fLOAD_005fRETURNED_005fBOUNDS-4339"></a></var><br>
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<blockquote><p>This hook is used by expand pass to emit insn to load bounds
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returned by function call in <var>slot</var>. Hook returns RTX holding
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loaded bounds.
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</p></blockquote></div>
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<div class="defun">
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— Target Hook: void <b>TARGET_STORE_RETURNED_BOUNDS</b> (<var>rtx slot, rtx bounds</var>)<var><a name="index-TARGET_005fSTORE_005fRETURNED_005fBOUNDS-4340"></a></var><br>
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<blockquote><p>This hook is used by expand pass to emit insn to store <var>bounds</var>
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returned by function call into <var>slot</var>.
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</p></blockquote></div>
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<div class="defun">
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— Target Hook: rtx <b>TARGET_CHKP_FUNCTION_VALUE_BOUNDS</b> (<var>const_tree ret_type, const_tree fn_decl_or_type, bool outgoing</var>)<var><a name="index-TARGET_005fCHKP_005fFUNCTION_005fVALUE_005fBOUNDS-4341"></a></var><br>
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<blockquote><p>Define this to return an RTX representing the place where a function
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returns bounds for returned pointers. Arguments meaning is similar to
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<code>TARGET_FUNCTION_VALUE</code>.
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</p></blockquote></div>
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<div class="defun">
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— Target Hook: void <b>TARGET_SETUP_INCOMING_VARARG_BOUNDS</b> (<var>cumulative_args_t args_so_far, enum machine_mode mode, tree type, int *pretend_args_size, int second_time</var>)<var><a name="index-TARGET_005fSETUP_005fINCOMING_005fVARARG_005fBOUNDS-4342"></a></var><br>
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<blockquote><p>Use it to store bounds for anonymous register arguments stored
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into the stack. Arguments meaning is similar to
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<code>TARGET_SETUP_INCOMING_VARARGS</code>.
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</p></blockquote></div>
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</body></html>
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