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<a name="Function-Entry"></a>
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<a name="Function-Entry-and-Exit"></a>
<h4 class="subsection">18.9.11 Function Entry and Exit</h4>
<a name="index-function-entry-and-exit"></a>
<a name="index-prologue"></a>
<a name="index-epilogue"></a>
<p>This section describes the macros that output function entry
(<em>prologue</em>) and exit (<em>epilogue</em>) code.
</p>
<dl>
<dt><a name="index-TARGET_005fASM_005fPRINT_005fPATCHABLE_005fFUNCTION_005fENTRY"></a>Target Hook: <em>void</em> <strong>TARGET_ASM_PRINT_PATCHABLE_FUNCTION_ENTRY</strong> <em>(FILE *<var>file</var>, unsigned HOST_WIDE_INT <var>patch_area_size</var>, bool <var>record_p</var>)</em></dt>
<dd><p>Generate a patchable area at the function start, consisting of
<var>patch_area_size</var> NOP instructions. If the target supports named
sections and if <var>record_p</var> is true, insert a pointer to the current
location in the table of patchable functions. The default implementation
of the hook places the table of pointers in the special section named
<code>__patchable_function_entries</code>.
</p></dd></dl>
<dl>
<dt><a name="index-TARGET_005fASM_005fFUNCTION_005fPROLOGUE"></a>Target Hook: <em>void</em> <strong>TARGET_ASM_FUNCTION_PROLOGUE</strong> <em>(FILE *<var>file</var>)</em></dt>
<dd><p>If defined, a function that outputs the assembler code for entry to a
function. The prologue is responsible for setting up the stack frame,
initializing the frame pointer register, saving registers that must be
saved, and allocating <var>size</var> additional bytes of storage for the
local variables. <var>file</var> is a stdio stream to which the assembler
code should be output.
</p>
<p>The label for the beginning of the function need not be output by this
macro. That has already been done when the macro is run.
</p>
<a name="index-regs_005fever_005flive"></a>
<p>To determine which registers to save, the macro can refer to the array
<code>regs_ever_live</code>: element <var>r</var> is nonzero if hard register
<var>r</var> is used anywhere within the function. This implies the function
prologue should save register <var>r</var>, provided it is not one of the
call-used registers. (<code>TARGET_ASM_FUNCTION_EPILOGUE</code> must likewise use
<code>regs_ever_live</code>.)
</p>
<p>On machines that have “register windows”, the function entry code does
not save on the stack the registers that are in the windows, even if
they are supposed to be preserved by function calls; instead it takes
appropriate steps to “push” the register stack, if any non-call-used
registers are used in the function.
</p>
<a name="index-frame_005fpointer_005fneeded"></a>
<p>On machines where functions may or may not have frame-pointers, the
function entry code must vary accordingly; it must set up the frame
pointer if one is wanted, and not otherwise. To determine whether a
frame pointer is in wanted, the macro can refer to the variable
<code>frame_pointer_needed</code>. The variable’s value will be 1 at run
time in a function that needs a frame pointer. See <a href="Elimination.html#Elimination">Elimination</a>.
</p>
<p>The function entry code is responsible for allocating any stack space
required for the function. This stack space consists of the regions
listed below. In most cases, these regions are allocated in the
order listed, with the last listed region closest to the top of the
stack (the lowest address if <code>STACK_GROWS_DOWNWARD</code> is defined, and
the highest address if it is not defined). You can use a different order
for a machine if doing so is more convenient or required for
compatibility reasons. Except in cases where required by standard
or by a debugger, there is no reason why the stack layout used by GCC
need agree with that used by other compilers for a machine.
</p></dd></dl>
<dl>
<dt><a name="index-TARGET_005fASM_005fFUNCTION_005fEND_005fPROLOGUE"></a>Target Hook: <em>void</em> <strong>TARGET_ASM_FUNCTION_END_PROLOGUE</strong> <em>(FILE *<var>file</var>)</em></dt>
<dd><p>If defined, a function that outputs assembler code at the end of a
prologue. This should be used when the function prologue is being
emitted as RTL, and you have some extra assembler that needs to be
emitted. See <a href="Standard-Names.html#prologue-instruction-pattern">prologue instruction pattern</a>.
</p></dd></dl>
<dl>
<dt><a name="index-TARGET_005fASM_005fFUNCTION_005fBEGIN_005fEPILOGUE"></a>Target Hook: <em>void</em> <strong>TARGET_ASM_FUNCTION_BEGIN_EPILOGUE</strong> <em>(FILE *<var>file</var>)</em></dt>
<dd><p>If defined, a function that outputs assembler code at the start of an
epilogue. This should be used when the function epilogue is being
emitted as RTL, and you have some extra assembler that needs to be
emitted. See <a href="Standard-Names.html#epilogue-instruction-pattern">epilogue instruction pattern</a>.
