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<title>GNU Compiler Collection (GCC) Internals: Stack Arguments</title>
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<a name="Stack-Arguments"></a>
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<div class="header">
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<p>
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Next: <a href="Register-Arguments.html#Register-Arguments" accesskey="n" rel="next">Register Arguments</a>, Previous: <a href="Elimination.html#Elimination" accesskey="p" rel="prev">Elimination</a>, Up: <a href="Stack-and-Calling.html#Stack-and-Calling" accesskey="u" rel="up">Stack and Calling</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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</div>
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<hr>
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<a name="Passing-Function-Arguments-on-the-Stack"></a>
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<h4 class="subsection">17.9.6 Passing Function Arguments on the Stack</h4>
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<a name="index-arguments-on-stack"></a>
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<a name="index-stack-arguments"></a>
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<p>The macros in this section control how arguments are passed
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on the stack. See the following section for other macros that
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control passing certain arguments in registers.
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</p>
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<dl>
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<dt><a name="index-TARGET_005fPROMOTE_005fPROTOTYPES"></a>Target Hook: <em>bool</em> <strong>TARGET_PROMOTE_PROTOTYPES</strong> <em>(const_tree <var>fntype</var>)</em></dt>
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<dd><p>This target hook returns <code>true</code> if an argument declared in a
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prototype as an integral type smaller than <code>int</code> should actually be
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passed as an <code>int</code>. In addition to avoiding errors in certain
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cases of mismatch, it also makes for better code on certain machines.
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The default is to not promote prototypes.
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</p></dd></dl>
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<dl>
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<dt><a name="index-PUSH_005fARGS"></a>Macro: <strong>PUSH_ARGS</strong></dt>
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<dd><p>A C expression. If nonzero, push insns will be used to pass
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outgoing arguments.
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If the target machine does not have a push instruction, set it to zero.
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That directs GCC to use an alternate strategy: to
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allocate the entire argument block and then store the arguments into
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it. When <code>PUSH_ARGS</code> is nonzero, <code>PUSH_ROUNDING</code> must be defined too.
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</p></dd></dl>
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<dl>
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<dt><a name="index-PUSH_005fARGS_005fREVERSED"></a>Macro: <strong>PUSH_ARGS_REVERSED</strong></dt>
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<dd><p>A C expression. If nonzero, function arguments will be evaluated from
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last to first, rather than from first to last. If this macro is not
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defined, it defaults to <code>PUSH_ARGS</code> on targets where the stack
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and args grow in opposite directions, and 0 otherwise.
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</p></dd></dl>
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<dl>
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<dt><a name="index-PUSH_005fROUNDING"></a>Macro: <strong>PUSH_ROUNDING</strong> <em>(<var>npushed</var>)</em></dt>
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<dd><p>A C expression that is the number of bytes actually pushed onto the
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stack when an instruction attempts to push <var>npushed</var> bytes.
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</p>
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<p>On some machines, the definition
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</p>
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<div class="smallexample">
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<pre class="smallexample">#define PUSH_ROUNDING(BYTES) (BYTES)
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</pre></div>
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<p>will suffice. But on other machines, instructions that appear
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to push one byte actually push two bytes in an attempt to maintain
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alignment. Then the definition should be
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</p>
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<div class="smallexample">
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<pre class="smallexample">#define PUSH_ROUNDING(BYTES) (((BYTES) + 1) & ~1)
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</pre></div>
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<p>If the value of this macro has a type, it should be an unsigned type.
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</p></dd></dl>
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<a name="index-outgoing_005fargs_005fsize"></a>
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<a name="index-crtl_002d_003eoutgoing_005fargs_005fsize"></a>
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<dl>
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<dt><a name="index-ACCUMULATE_005fOUTGOING_005fARGS"></a>Macro: <strong>ACCUMULATE_OUTGOING_ARGS</strong></dt>
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<dd><p>A C expression. If nonzero, the maximum amount of space required for outgoing arguments
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will be computed and placed into
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<code>crtl->outgoing_args_size</code>. No space will be pushed
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onto the stack for each call; instead, the function prologue should
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increase the stack frame size by this amount.
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</p>
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<p>Setting both <code>PUSH_ARGS</code> and <code>ACCUMULATE_OUTGOING_ARGS</code>
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is not proper.
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</p></dd></dl>
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<dl>
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<dt><a name="index-REG_005fPARM_005fSTACK_005fSPACE"></a>Macro: <strong>REG_PARM_STACK_SPACE</strong> <em>(<var>fndecl</var>)</em></dt>
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<dd><p>Define this macro if functions should assume that stack space has been
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allocated for arguments even when their values are passed in
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registers.
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</p>
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<p>The value of this macro is the size, in bytes, of the area reserved for
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arguments passed in registers for the function represented by <var>fndecl</var>,
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which can be zero if GCC is calling a library function.
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The argument <var>fndecl</var> can be the FUNCTION_DECL, or the type itself
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of the function.
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</p>
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<p>This space can be allocated by the caller, or be a part of the
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machine-dependent stack frame: <code>OUTGOING_REG_PARM_STACK_SPACE</code> says
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which.
