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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>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<meta name="description"
content="Exception Handling in LLVM.">
<link rel="stylesheet" href="llvm.css" type="text/css">
</head>
<body>
<h1>Exception Handling in LLVM</h1>
<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="#sjlj">Setjmp/Longjmp 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_typeid_for"><tt>llvm.eh.typeid.for</tt></a></li>
<li><a href="#llvm_eh_sjlj_setjmp"><tt>llvm.eh.sjlj.setjmp</tt></a></li>
<li><a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a></li>
<li><a href="#llvm_eh_sjlj_lsda"><tt>llvm.eh.sjlj.lsda</tt></a></li>
<li><a href="#llvm_eh_sjlj_callsite"><tt>llvm.eh.sjlj.callsite</tt></a></li>
<li><a href="#llvm_eh_sjlj_dispatchsetup"><tt>llvm.eh.sjlj.dispatchsetup</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>
</ul>
</td>
</tr></table>
<div class="doc_author">
<p>Written by <a href="mailto:jlaskey@mac.com">Jim Laskey</a></p>
</div>
<!-- *********************************************************************** -->
<h2><a name="introduction">Introduction</a></h2>
<!-- *********************************************************************** -->
<div>
<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
in C and C++.</p>
<!-- ======================================================================= -->
<h3>
<a name="itanium">Itanium ABI Zero-cost Exception Handling</a>
</h3>
<div>
<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 4 specification at
<a href="http://dwarfstd.org/Dwarf4Std.php">DWARF 4 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>
<!-- ======================================================================= -->
<h3>
<a name="sjlj">Setjmp/Longjmp Exception Handling</a>
</h3>
<div>
<p>Setjmp/Longjmp (SJLJ) based exception handling uses LLVM intrinsics
<a href="#llvm_eh_sjlj_setjmp"><tt>llvm.eh.sjlj.setjmp</tt></a> and
<a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a> to
handle control flow for exception handling.</p>
<p>For each function which does exception processing — be
it <tt>try</tt>/<tt>catch</tt> blocks or cleanups — that function
registers itself on a global frame list. When exceptions are unwinding, the
runtime uses this list to identify which functions need processing.<p>
<p>Landing pad selection is encoded in the call site entry of the function
context. The runtime returns to the function via
<a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a>, where
a switch table transfers control to the appropriate landing pad based on
the index stored in the function context.</p>
<p>In contrast to DWARF exception handling, which encodes exception regions
and frame information in out-of-line tables, SJLJ exception handling
builds and removes the unwind frame context at runtime. This results in
faster exception handling at the expense of slower execution when no
exceptions are thrown. As exceptions are, by their nature, intended for
uncommon code paths, DWARF exception handling is generally preferred to
SJLJ.</p>
</div>
<!-- ======================================================================= -->
<h3>
<a name="overview">Overview</a>
</h3>
<div>
<p>When an exception is thrown in LLVM code, the runtime does its best to find a
handler suited to processing 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
supports exception handling (e.g. C++), the exception frame contains a
reference to an exception table describing how to process the exception. If
the language does not support exception handling (e.g. C), 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>Because 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> (e.g. <tt>__gxx_personality_v0</tt>
in C++), 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 roughly to the code found in the <tt>catch</tt>
portion of a <tt>try</tt>/<tt>catch</tt> sequence. When execution resumes at
a landing pad, it receives an <i>exception structure</i> and a
<i>selector value</i> corresponding to the <i>type</i> of exception
thrown. The selector is then used to determine which <i>catch</i> should
actually process the exception.</p>
</div>
</div>
<!-- ======================================================================= -->
<h2>
<a name="codegen">LLVM Code Generation</a>
</h2>
<div>
<p>From a C++ developer's perspective, exceptions are defined in terms of the
<tt>throw</tt> and <tt>try</tt>/<tt>catch</tt> statements. In this section
we will describe the implementation of LLVM exception handling in terms of
C++ examples.</p>
<!-- ======================================================================= -->
<h3>
<a name="throw">Throw</a>
</h3>
<div>
<p>Languages that support exception handling typically provide a <tt>throw</tt>
operation to initiate the exception process. Internally, a <tt>throw</tt>
operation breaks down into two steps.</p>
<ol>
<li>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.</li>
<li>A call is made to the runtime to raise the exception, passing the
exception structure as an argument.</li>
</ol>
<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 structure.</p>
</div>
<!-- ======================================================================= -->
<h3>
<a name="try_catch">Try/Catch</a>
</h3>
<div>
<p>A call within the scope of a <i>try</i> 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 <tt>invoke</tt> has