</p></dd></dl>
<dl>
<dt><a name="index-TARGET_005fASM_005fFUNCTION_005fEPILOGUE"></a>Target Hook: <em>void</em> <strong>TARGET_ASM_FUNCTION_EPILOGUE</strong> <em>(FILE *<var>file</var>)</em></dt>
<dd><p>If defined, a function that outputs the assembler code for exit from a
function. The epilogue is responsible for restoring the saved
registers and stack pointer to their values when the function was
called, and returning control to the caller. This macro takes the
same argument as the macro <code>TARGET_ASM_FUNCTION_PROLOGUE</code>, and the
registers to restore are determined from <code>regs_ever_live</code> and
<code>CALL_USED_REGISTERS</code> in the same way.
</p>
<p>On some machines, there is a single instruction that does all the work
of returning from the function. On these machines, give that
instruction the name ‘<samp>return</samp>’ and do not define the macro
<code>TARGET_ASM_FUNCTION_EPILOGUE</code> at all.
</p>
<p>Do not define a pattern named ‘<samp>return</samp>’ if you want the
<code>TARGET_ASM_FUNCTION_EPILOGUE</code> to be used. If you want the target
switches to control whether return instructions or epilogues are used,
define a ‘<samp>return</samp>’ pattern with a validity condition that tests the
target switches appropriately. If the ‘<samp>return</samp>’ pattern’s validity
condition is false, epilogues will be used.
</p>
<p>On machines where functions may or may not have frame-pointers, the
function exit code must vary accordingly. Sometimes the code for these
two cases is completely different. To determine whether a frame pointer
is wanted, the macro can refer to the variable
<code>frame_pointer_needed</code>. The variable’s value will be 1 when compiling
a function that needs a frame pointer.
</p>
<p>Normally, <code>TARGET_ASM_FUNCTION_PROLOGUE</code> and
<code>TARGET_ASM_FUNCTION_EPILOGUE</code> must treat leaf functions specially.
The C variable <code>current_function_is_leaf</code> is nonzero for such a
function. See <a href="Leaf-Functions.html#Leaf-Functions">Leaf Functions</a>.
</p>
<p>On some machines, some functions pop their arguments on exit while
others leave that for the caller to do. For example, the 68020 when
given <samp>-mrtd</samp> pops arguments in functions that take a fixed
number of arguments.
</p>
<a name="index-pops_005fargs"></a>
<a name="index-crtl_002d_003eargs_002epops_005fargs"></a>
<p>Your definition of the macro <code>RETURN_POPS_ARGS</code> decides which
functions pop their own arguments. <code>TARGET_ASM_FUNCTION_EPILOGUE</code>
needs to know what was decided. The number of bytes of the current
function’s arguments that this function should pop is available in
<code>crtl->args.pops_args</code>. See <a href="Scalar-Return.html#Scalar-Return">Scalar Return</a>.
</p></dd></dl>
<ul>
<li> <a name="index-pretend_005fargs_005fsize"></a>
<a name="index-crtl_002d_003eargs_002epretend_005fargs_005fsize"></a>
A region of <code>crtl->args.pretend_args_size</code> bytes of
uninitialized space just underneath the first argument arriving on the
stack. (This may not be at the very start of the allocated stack region
if the calling sequence has pushed anything else since pushing the stack
arguments. But usually, on such machines, nothing else has been pushed
yet, because the function prologue itself does all the pushing.) This
region is used on machines where an argument may be passed partly in
registers and partly in memory, and, in some cases to support the
features in <code><stdarg.h></code>.
</li><li> An area of memory used to save certain registers used by the function.
The size of this area, which may also include space for such things as
the return address and pointers to previous stack frames, is
machine-specific and usually depends on which registers have been used
in the function. Machines with register windows often do not require
a save area.
</li><li> A region of at least <var>size</var> bytes, possibly rounded up to an allocation
boundary, to contain the local variables of the function. On some machines,
this region and the save area may occur in the opposite order, with the
save area closer to the top of the stack.
</li><li> <a name="index-ACCUMULATE_005fOUTGOING_005fARGS-and-stack-frames"></a>
Optionally, when <code>ACCUMULATE_OUTGOING_ARGS</code> is defined, a region of
<code>crtl->outgoing_args_size</code> bytes to be used for outgoing
argument lists of the function. See <a href="Stack-Arguments.html#Stack-Arguments">Stack Arguments</a>.
