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llvm-mirror/docs/ExceptionHandling.html
Duncan Sands fb6f54629f Clarify a bit. Based on feedback by Talin. 
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<!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 a pointer to the exception structure. This only returns a
sensible value if called after an invoke has branched to a landing pad. Due to
codegen limitations, it must currently be called in the landing pad itself.</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>.
This only returns a sensible value if called after an invoke has branched to a
landing pad. Due to codegen limitations, it must currently be called in the
landing pad itself.
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 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 a
reference to the exception structure, a reference to the personality function,
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 requires a reference to the
exception structure. Note that the most general form of an
<a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> call can contain
any number of type infos, filter expressions and cleanups (though having more
than one cleanup is pointless). The LLVM C++ front-end can generate such
<a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> calls due to inlining
creating nested exception handling scopes.</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 returns a pointer to the exception structure.</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 is used to compare the exception with the given type infos,
filters and cleanups.</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>
<!-- *********************************************************************** -->
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<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
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