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