You cannot select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
239 lines
12 KiB
HTML
239 lines
12 KiB
HTML
4 years ago
|
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
||
|
|
<html>
|
||
|
|
<!-- Copyright (C) 1988-2018 Free Software Foundation, Inc.
|
||
|
|
|
||
|
|
Permission is granted to copy, distribute and/or modify this document
|
||
|
|
under the terms of the GNU Free Documentation License, Version 1.3 or
|
||
|
|
any later version published by the Free Software Foundation; with the
|
||
|
|
Invariant Sections being "Funding Free Software", the Front-Cover
|
||
|
|
Texts being (a) (see below), and with the Back-Cover Texts being (b)
|
||
|
|
(see below). A copy of the license is included in the section entitled
|
||
|
|
"GNU Free Documentation License".
|
||
|
|
|
||
|
|
(a) The FSF's Front-Cover Text is:
|
||
|
|
|
||
|
|
A GNU Manual
|
||
|
|
|
||
|
|
(b) The FSF's Back-Cover Text is:
|
||
|
|
|
||
|
|
You have freedom to copy and modify this GNU Manual, like GNU
|
||
|
|
software. Copies published by the Free Software Foundation raise
|
||
|
|
funds for GNU development. -->
|
||
|
|
<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
|
||
|
|
<head>
|
||
|
|
<title>Trampolines (GNU Compiler Collection (GCC) Internals)</title>
|
||
|
|
|
||
|
|
<meta name="description" content="Trampolines (GNU Compiler Collection (GCC) Internals)">
|
||
|
|
<meta name="keywords" content="Trampolines (GNU Compiler Collection (GCC) Internals)">
|
||
|
|
<meta name="resource-type" content="document">
|
||
|
|
<meta name="distribution" content="global">
|
||
|
|
<meta name="Generator" content="makeinfo">
|
||
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
||
|
|
<link href="index.html#Top" rel="start" title="Top">
|
||
|
|
<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
|
||
|
|
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
|
||
|
|
<link href="Target-Macros.html#Target-Macros" rel="up" title="Target Macros">
|
||
|
|
<link href="Library-Calls.html#Library-Calls" rel="next" title="Library Calls">
|
||
|
|
<link href="Varargs.html#Varargs" rel="prev" title="Varargs">
|
||
|
|
<style type="text/css">
|
||
|
|
<!--
|
||
|
|
a.summary-letter {text-decoration: none}
|
||
|
|
blockquote.indentedblock {margin-right: 0em}
|
||
|
|
blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
|
||
|
|
blockquote.smallquotation {font-size: smaller}
|
||
|
|
div.display {margin-left: 3.2em}
|
||
|
|
div.example {margin-left: 3.2em}
|
||
|
|
div.lisp {margin-left: 3.2em}
|
||
|
|
div.smalldisplay {margin-left: 3.2em}
|
||
|
|
div.smallexample {margin-left: 3.2em}
|
||
|
|
div.smalllisp {margin-left: 3.2em}
|
||
|
|
kbd {font-style: oblique}
|
||
|
|
pre.display {font-family: inherit}
|
||
|
|
pre.format {font-family: inherit}
|
||
|
|
pre.menu-comment {font-family: serif}
|
||
|
|
pre.menu-preformatted {font-family: serif}
|
||
|
|
pre.smalldisplay {font-family: inherit; font-size: smaller}
|
||
|
|
pre.smallexample {font-size: smaller}
|
||
|
|
pre.smallformat {font-family: inherit; font-size: smaller}
|
||
|
|
pre.smalllisp {font-size: smaller}
|
||
|
|
span.nolinebreak {white-space: nowrap}
|
||
|
|
span.roman {font-family: initial; font-weight: normal}
|
||
|
|
span.sansserif {font-family: sans-serif; font-weight: normal}
|
||
|
|
ul.no-bullet {list-style: none}
|
||
|
|
-->
|
||
|
|
</style>
|
||
|
|
|
||
|
|
|
||
|
|
</head>
|
||
|
|
|
||
|
|
<body lang="en">
|
||
|
|
<a name="Trampolines"></a>
|
||
|
|
<div class="header">
|
||
|
|
<p>
|
||
|
|
Next: <a href="Library-Calls.html#Library-Calls" accesskey="n" rel="next">Library Calls</a>, Previous: <a href="Varargs.html#Varargs" accesskey="p" rel="prev">Varargs</a>, Up: <a href="Target-Macros.html#Target-Macros" accesskey="u" rel="up">Target Macros</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>
|
||
|
|
</div>
|
||
|
|
<hr>
|
||
|
|
<a name="Trampolines-for-Nested-Functions"></a>
|
||
|
|
<h3 class="section">18.11 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>
|
||
|
|
|
||
|
|
<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>
|
||
|
|
|
||
|
|
<hr>
|
||
|
|
<div class="header">
|
||
|
|
<p>
|
||
|
|
Next: <a href="Library-Calls.html#Library-Calls" accesskey="n" rel="next">Library Calls</a>, Previous: <a href="Varargs.html#Varargs" accesskey="p" rel="prev">Varargs</a>, Up: <a href="Target-Macros.html#Target-Macros" accesskey="u" rel="up">Target Macros</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>
|
||
|
|
</div>
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
</body>
|
||
|
|
</html>
|