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</p></dd></dl>
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<dl>
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<dt><a name="index-INCOMING_005fREG_005fPARM_005fSTACK_005fSPACE"></a>Macro: <strong>INCOMING_REG_PARM_STACK_SPACE</strong> <em>(<var>fndecl</var>)</em></dt>
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<dd><p>Like <code>REG_PARM_STACK_SPACE</code>, but for incoming register arguments.
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Define this macro if space guaranteed when compiling a function body
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is different to space required when making a call, a situation that
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can arise with K&R style function definitions.
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</p></dd></dl>
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<dl>
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<dt><a name="index-OUTGOING_005fREG_005fPARM_005fSTACK_005fSPACE"></a>Macro: <strong>OUTGOING_REG_PARM_STACK_SPACE</strong> <em>(<var>fntype</var>)</em></dt>
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<dd><p>Define this to a nonzero value if it is the responsibility of the
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caller to allocate the area reserved for arguments passed in registers
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when calling a function of <var>fntype</var>. <var>fntype</var> may be NULL
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if the function called is a library function.
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</p>
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<p>If <code>ACCUMULATE_OUTGOING_ARGS</code> is defined, this macro controls
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whether the space for these arguments counts in the value of
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<code>crtl->outgoing_args_size</code>.
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</p></dd></dl>
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<dl>
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<dt><a name="index-STACK_005fPARMS_005fIN_005fREG_005fPARM_005fAREA"></a>Macro: <strong>STACK_PARMS_IN_REG_PARM_AREA</strong></dt>
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<dd><p>Define this macro if <code>REG_PARM_STACK_SPACE</code> is defined, but the
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stack parameters don’t skip the area specified by it.
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</p>
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<p>Normally, when a parameter is not passed in registers, it is placed on the
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stack beyond the <code>REG_PARM_STACK_SPACE</code> area. Defining this macro
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suppresses this behavior and causes the parameter to be passed on the
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stack in its natural location.
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</p></dd></dl>
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<dl>
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<dt><a name="index-TARGET_005fRETURN_005fPOPS_005fARGS"></a>Target Hook: <em>int</em> <strong>TARGET_RETURN_POPS_ARGS</strong> <em>(tree <var>fundecl</var>, tree <var>funtype</var>, int <var>size</var>)</em></dt>
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<dd><p>This target hook returns the number of bytes of its own arguments that
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a function pops on returning, or 0 if the function pops no arguments
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and the caller must therefore pop them all after the function returns.
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</p>
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<p><var>fundecl</var> is a C variable whose value is a tree node that describes
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the function in question. Normally it is a node of type
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<code>FUNCTION_DECL</code> that describes the declaration of the function.
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From this you can obtain the <code>DECL_ATTRIBUTES</code> of the function.
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</p>
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<p><var>funtype</var> is a C variable whose value is a tree node that
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describes the function in question. Normally it is a node of type
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<code>FUNCTION_TYPE</code> that describes the data type of the function.
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From this it is possible to obtain the data types of the value and
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arguments (if known).
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</p>
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<p>When a call to a library function is being considered, <var>fundecl</var>
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will contain an identifier node for the library function. Thus, if
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you need to distinguish among various library functions, you can do so
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by their names. Note that “library function” in this context means
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a function used to perform arithmetic, whose name is known specially
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in the compiler and was not mentioned in the C code being compiled.
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</p>
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<p><var>size</var> is the number of bytes of arguments passed on the
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stack. If a variable number of bytes is passed, it is zero, and
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argument popping will always be the responsibility of the calling function.
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</p>
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<p>On the VAX, all functions always pop their arguments, so the definition
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of this macro is <var>size</var>. On the 68000, using the standard
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calling convention, no functions pop their arguments, so the value of
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the macro is always 0 in this case. But an alternative calling
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convention is available in which functions that take a fixed number of
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arguments pop them but other functions (such as <code>printf</code>) pop
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nothing (the caller pops all). When this convention is in use,
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<var>funtype</var> is examined to determine whether a function takes a fixed
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number of arguments.
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</p></dd></dl>
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<dl>
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<dt><a name="index-CALL_005fPOPS_005fARGS"></a>Macro: <strong>CALL_POPS_ARGS</strong> <em>(<var>cum</var>)</em></dt>
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<dd><p>A C expression that should indicate the number of bytes a call sequence
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pops off the stack. It is added to the value of <code>RETURN_POPS_ARGS</code>
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when compiling a function call.
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</p>
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<p><var>cum</var> is the variable in which all arguments to the called function
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have been accumulated.
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</p>
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<p>On certain architectures, such as the SH5, a call trampoline is used
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that pops certain registers off the stack, depending on the arguments
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that have been passed to the function. Since this is a property of the
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call site, not of the called function, <code>RETURN_POPS_ARGS</code> is not
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appropriate.
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</p></dd></dl>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Register-Arguments.html#Register-Arguments" accesskey="n" rel="next">Register Arguments</a>, Previous: <a href="Elimination.html#Elimination" accesskey="p" rel="prev">Elimination</a>, Up: <a href="Stack-and-Calling.html#Stack-and-Calling" accesskey="u" rel="up">Stack and Calling</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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