mirror of
https://github.com/c64scene-ar/llvm-6502.git
synced 2025-11-03 14:21:30 +00:00
d40bc4ad43
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@42232 91177308-0d34-0410-b5e6-96231b3b80d8
479 lines
20 KiB
HTML
479 lines
20 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
|
|
"http://www.w3.org/TR/html4/strict.dtd">
|
|
<html>
|
|
<head>
|
|
<title>Exception Handling in LLVM</title>
|
|
<link rel="stylesheet" href="llvm.css" type="text/css">
|
|
</head>
|
|
<body>
|
|
|
|
<div class="doc_title">Exception Handling in LLVM</div>
|
|
|
|
<table class="layout" style="width:100%">
|
|
<tr class="layout">
|
|
<td class="left">
|
|
<ul>
|
|
<li><a href="#introduction">Introduction</a>
|
|
<ol>
|
|
<li><a href="#itanium">Itanium ABI Zero-cost Exception Handling</a></li>
|
|
<li><a href="#overview">Overview</a></li>
|
|
</ol></li>
|
|
<li><a href="#codegen">LLVM Code Generation</a>
|
|
<ol>
|
|
<li><a href="#throw">Throw</a></li>
|
|
<li><a href="#try_catch">Try/Catch</a></li>
|
|
<li><a href="#cleanups">Cleanups</a></li>
|
|
<li><a href="#throw_filters">Throw Filters</a></li>
|
|
<li><a href="#restrictions">Restrictions</a></li>
|
|
</ol></li>
|
|
<li><a href="#format_common_intrinsics">Exception Handling Intrinsics</a>
|
|
<ol>
|
|
<li><a href="#llvm_eh_exception"><tt>llvm.eh.exception</tt></a></li>
|
|
<li><a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a></li>
|
|
<li><a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a></li>
|
|
</ol></li>
|
|
<li><a href="#asm">Asm Table Formats</a>
|
|
<ol>
|
|
<li><a href="#unwind_tables">Exception Handling Frame</a></li>
|
|
<li><a href="#exception_tables">Exception Tables</a></li>
|
|
</ol></li>
|
|
<li><a href="#todo">ToDo</a></li>
|
|
</ul>
|
|
</td>
|
|
</tr></table>
|
|
|
|
<div class="doc_author">
|
|
<p>Written by <a href="mailto:jlaskey@mac.com">Jim Laskey</a></p>
|
|
</div>
|
|
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section"><a name="introduction">Introduction</a></div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>This document is the central repository for all information pertaining to
|
|
exception handling in LLVM. It describes the format that LLVM exception
|
|
handling information takes, which is useful for those interested in creating
|
|
front-ends or dealing directly with the information. Further, this document
|
|
provides specific examples of what exception handling information is used for
|
|
C/C++.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="itanium">Itanium ABI Zero-cost Exception Handling</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>Exception handling for most programming languages is designed to recover from
|
|
conditions that rarely occur during general use of an application. To that end,
|
|
exception handling should not interfere with the main flow of an
|
|
application's algorithm by performing checkpointing tasks such as saving
|
|
the current pc or register state.</p>
|
|
|
|
<p>The Itanium ABI Exception Handling Specification defines a methodology for
|
|
providing outlying data in the form of exception tables without inlining
|
|
speculative exception handling code in the flow of an application's main
|
|
algorithm. Thus, the specification is said to add "zero-cost" to the normal
|
|
execution of an application.</p>
|
|
|
|
<p>A more complete description of the Itanium ABI exception handling runtime
|
|
support of can be found at <a
|
|
href="http://www.codesourcery.com/cxx-abi/abi-eh.html">Itanium C++ ABI:
|
|
Exception Handling.</a> A description of the exception frame format can be found
|
|
at <a href="http://refspecs.freestandards.org/LSB_3.0.0/LSB-Core-generic/LSB-
|
|
Core-generic/ehframechpt.html">Exception Frames</a>, with details of the Dwarf
|
|
specification at <a href="http://www.eagercon.com/dwarf/dwarf3std.htm">Dwarf 3
|
|
Standard.</a> A description for the C++ exception table formats can be found at
|
|
<a href="http://www.codesourcery.com/cxx-abi/exceptions.pdf">Exception Handling
|
|
Tables.</a></p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="overview">Overview</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>When an exception is thrown in llvm code, the runtime does a best effort to
|
|
find a handler suited to process the circumstance.</p>
|
|
|
|
<p>The runtime first attempts to find an <i>exception frame</i> corresponding to
|
|
the function where the exception was thrown. If the programming language (ex.