two potential continuation points:</p>
<ol>
<li>where to continue when the call succeeds as per normal, and</li>
<li>where to continue if the call raises an exception, either by a throw or
the unwinding of a throw</li>
</ol>
<p>The term used to define a the place where an <tt>invoke</tt> continues after
an exception is called a <i>landing pad</i>. LLVM landing pads are
conceptually alternative function entry points where an 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>The LLVM <a href="LangRef.html#i_landingpad"><tt>landingpad</tt>
instruction</a> is used to convey information about the landing pad to the
back end. For C++, the <tt>landingpad</tt> instruction returns a pointer and
integer pair corresponding to the pointer to the <i>exception structure</i>
and the <i>selector value</i> respectively.</p>
<p>The <tt>landingpad</tt> instruction takes a reference to the personality
function to be used for this <tt>try</tt>/<tt>catch</tt> sequence. The
remainder of the instruction is a list of <i>cleanup</i>, <i>catch</i>,
and <i>filter</i> clauses. The exception is tested against the clauses
sequentially from first to last. The selector value 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 behavior 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>
<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 infos have been gathered in the
backend, the catch code must 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.</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>.</p>
<ul>
<li><tt>__cxa_begin_catch</tt> takes an exception structure reference as an
argument and returns the value of the exception object.</li>
<li><tt>__cxa_end_catch</tt> takes no arguments. This function:<br><br>
<ol>
<li>Locates the most recently caught exception and decrements its handler
count,</li>
<li>Removes the exception from the <i>caught</i> stack if the handler
count goes to zero, and</li>
<li>Destroys the exception if the handler count goes to zero and the
exception was not re-thrown by throw.</li>
</ol>
<p><b>Note:</b> a rethrow from within the catch may replace this call with
a <tt>__cxa_rethrow</tt>.</p></li>
</ul>
</div>
<!-- ======================================================================= -->
<h3>
<a name="cleanups">Cleanups</a>
</h3>
<div>
<p>A cleanup is extra code which needs to be run as part of unwinding a scope.
C++ destructors are a typical example, but other languages and language
extensions provide a variety of different kinds of cleanups. In general, a
landing pad may need to run arbitrary amounts of cleanup code before actually
entering a catch block. To indicate the presence of cleanups, a
<a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a>
should have a <i>cleanup</i> clause. Otherwise, the unwinder will not stop at
the landing pad if there are no catches or filters that require it to.</p>
<p><b>Note:</b> Do not allow a new exception to propagate out of the execution
of a cleanup. This can corrupt the internal state of the unwinder.
Different languages describe different high-level semantics for these
situations: for example, C++ requires that the process be terminated, whereas
Ada cancels both exceptions and throws a third.</p>
<p>When all cleanups are finished, if the exception is not handled by the
current function, resume unwinding by calling the
<a href="LangRef.html#i_resume"><tt>resume</tt> instruction</a>, passing in
the result of the <tt>landingpad</tt> instruction for the original landing
pad.</p>
</div>
<!-- ======================================================================= -->
<h3>
<a name="throw_filters">Throw Filters</a>
</h3>
<div>
<p>C++ allows the specification of which exception types may 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
<a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a> will
have a filter clause. The clause consists of an array of type infos.
<tt>landingpad</tt> 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 a
<a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a> can
have any number of catch, cleanup, and filter clauses (though having more
than one cleanup is pointless). The LLVM C++ front-end can generate such
<a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instructions</a> due
to inlining creating nested exception handling scopes.</p>
</div>
<!-- ======================================================================= -->
<h3>
<a name="restrictions">Restrictions</a>
</h3>
<div>
<p>The unwinder delegates the decision of whether to stop in a call frame to
that call frame's language-specific personality function. Not all unwinders
guarantee that they will stop to perform cleanups. For example, the GNU C++
unwinder doesn't do so unless the exception is actually caught somewhere
further up the stack.</p>
<p>In order for inlining to behave correctly, landing pads must be prepared to
handle selector results that they did not originally advertise. Suppose that
a function catches exceptions of type <tt>A</tt>, and it's inlined into a
function that catches exceptions of type <tt>B</tt>. The inliner will update
the <tt>landingpad</tt> instruction for the inlined landing pad to include
the fact that <tt>B</tt> is also caught. If that landing pad assumes that it
will only be entered to catch an <tt>A</tt>, it's in for a rude awakening.
