docs/ExceptionHandling.html - fp2-dev/platform/external/llvm - Gitiles

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   <title>Exception Handling in LLVM</title>
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 <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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	<!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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