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| <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—the static chain value and the function address—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—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’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> |
| |
| <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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