Consequently, landing pads must test for the selector results they understand
and then resume exception propagation with the
<a href="LangRef.html#i_resume"><tt>resume</tt> instruction</a> if none of
the conditions match.</p>
</div>
</div>
<!-- ======================================================================= -->
<h2>
<a name="format_common_intrinsics">Exception Handling Intrinsics</a>
</h2>
<div>
<p>In addition to the
<a href="LangRef.html#i_landingpad"><tt>landingpad</tt></a> and
<a href="LangRef.html#i_resume"><tt>resume</tt></a> instructions, LLVM uses
several intrinsic functions (name prefixed with <i><tt>llvm.eh</tt></i>) to
provide exception handling information at various points in generated
code.</p>
<!-- ======================================================================= -->
<h4>
<a name="llvm_eh_typeid_for">llvm.eh.typeid.for</a>
</h4>
<div>
<pre>
i32 @llvm.eh.typeid.for(i8* %type_info)
</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="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a>.
The single argument is a reference to a type info.</p>
</div>
<!-- ======================================================================= -->
<h4>
<a name="llvm_eh_sjlj_setjmp">llvm.eh.sjlj.setjmp</a>
</h4>
<div>
<pre>
i32 @llvm.eh.sjlj.setjmp(i8* %setjmp_buf)
</pre>
<p>For SJLJ based exception handling, this intrinsic forces register saving for
the current function and stores the address of the following instruction for
use as a destination address
by <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a>. The
buffer format and the overall functioning of this intrinsic is compatible
with the GCC <tt>__builtin_setjmp</tt> implementation allowing code built
with the clang and GCC to interoperate.</p>
<p>The single parameter is a pointer to a five word buffer in which the calling
context is saved. The front end places the frame pointer in the first word,
and the target implementation of this intrinsic should place the destination
address for a
<a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a> in the
second word. The following three words are available for use in a
target-specific manner.</p>
</div>
<!-- ======================================================================= -->
<h4>
<a name="llvm_eh_sjlj_longjmp">llvm.eh.sjlj.longjmp</a>
</h4>
<div>
<pre>
void @llvm.eh.sjlj.longjmp(i8* %setjmp_buf)
</pre>
<p>For SJLJ based exception handling, the <tt>llvm.eh.sjlj.longjmp</tt>
intrinsic is used to implement <tt>__builtin_longjmp()</tt>. The single
parameter is a pointer to a buffer populated
by <a href="#llvm_eh_sjlj_setjmp"><tt>llvm.eh.sjlj.setjmp</tt></a>. The frame
pointer and stack pointer are restored from the buffer, then control is
transferred to the destination address.</p>
</div>
<!-- ======================================================================= -->
<h4>
<a name="llvm_eh_sjlj_lsda">llvm.eh.sjlj.lsda</a>
</h4>
<div>
<pre>
i8* @llvm.eh.sjlj.lsda()
</pre>
<p>For SJLJ based exception handling, the <tt>llvm.eh.sjlj.lsda</tt> intrinsic
returns the address of the Language Specific Data Area (LSDA) for the current
function. The SJLJ front-end code stores this address in the exception
handling function context for use by the runtime.</p>
</div>
<!-- ======================================================================= -->
<h4>
<a name="llvm_eh_sjlj_callsite">llvm.eh.sjlj.callsite</a>
</h4>
<div>
<pre>
void @llvm.eh.sjlj.callsite(i32 %call_site_num)
</pre>
<p>For SJLJ based exception handling, the <tt>llvm.eh.sjlj.callsite</tt>
intrinsic identifies the callsite value associated with the
following <tt>invoke</tt> instruction. This is used to ensure that landing
pad entries in the LSDA are generated in matching order.</p>
</div>
<!-- ======================================================================= -->
<h4>
<a name="llvm_eh_sjlj_dispatchsetup">llvm.eh.sjlj.dispatchsetup</a>
</h4>
<div>
<pre>
void @llvm.eh.sjlj.dispatchsetup(i32 %dispatch_value)
</pre>
<p>For SJLJ based exception handling, the <tt>llvm.eh.sjlj.dispatchsetup</tt>
intrinsic is used by targets to do any unwind edge setup they need. By
default, no action is taken.</p>
</div>
</div>
<!-- ======================================================================= -->
<h2>
<a name="asm">Asm Table Formats</a>
</h2>
<div>
<p>There are two tables that are used by the exception handling runtime to
determine which actions should be taken when an exception is thrown.</p>
<!-- ======================================================================= -->
<h3>
<a name="unwind_tables">Exception Handling Frame</a>
</h3>
<div>
<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>
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<h3>
<a name="exception_tables">Exception Tables</a>
</h3>
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<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 functions and functions that have
calls only to non-throwing functions. They do not need an exception
table.</p>
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<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
<a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>
Last modified: $Date: 2011-09-27 13:16:57 -0700 (Tue, 27 Sep 2011) $
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