</li></ul>
<dl>
<dt><a name="index-EXIT_005fIGNORE_005fSTACK"></a>Macro: <strong>EXIT_IGNORE_STACK</strong></dt>
<dd><p>Define this macro as a C expression that is nonzero if the return
instruction or the function epilogue ignores the value of the stack
pointer; in other words, if it is safe to delete an instruction to
adjust the stack pointer before a return from the function. The
default is 0.
</p>
<p>Note that this macro’s value is relevant only for functions for which
frame pointers are maintained. It is never safe to delete a final
stack adjustment in a function that has no frame pointer, and the
compiler knows this regardless of <code>EXIT_IGNORE_STACK</code>.
</p></dd></dl>
<dl>
<dt><a name="index-EPILOGUE_005fUSES"></a>Macro: <strong>EPILOGUE_USES</strong> <em>(<var>regno</var>)</em></dt>
<dd><p>Define this macro as a C expression that is nonzero for registers that are
used by the epilogue or the ‘<samp>return</samp>’ pattern. The stack and frame
pointer registers are already assumed to be used as needed.
</p></dd></dl>
<dl>
<dt><a name="index-EH_005fUSES"></a>Macro: <strong>EH_USES</strong> <em>(<var>regno</var>)</em></dt>
<dd><p>Define this macro as a C expression that is nonzero for registers that are
used by the exception handling mechanism, and so should be considered live
on entry to an exception edge.
</p></dd></dl>
<dl>
<dt><a name="index-TARGET_005fASM_005fOUTPUT_005fMI_005fTHUNK"></a>Target Hook: <em>void</em> <strong>TARGET_ASM_OUTPUT_MI_THUNK</strong> <em>(FILE *<var>file</var>, tree <var>thunk_fndecl</var>, HOST_WIDE_INT <var>delta</var>, HOST_WIDE_INT <var>vcall_offset</var>, tree <var>function</var>)</em></dt>
<dd><p>A function that outputs the assembler code for a thunk
function, used to implement C++ virtual function calls with multiple
inheritance. The thunk acts as a wrapper around a virtual function,
adjusting the implicit object parameter before handing control off to
the real function.
</p>
<p>First, emit code to add the integer <var>delta</var> to the location that
contains the incoming first argument. Assume that this argument
contains a pointer, and is the one used to pass the <code>this</code> pointer
in C++. This is the incoming argument <em>before</em> the function prologue,
e.g. ‘<samp>%o0</samp>’ on a sparc. The addition must preserve the values of
all other incoming arguments.
</p>
<p>Then, if <var>vcall_offset</var> is nonzero, an additional adjustment should be
made after adding <code>delta</code>. In particular, if <var>p</var> is the
adjusted pointer, the following adjustment should be made:
</p>
<div class="smallexample">
<pre class="smallexample">p += (*((ptrdiff_t **)p))[vcall_offset/sizeof(ptrdiff_t)]
</pre></div>
<p>After the additions, emit code to jump to <var>function</var>, which is a
<code>FUNCTION_DECL</code>. This is a direct pure jump, not a call, and does
not touch the return address. Hence returning from <var>FUNCTION</var> will
return to whoever called the current ‘<samp>thunk</samp>’.
</p>
<p>The effect must be as if <var>function</var> had been called directly with
the adjusted first argument. This macro is responsible for emitting all
of the code for a thunk function; <code>TARGET_ASM_FUNCTION_PROLOGUE</code>
and <code>TARGET_ASM_FUNCTION_EPILOGUE</code> are not invoked.
</p>
<p>The <var>thunk_fndecl</var> is redundant. (<var>delta</var> and <var>function</var>
have already been extracted from it.) It might possibly be useful on
some targets, but probably not.
</p>
<p>If you do not define this macro, the target-independent code in the C++
front end will generate a less efficient heavyweight thunk that calls
<var>function</var> instead of jumping to it. The generic approach does
not support varargs.
</p></dd></dl>
<dl>
<dt><a name="index-TARGET_005fASM_005fCAN_005fOUTPUT_005fMI_005fTHUNK"></a>Target Hook: <em>bool</em> <strong>TARGET_ASM_CAN_OUTPUT_MI_THUNK</strong> <em>(const_tree <var>thunk_fndecl</var>, HOST_WIDE_INT <var>delta</var>, HOST_WIDE_INT <var>vcall_offset</var>, const_tree <var>function</var>)</em></dt>
<dd><p>A function that returns true if TARGET_ASM_OUTPUT_MI_THUNK would be able
to output the assembler code for the thunk function specified by the
arguments it is passed, and false otherwise. In the latter case, the
generic approach will be used by the C++ front end, with the limitations
previously exposed.
</p></dd></dl>
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