|
|
C++) supports exception handling, the exception frame contains a reference to an
|
|
exception table describing how to process the exception. If the language (ex.
|
|
C) does not support exception handling or if the exception needs to be forwarded
|
|
to a prior activation, the exception frame contains information about how to
|
|
unwind the current activation and restore the state of the prior activation.
|
|
This process is repeated until the exception is handled. If the exception is
|
|
not handled and no activations remain, then the application is terminated with
|
|
an appropriate error message.</p>
|
|
|
|
<p>Since different programming languages have different behaviors when handling
|
|
exceptions, the exception handling ABI provides a mechanism for supplying
|
|
<i>personalities.</i> An exception handling personality is defined by way of a
|
|
<i>personality function</i> (ex. for C++ <tt>__gxx_personality_v0</tt>) which
|
|
receives the context of the exception, an <i>exception structure</i> containing
|
|
the exception object type and value, and a reference to the exception table for
|
|
the current function. The personality function for the current compile unit is
|
|
specified in a <i>common exception frame</i>.</p>
|
|
|
|
<p>The organization of an exception table is language dependent. For C++, an
|
|
exception table is organized as a series of code ranges defining what to do if
|
|
an exception occurs in that range. Typically, the information associated with a
|
|
range defines which types of exception objects (using C++ <i>type info</i>) that
|
|
are handled in that range, and an associated action that should take place.
|
|
Actions typically pass control to a <i>landing pad</i>.</p>
|
|
|
|
<p>A landing pad corresponds to the code found in the catch portion of a
|
|
try/catch sequence. When execution resumes at a landing pad, it receives the
|
|
exception structure and a selector corresponding to the <i>type</i> of exception
|
|
thrown. The selector is then used to determine which catch should actually
|
|
process the exception.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_section">
|
|
<a name="codegen">LLVM Code Generation</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>At the time of this writing, only C++ exception handling support is available
|
|
in LLVM. So the remainder of this document will be somewhat C++-centric.</p>
|
|
|
|
<p>From the C++ developers perspective, exceptions are defined in terms of the
|
|
<tt>throw</tt> and <tt>try/catch</tt> statements. In this section we will
|
|
describe the implementation of llvm exception handling in terms of C++
|
|
examples.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="throw">Throw</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>Languages that support exception handling typically provide a <tt>throw</tt>
|
|
operation to initiate the exception process. Internally, a throw operation
|
|
breaks down into two steps. First, a request is made to allocate exception
|
|
space for an exception structure. This structure needs to survive beyond the
|
|
current activation. This structure will contain the type and value of the
|
|
object being thrown. Second, a call is made to the runtime to raise the
|
|
exception, passing the exception structure as an argument.</p>
|
|
|
|
<p>In C++, the allocation of the exception structure is done by the
|
|
<tt>__cxa_allocate_exception</tt> runtime function. The exception raising is
|
|
handled by <tt>__cxa_throw</tt>. The type of the exception is represented using
|
|
a C++ RTTI type info structure.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="try_catch">Try/Catch</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>A call within the scope of a try statement can potentially raise an exception.
|
|
In those circumstances, the LLVM C++ front-end replaces the call with an
|
|
<tt>invoke</tt> instruction. Unlike a call, the invoke has two potential
|
|
continuation points; where to continue when the call succeeds as per normal, and
|
|
where to continue if the call raises an exception, either by a throw or the
|
|
unwinding of a throw.</p>
|
|
|
|
<p>The term used to define a the place where an invoke continues after an
|
|
exception is called a <i>landing pad</i>. LLVM landing pads are conceptually
|
|
alternative function entry points where a exception structure reference and a type
|
|
info index are passed in as arguments. The landing pad saves the exception
|
|
structure reference and then proceeds to select the catch block that corresponds
|
|
to the type info of the exception object.</p>
|
|
|
|
<p>Two llvm intrinsic functions are used convey information about the landing
|
|
pad to the back end.</p>
|
|
|
|
<p><a href="#llvm_eh_exception"><tt>llvm.eh.exception</tt></a> takes no
|
|
arguments and returns the exception structure reference. The backend replaces
|
|
this intrinsic with the code that accesses the first argument of a call. The
|
|
LLVM C++ front end generates code to save this value in an alloca location for
|
|
further use in the landing pad and catch code.</p>
|
|
|
|
<p><a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> takes a minimum of
|
|
three arguments. The first argument is the reference to the exception
|
|
structure. The second argument is a reference to the personality function to be
|
|
used for this try catch sequence. Each of the remaining arguments is either a
|
|
reference to the type info for a catch statement,
|
|
a <a href="#throw_filters">filter</a> expression,
|
|
or the number zero representing a <a href="#cleanups">cleanup</a>.
