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<a name="Trampolines"></a>
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<hr>
<a name="Trampolines-for-Nested-Functions"></a>
<h3 class="section">17.11 Trampolines for Nested Functions</h3>
<a name="index-trampolines-for-nested-functions"></a>
<a name="index-nested-functions_002c-trampolines-for"></a>

<p>A <em>trampoline</em> is a small piece of code that is created at run time
when the address of a nested function is taken.  It normally resides on
the stack, in the stack frame of the containing function.  These macros
tell GCC how to generate code to allocate and initialize a
trampoline.
</p>
<p>The instructions in the trampoline must do two things: load a constant
address into the static chain register, and jump to the real address of
the nested function.  On CISC machines such as the m68k, this requires
two instructions, a move immediate and a jump.  Then the two addresses
exist in the trampoline as word-long immediate operands.  On RISC
machines, it is often necessary to load each address into a register in
two parts.  Then pieces of each address form separate immediate
operands.
</p>
<p>The code generated to initialize the trampoline must store the variable
parts&mdash;the static chain value and the function address&mdash;into the
immediate operands of the instructions.  On a CISC machine, this is
simply a matter of copying each address to a memory reference at the
proper offset from the start of the trampoline.  On a RISC machine, it
may be necessary to take out pieces of the address and store them
separately.
</p>
<dl>
<dt><a name="index-TARGET_005fASM_005fTRAMPOLINE_005fTEMPLATE"></a>Target Hook: <em>void</em> <strong>TARGET_ASM_TRAMPOLINE_TEMPLATE</strong> <em>(FILE *<var>f</var>)</em></dt>
<dd><p>This hook is called by <code>assemble_trampoline_template</code> to output,
on the stream <var>f</var>, assembler code for a block of data that contains
the constant parts of a trampoline.  This code should not include a
label&mdash;the label is taken care of automatically.
</p>
<p>If you do not define this hook, it means no template is needed
for the target.  Do not define this hook on systems where the block move
code to copy the trampoline into place would be larger than the code
to generate it on the spot.
</p></dd></dl>

<dl>
<dt><a name="index-TRAMPOLINE_005fSECTION"></a>Macro: <strong>TRAMPOLINE_SECTION</strong></dt>
<dd><p>Return the section into which the trampoline template is to be placed
(see <a href="Sections.html#Sections">Sections</a>).  The default value is <code>readonly_data_section</code>.
</p></dd></dl>

<dl>
<dt><a name="index-TRAMPOLINE_005fSIZE"></a>Macro: <strong>TRAMPOLINE_SIZE</strong></dt>
<dd><p>A C expression for the size in bytes of the trampoline, as an integer.
</p></dd></dl>

