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Clarify a bit. Based on feedback by Talin. 

llvm-svn: 61470
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Duncan Sands 2008-12-29 15:27:32 +00:00
parent c125d6a3d3
commit 660b053fac
1 changed files with 21 additions and 20 deletions
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llvm/docs/ExceptionHandling.html
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@ -203,10 +203,9 @@ to the type info of the exception object.</p>
pad to the back end.</p>
<p><a href="#llvm_eh_exception"><tt>llvm.eh.exception</tt></a> takes no
arguments and returns the exception structure reference. The backend replaces
this intrinsic with the code that accesses the first argument of a call. The
LLVM C++ front end generates code to save this value in an alloca location for
further use in the landing pad and catch code.</p>
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
@ -220,8 +219,9 @@ The result of the <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> is a
positive number if the exception matched a type info, a negative number if it matched
a filter, and zero if it matched a cleanup. If nothing is matched, the behaviour of
the program is <a href="#restrictions">undefined</a>.
The LLVM C++ front end generates code to save the selector value in an alloca
location for further use in the landing pad and catch code.
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>
@ -276,17 +276,23 @@ instead.
<div class="doc_text">
<p>C++ allows the specification of which exception types that can be thrown from
<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 the
length of the filter expression (the number of type infos plus one), followed by
the type infos themselves.
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 require a reference to the
exception structure.</p>
<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>
@ -340,10 +346,7 @@ provide exception handling information at various points in generated code.</p>
i8* %<a href="#llvm_eh_exception">llvm.eh.exception</a>( )
</pre>
<p>This intrinsic indicates that the exception structure is available at this
point in the code. The backend will replace this intrinsic with code to fetch
the first argument of a call. The effect is that the intrinsic result is the
exception structure reference.</p>
<p>This intrinsic returns a pointer to the exception structure.</p>
</div>
@ -358,10 +361,8 @@ exception structure reference.</p>
i64 %<a href="#llvm_eh_selector">llvm.eh.selector.i64</a>(i8*, i8*, i8*, ...)
</pre>
<p>This intrinsic indicates that the exception selector is available at this
point in the code. The backend will replace this intrinsic with code to fetch
the second argument of a call. The effect is that the intrinsic result is the
exception selector.</p>
<p>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