|
|
The exception is tested against the arguments sequentially from first to last.
|
|
The result of the <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> is a
|
|
positive number if the exception matched a type info, a negative number if it matched
|
|
a filter, and zero if it matched a cleanup. If nothing is matched, the behaviour of
|
|
the program is <a href="#restrictions">undefined</a>.
|
|
The LLVM C++ front end generates code to save the selector value in an alloca
|
|
location for further use in the landing pad and catch code.
|
|
If a type info matched then the selector value is the index of the type info in
|
|
the exception table, which can be obtained using the
|
|
<a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a> intrinsic.</p>
|
|
|
|
<p>Once the landing pad has the type info selector, the code branches to the
|
|
code for the first catch. The catch then checks the value of the type info
|
|
selector against the index of type info for that catch. Since the type info
|
|
index is not known until all the type info have been gathered in the backend,
|
|
the catch code will call the <a
|
|
href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a> intrinsic to
|
|
determine the index for a given type info. If the catch fails to match the
|
|
selector then control is passed on to the next catch. Note: Since the landing
|
|
pad will not be used if there is no match in the list of type info on the call
|
|
to <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a>, then neither the
|
|
last catch nor <i>catch all</i> need to perform the the check against the
|
|
selector.</p>
|
|
|
|
<p>Finally, the entry and exit of catch code is bracketed with calls to
|
|
<tt>__cxa_begin_catch</tt> and <tt>__cxa_end_catch</tt>.
|
|
<tt>__cxa_begin_catch</tt> takes a exception structure reference as an argument
|
|
and returns the value of the exception object. <tt>__cxa_end_catch</tt>
|
|
takes a exception structure reference as an argument. This function clears the
|
|
exception from the exception space. Note: a rethrow from within the catch may
|
|
replace this call with a <tt>__cxa_rethrow</tt>.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="cleanups">Cleanups</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>To handle destructors and cleanups in try code, control may not run directly
|
|
from a landing pad to the first catch. Control may actually flow from the
|
|
landing pad to clean up code and then to the first catch. Since the required
|
|
clean up for each invoke in a try may be different (ex., intervening
|
|
constructor), there may be several landing pads for a given try. If cleanups
|
|
need to be run, the number zero should be passed as the last
|
|
<a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> argument.
|
|
However for C++ a <tt>null i8*</tt> <a href="#restrictions">must</a> be passed
|
|
instead.
|
|
</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="throw_filters">Throw Filters</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>C++ allows the specification of which exception types that can be thrown from
|
|
a function. To represent this a top level landing pad may exist to filter out
|
|
invalid types. To express this in LLVM code the landing pad will call <a
|
|
href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a>. The arguments are the
|
|
length of the filter expression (the number of type infos plus one), followed by
|
|
the type infos themselves.
|
|
<a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> will return a negative
|
|
value if the exception does not match any of the type infos. If no match is
|
|
found then a call to <tt>__cxa_call_unexpected</tt> should be made, otherwise
|
|
<tt>_Unwind_Resume</tt>. Each of these functions require a reference to the
|
|
exception structure.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="restrictions">Restrictions</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The semantics of the invoke instruction require that any exception that
|
|
unwinds through an invoke call should result in a branch to the invoke's unwind
|
|
label. However such a branch will only happen if the
|
|
<a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> matches.
|
|
Thus in order to ensure correct operation, the front-end must only generate
|
|
<a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> calls that are
|
|
guaranteed to always match whatever exception unwinds through the invoke.
|
|
For most languages it is enough to pass zero, indicating the presence of
|
|
a <a href="#cleanups">cleanup</a>, as the last
|
|
<a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> argument.
|
|
However for C++ this is not sufficient, because the C++ personality function
|
|
will terminate the program if it detects that unwinding the exception only
|
|
results in matches with cleanups. For C++ a <tt>null i8*</tt> should
|
|
be passed as the last
|
|
<a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> argument instead.
