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189 lines
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189 lines
9.5 KiB
HTML
4 years ago
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<html lang="en">
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<title>Trampolines - GNU Compiler Collection (GCC) Internals</title>
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<link rel="up" href="Target-Macros.html#Target-Macros" title="Target Macros">
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<link rel="prev" href="Varargs.html#Varargs" title="Varargs">
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<link rel="next" href="Library-Calls.html#Library-Calls" title="Library Calls">
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Copyright (C) 1988-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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</head>
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<body>
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<div class="node">
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<a name="Trampolines"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Library-Calls.html#Library-Calls">Library Calls</a>,
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Previous: <a rel="previous" accesskey="p" href="Varargs.html#Varargs">Varargs</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">17.11 Trampolines for Nested Functions</h3>
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<p><a name="index-trampolines-for-nested-functions-4343"></a><a name="index-nested-functions_002c-trampolines-for-4344"></a>
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A <dfn>trampoline</dfn> is a small piece of code that is created at run time
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when the address of a nested function is taken. It normally resides on
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the stack, in the stack frame of the containing function. These macros
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tell GCC how to generate code to allocate and initialize a
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trampoline.
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<p>The instructions in the trampoline must do two things: load a constant
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address into the static chain register, and jump to the real address of
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the nested function. On CISC machines such as the m68k, this requires
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two instructions, a move immediate and a jump. Then the two addresses
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exist in the trampoline as word-long immediate operands. On RISC
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machines, it is often necessary to load each address into a register in
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two parts. Then pieces of each address form separate immediate
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operands.
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<p>The code generated to initialize the trampoline must store the variable
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parts—the static chain value and the function address—into the
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immediate operands of the instructions. On a CISC machine, this is
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simply a matter of copying each address to a memory reference at the
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proper offset from the start of the trampoline. On a RISC machine, it
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may be necessary to take out pieces of the address and store them
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separately.
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<div class="defun">
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— Target Hook: void <b>TARGET_ASM_TRAMPOLINE_TEMPLATE</b> (<var>FILE *f</var>)<var><a name="index-TARGET_005fASM_005fTRAMPOLINE_005fTEMPLATE-4345"></a></var><br>
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<blockquote><p>This hook is called by <code>assemble_trampoline_template</code> to output,
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on the stream <var>f</var>, assembler code for a block of data that contains
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the constant parts of a trampoline. This code should not include a
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label—the label is taken care of automatically.
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<p>If you do not define this hook, it means no template is needed
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for the target. Do not define this hook on systems where the block move
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code to copy the trampoline into place would be larger than the code
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to generate it on the spot.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>TRAMPOLINE_SECTION</b><var><a name="index-TRAMPOLINE_005fSECTION-4346"></a></var><br>
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<blockquote><p>Return the section into which the trampoline template is to be placed
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(see <a href="Sections.html#Sections">Sections</a>). The default value is <code>readonly_data_section</code>.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>TRAMPOLINE_SIZE</b><var><a name="index-TRAMPOLINE_005fSIZE-4347"></a></var><br>
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<blockquote><p>A C expression for the size in bytes of the trampoline, as an integer.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>TRAMPOLINE_ALIGNMENT</b><var><a name="index-TRAMPOLINE_005fALIGNMENT-4348"></a></var><br>
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<blockquote><p>Alignment required for trampolines, in bits.
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<p>If you don't define this macro, the value of <code>FUNCTION_ALIGNMENT</code>
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is used for aligning trampolines.
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</p></blockquote></div>
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<div class="defun">
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— Target Hook: void <b>TARGET_TRAMPOLINE_INIT</b> (<var>rtx m_tramp, tree fndecl, rtx static_chain</var>)<var><a name="index-TARGET_005fTRAMPOLINE_005fINIT-4349"></a></var><br>
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<blockquote><p>This hook is called to initialize a trampoline.
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<var>m_tramp</var> is an RTX for the memory block for the trampoline; <var>fndecl</var>
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is the <code>FUNCTION_DECL</code> for the nested function; <var>static_chain</var> is an
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RTX for the static chain value that should be passed to the function
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when it is called.
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<p>If the target defines <code>TARGET_ASM_TRAMPOLINE_TEMPLATE</code>, then the
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first thing this hook should do is emit a block move into <var>m_tramp</var>
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from the memory block returned by <code>assemble_trampoline_template</code>.
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Note that the block move need only cover the constant parts of the
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trampoline. If the target isolates the variable parts of the trampoline
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to the end, not all <code>TRAMPOLINE_SIZE</code> bytes need be copied.
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<p>If the target requires any other actions, such as flushing caches or
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enabling stack execution, these actions should be performed after
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initializing the trampoline proper.
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</p></blockquote></div>
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<div class="defun">
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— Target Hook: rtx <b>TARGET_TRAMPOLINE_ADJUST_ADDRESS</b> (<var>rtx addr</var>)<var><a name="index-TARGET_005fTRAMPOLINE_005fADJUST_005fADDRESS-4350"></a></var><br>
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<blockquote><p>This hook should perform any machine-specific adjustment in
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the address of the trampoline. Its argument contains the address of the
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memory block that was passed to <code>TARGET_TRAMPOLINE_INIT</code>. In case
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the address to be used for a function call should be different from the
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address at which the template was stored, the different address should
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be returned; otherwise <var>addr</var> should be returned unchanged.
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If this hook is not defined, <var>addr</var> will be used for function calls.
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</p></blockquote></div>
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<p>Implementing trampolines is difficult on many machines because they have
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separate instruction and data caches. Writing into a stack location
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fails to clear the memory in the instruction cache, so when the program
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jumps to that location, it executes the old contents.
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<p>Here are two possible solutions. One is to clear the relevant parts of
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the instruction cache whenever a trampoline is set up. The other is to
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make all trampolines identical, by having them jump to a standard
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subroutine. The former technique makes trampoline execution faster; the
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latter makes initialization faster.
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<p>To clear the instruction cache when a trampoline is initialized, define
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the following macro.
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<div class="defun">
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— Macro: <b>CLEAR_INSN_CACHE</b> (<var>beg, end</var>)<var><a name="index-CLEAR_005fINSN_005fCACHE-4351"></a></var><br>
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<blockquote><p>If defined, expands to a C expression clearing the <em>instruction
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cache</em> in the specified interval. The definition of this macro would
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typically be a series of <code>asm</code> statements. Both <var>beg</var> and
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<var>end</var> are both pointer expressions.
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</p></blockquote></div>
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<p>To use a standard subroutine, define the following macro. In addition,
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you must make sure that the instructions in a trampoline fill an entire
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cache line with identical instructions, or else ensure that the
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beginning of the trampoline code is always aligned at the same point in
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its cache line. Look in <samp><span class="file">m68k.h</span></samp> as a guide.
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<div class="defun">
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— Macro: <b>TRANSFER_FROM_TRAMPOLINE</b><var><a name="index-TRANSFER_005fFROM_005fTRAMPOLINE-4352"></a></var><br>
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<blockquote><p>Define this macro if trampolines need a special subroutine to do their
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work. The macro should expand to a series of <code>asm</code> statements
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which will be compiled with GCC. They go in a library function named
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<code>__transfer_from_trampoline</code>.
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<p>If you need to avoid executing the ordinary prologue code of a compiled
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C function when you jump to the subroutine, you can do so by placing a
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special label of your own in the assembler code. Use one <code>asm</code>
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statement to generate an assembler label, and another to make the label
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global. Then trampolines can use that label to jump directly to your
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special assembler code.
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</p></blockquote></div>
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</body></html>
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