<dl>
<dt><a name="index-TRAMPOLINE_005fALIGNMENT"></a>Macro: <strong>TRAMPOLINE_ALIGNMENT</strong></dt>
<dd><p>Alignment required for trampolines, in bits.
</p>
<p>If you don&rsquo;t define this macro, the value of <code>FUNCTION_ALIGNMENT</code>
is used for aligning trampolines.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fTRAMPOLINE_005fINIT"></a>Target Hook: <em>void</em> <strong>TARGET_TRAMPOLINE_INIT</strong> <em>(rtx <var>m_tramp</var>, tree <var>fndecl</var>, rtx <var>static_chain</var>)</em></dt>
<dd><p>This hook is called to initialize a trampoline.
<var>m_tramp</var> is an RTX for the memory block for the trampoline; <var>fndecl</var>
is the <code>FUNCTION_DECL</code> for the nested function; <var>static_chain</var> is an
RTX for the static chain value that should be passed to the function
when it is called.
</p>
<p>If the target defines <code>TARGET_ASM_TRAMPOLINE_TEMPLATE</code>, then the
first thing this hook should do is emit a block move into <var>m_tramp</var>
from the memory block returned by <code>assemble_trampoline_template</code>.
Note that the block move need only cover the constant parts of the
trampoline.  If the target isolates the variable parts of the trampoline
to the end, not all <code>TRAMPOLINE_SIZE</code> bytes need be copied.
</p>
<p>If the target requires any other actions, such as flushing caches or
enabling stack execution, these actions should be performed after
initializing the trampoline proper.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fTRAMPOLINE_005fADJUST_005fADDRESS"></a>Target Hook: <em>rtx</em> <strong>TARGET_TRAMPOLINE_ADJUST_ADDRESS</strong> <em>(rtx <var>addr</var>)</em></dt>
<dd><p>This hook should perform any machine-specific adjustment in
the address of the trampoline.  Its argument contains the address of the
memory block that was passed to <code>TARGET_TRAMPOLINE_INIT</code>.  In case
the address to be used for a function call should be different from the
address at which the template was stored, the different address should
be returned; otherwise <var>addr</var> should be returned unchanged.
If this hook is not defined, <var>addr</var> will be used for function calls.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fCUSTOM_005fFUNCTION_005fDESCRIPTORS"></a>Target Hook: <em>int</em> <strong>TARGET_CUSTOM_FUNCTION_DESCRIPTORS</strong></dt>
<dd><p>This hook should be defined to a power of 2 if the target will benefit
from the use of custom descriptors for nested functions instead of the
standard trampolines.  Such descriptors are created at run time on the
stack and made up of data only, but they are non-standard so the generated
code must be prepared to deal with them.  This hook should be defined to 0
if the target uses function descriptors for its standard calling sequence,
like for example HP-PA or IA-64.  Using descriptors for nested functions
eliminates the need for trampolines that reside on the stack and require
it to be made executable.
</p>
<p>The value of the macro is used to parameterize the run-time identification
scheme implemented to distinguish descriptors from function addresses: it
gives the number of bytes by which their address is misaligned compared
with function addresses.  The value of 1 will generally work, unless it is
already reserved by the target for another purpose, like for example on ARM.
</p></dd></dl>

<p>Implementing trampolines is difficult on many machines because they have
separate instruction and data caches.  Writing into a stack location
fails to clear the memory in the instruction cache, so when the program
jumps to that location, it executes the old contents.
</p>
<p>Here are two possible solutions.  One is to clear the relevant parts of
the instruction cache whenever a trampoline is set up.  The other is to
make all trampolines identical, by having them jump to a standard
subroutine.  The former technique makes trampoline execution faster; the
latter makes initialization faster.
</p>
<p>To clear the instruction cache when a trampoline is initialized, define
the following macro.
</p>
<dl>
<dt><a name="index-CLEAR_005fINSN_005fCACHE"></a>Macro: <strong>CLEAR_INSN_CACHE</strong> <em>(<var>beg</var>, <var>end</var>)</em></dt>
<dd><p>If defined, expands to a C expression clearing the <em>instruction
cache</em> in the specified interval.  The definition of this macro would
typically be a series of <code>asm</code> statements.  Both <var>beg</var> and
<var>end</var> are both pointer expressions.
</p></dd></dl>

<p>To use a standard subroutine, define the following macro.  In addition,
you must make sure that the instructions in a trampoline fill an entire
cache line with identical instructions, or else ensure that the
beginning of the trampoline code is always aligned at the same point in
its cache line.  Look in <samp>m68k.h</samp> as a guide.
</p>
<dl>
<dt><a name="index-TRANSFER_005fFROM_005fTRAMPOLINE"></a>Macro: <strong>TRANSFER_FROM_TRAMPOLINE</strong></dt>
<dd><p>Define this macro if trampolines need a special subroutine to do their
work.  The macro should expand to a series of <code>asm</code> statements
which will be compiled with GCC.  They go in a library function named
<code>__transfer_from_trampoline</code>.
</p>
<p>If you need to avoid executing the ordinary prologue code of a compiled
C function when you jump to the subroutine, you can do so by placing a
special label of your own in the assembler code.  Use one <code>asm</code>
statement to generate an assembler label, and another to make the label
global.  Then trampolines can use that label to jump directly to your
special assembler code.
</p></dd></dl>

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