|
|
This is interpreted as a catch-all by the C++ personality function, and will
|
|
always match.
|
|
</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_section">
|
|
<a name="format_common_intrinsics">Exception Handling Intrinsics</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>LLVM uses several intrinsic functions (name prefixed with "llvm.eh") to
|
|
provide exception handling information at various points in generated code.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsubsection">
|
|
<a name="llvm_eh_exception">llvm.eh.exception</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
<pre>
|
|
i8* %<a href="#llvm_eh_exception">llvm.eh.exception</a>( )
|
|
</pre>
|
|
|
|
<p>This intrinsic indicates that the exception structure is available at this
|
|
point in the code. The backend will replace this intrinsic with code to fetch
|
|
the first argument of a call. The effect is that the intrinsic result is the
|
|
exception structure reference.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsubsection">
|
|
<a name="llvm_eh_selector">llvm.eh.selector</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
<pre>
|
|
i32 %<a href="#llvm_eh_selector">llvm.eh.selector.i32</a>(i8*, i8*, i8*, ...)
|
|
i64 %<a href="#llvm_eh_selector">llvm.eh.selector.i64</a>(i8*, i8*, i8*, ...)
|
|
</pre>
|
|
|
|
<p>This intrinsic indicates that the exception selector is available at this
|
|
point in the code. The backend will replace this intrinsic with code to fetch
|
|
the second argument of a call. The effect is that the intrinsic result is the
|
|
exception selector.</p>
|
|
|
|
<p><a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> takes a minimum of
|
|
three arguments. The first argument is the reference to the exception
|
|
structure. The second argument is a reference to the personality function to be
|
|
used for this try catch sequence. Each of the remaining arguments is either a
|
|
reference to the type info for a catch statement,
|
|
a <a href="#throw_filters">filter</a> expression,
|
|
or the number zero representing a <a href="#cleanups">cleanup</a>.
|
|
The exception is tested against the arguments sequentially from first to last.
|
|
The result of the <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> is a
|
|
positive number if the exception matched a type info, a negative number if it matched
|
|
a filter, and zero if it matched a cleanup. If nothing is matched, the behaviour of
|
|
the program is <a href="#restrictions">undefined</a>.
|
|
If a type info matched then the selector value is the index of the type info in
|
|
the exception table, which can be obtained using the
|
|
<a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a> intrinsic.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsubsection">
|
|
<a name="llvm_eh_typeid_for">llvm.eh.typeid.for</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
<pre>
|
|
i32 %<a href="#llvm_eh_typeid_for">llvm.eh.typeid.for.i32</a>(i8*)
|
|
i64 %<a href="#llvm_eh_typeid_for">llvm.eh.typeid.for.i64</a>(i8*)
|
|
</pre>
|
|
|
|
<p>This intrinsic returns the type info index in the exception table of the
|
|
current function. This value can be used to compare against the result of <a
|
|
href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a>. The single argument is
|
|
a reference to a type info.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_section">
|
|
<a name="asm">Asm Table Formats</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>There are two tables that are used by the exception handling runtime to
|
|
determine which actions should take place when an exception is thrown.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="unwind_tables">Exception Handling Frame</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>An exception handling frame <tt>eh_frame</tt> is very similar to the unwind
|
|
frame used by dwarf debug info. The frame contains all the information
|
|
necessary to tear down the current frame and restore the state of the prior
|
|
frame. There is an exception handling frame for each function in a compile
|
|
unit, plus a common exception handling frame that defines information common to
|
|
all functions in the unit.</p>
|
|
|
|
<p>Todo - Table details here.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="exception_tables">Exception Tables</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>An exception table contains information about what actions to take when an
|
|
exception is thrown in a particular part of a function's code. There is
|
|
one exception table per function except leaf routines and functions that have
|
|
only calls to non-throwing functions will not need an exception table.</p>
|
|
|
|
<p>Todo - Table details here.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_section">
|
|
<a name="todo">ToDo</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<ol>
|
|
|
|
<li><p>Testing/Testing/Testing.</p></li>
|
|
|
|
</ol>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
<hr>
|
|
<address>
|
|
<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
|
|
src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
|
|
<a href="http://validator.w3.org/check/referer"><img
|
|
src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
|
|
|
|
<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
|
|
<a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
|
|
Last modified: $Date$
|
|
</address>
|
|
|
|
</body>
|
|
</html>
|