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Add basic information about SJLJ EH

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@79714 91177308-0d34-0410-b5e6-96231b3b80d8
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Jim Grosbach 2009-08-22 01:42:39 +00:00
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docs/ExceptionHandling.html
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@ -20,6 +20,7 @@
<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>
@ -102,6 +103,38 @@
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="sjlj">Setjmp/Longjmp Exception Handling</a>
</div>
<div class="doc_text">
<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 try/catch blocks
or cleanups, that function registers itself on a global frame list. When
exceptions are being unwound, 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>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="overview">Overview</a>
@ -282,10 +315,10 @@
from the landing pad to clean up code and then to the first catch. Since the
required clean up for each <tt>invoke</tt> in a <tt>try</tt> may be different
(e.g. 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
try. If cleanups need to be run, an <tt>i32 0</tt> 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> <b><a href="#restrictions">must</a></b>
be passed instead.</p>
However, when using DWARF exception handling with C++, a <tt>i8* null</tt>
<a href="#restrictions">must</a> be passed instead.</p>
</div>