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Update the EH doc to reflect the new EH model.

Browse Source 
This basically involved removing references to llvm.eh.exception,
llvm.eh.selector, and llvm.eh.resume and replacing them with references to the
landingpad and resume instructions.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@140128 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Bill Wendling
2011-09-20 01:08:53 +00:00
parent 50172e77bc
commit 4a065ddc59
1 changed files with 93 additions and 193 deletions
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docs/ExceptionHandling.html
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@@ -96,8 +96,8 @@
Exception Handling</a>. A description of the exception frame format can be Exception Handling</a>. A description of the exception frame format can be
found at found at
<a href="http://refspecs.freestandards.org/LSB_3.0.0/LSB-Core-generic/LSB-Core-generic/ehframechpt.html">Exception <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 3 specification at Frames</a>, with details of the DWARF 4 specification at
<a href="http://www.eagercon.com/dwarf/dwarf3std.htm">DWARF 3 Standard</a>. <a href="http://dwarfstd.org/Dwarf4Std.php">DWARF 4 Standard</a>.
A description for the C++ exception table formats can be found at A description for the C++ exception table formats can be found at
<a href="http://www.codesourcery.com/cxx-abi/exceptions.pdf">Exception Handling <a href="http://www.codesourcery.com/cxx-abi/exceptions.pdf">Exception Handling
Tables</a>.</p> Tables</a>.</p>
@@ -116,10 +116,10 @@
<a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a> to <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a> to
handle control flow for exception handling.</p> handle control flow for exception handling.</p>
<p>For each function which does exception processing, be it try/catch blocks <p>For each function which does exception processing &mdash; be
or cleanups, that function registers itself on a global frame list. When it <tt>try</tt>/<tt>catch</tt> blocks or cleanups &mdash; that function
exceptions are being unwound, the runtime uses this list to identify which registers itself on a global frame list. When exceptions are unwinding, the
functions need processing.<p> 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 <p>Landing pad selection is encoded in the call site entry of the function
context. The runtime returns to the function via context. The runtime returns to the function via
@@ -134,6 +134,7 @@
exceptions are thrown. As exceptions are, by their nature, intended for exceptions are thrown. As exceptions are, by their nature, intended for
uncommon code paths, DWARF exception handling is generally preferred to uncommon code paths, DWARF exception handling is generally preferred to
SJLJ.</p> SJLJ.</p>
</div> </div>
<!-- ======================================================================= --> <!-- ======================================================================= -->
@@ -176,8 +177,8 @@
should take place. Actions typically pass control to a <i>landing should take place. Actions typically pass control to a <i>landing
pad</i>.</p> pad</i>.</p>
<p>A landing pad corresponds to the code found in the <i>catch</i> portion of <p>A landing pad corresponds to the code found in the <tt>catch</tt> portion of
a <i>try</i>/<i>catch</i> sequence. When execution resumes at a landing a <tt>try</tt>/<tt>catch</tt> sequence. When execution resumes at a landing
pad, it receives the exception structure and a selector corresponding to 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 the <i>type</i> of exception thrown. The selector is then used to determine
which <i>catch</i> should actually process the exception.</p> which <i>catch</i> should actually process the exception.</p>
@@ -193,11 +194,8 @@
<div> <div>
<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 <p>From the C++ developers perspective, exceptions are defined in terms of the
<tt>throw</tt> and <tt>try</tt>/<tt>catch</tt> statements. In this section <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 we will describe the implementation of LLVM exception handling in terms of
C++ examples.</p> C++ examples.</p>
@@ -210,16 +208,19 @@
<p>Languages that support exception handling typically provide a <tt>throw</tt> <p>Languages that support exception handling typically provide a <tt>throw</tt>
operation to initiate the exception process. Internally, a throw operation operation to initiate the exception process. Internally, a throw operation
breaks down into two steps. First, a request is made to allocate exception breaks down into two steps.</p>
space for an exception structure. This structure needs to survive beyond the <ol>
current activation. This structure will contain the type and value of the <li>A request is made to allocate exception space for an exception structure.
object being thrown. Second, a call is made to the runtime to raise the This structure needs to survive beyond the current activation. This
exception, passing the exception structure as an argument.</p> 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 <p>In C++, the allocation of the exception structure is done by then
the <tt>__cxa_allocate_exception</tt> runtime function. The exception <tt>__cxa_allocate_exception</tt> runtime function. The exception raising is
raising is handled by <tt>__cxa_throw</tt>. The type of the exception is handled by <tt>__cxa_throw</tt>. The type of the exception is represented
represented using a C++ RTTI structure.</p> using a C++ RTTI structure.</p>
</div> </div>
@@ -244,50 +245,36 @@
saves the exception structure reference and then proceeds to select the catch saves the exception structure reference and then proceeds to select the catch
block that corresponds to the type info of the exception object.</p> block that corresponds to the type info of the exception object.</p>
<p>Two LLVM intrinsic functions are used to convey information about the landing <p>The LLVM <a href="LangRef.html#i_landingpad"><tt>landingpad</tt>
pad to the back end.</p> 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 exception structure and the
"selector value" respectively.</p>
<ol> <p>The <tt>landingpad</tt> instruction takes a reference to the personality
<li><a href="#llvm_eh_exception"><tt>llvm.eh.exception</tt></a> takes no function to be used for this <tt>try</tt>/<tt>catch</tt> sequence. The
arguments and returns a pointer to the exception structure. This only remainder of the instruction is a list of <i>catch</i> and <i>filter</i>
returns a sensible value if called after an <tt>invoke</tt> has branched clauses. The exception is tested against the clauses sequentially from first
to a landing pad. Due to code generation limitations, it must currently to last. The selector value is a positive number if the exception matched a
be called in the landing pad itself.</li> 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
<li><a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> takes a minimum is <a href="#restrictions">undefined</a>. If a type info matched, then the
of three arguments. The first argument is the reference to the exception selector value is the index of the type info in the exception table, which
structure. The second argument is a reference to the personality function can be obtained using the
to be used for this <tt>try</tt>/<tt>catch</tt> sequence. Each of the <a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a> intrinsic.</p>
remaining arguments is either a reference to the type info for
a <tt>catch</tt> statement, a <a href="#throw_filters">filter</a>
expression, or the number zero (<tt>0</tt>) 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 <tt>invoke</tt> 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.</li>
</ol>
<p>Once the landing pad has the type info selector, the code branches to the <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 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 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, index is not known until all the type info have been gathered in the backend,
the catch code will call the the catch code will call the
<a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a> intrinsic <a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a> intrinsic to
to determine the index for a given type info. If the catch fails to match determine the index for a given type info. If the catch fails to match the
the selector then control is passed on to the next catch. Note: Since the selector then control is passed on to the next catch. Note: Since the landing
landing pad will not be used if there is no match in the list of type info on pad will not be used if there is no match in the list of type info on the
the call to <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a>, then call to the <a href="LangRef.html#i_landingpad"><tt>landingpad</tt>
neither the last catch nor <i>catch all</i> need to perform the check instruction</a>, then neither the last catch nor <i>catch all</i> need to
against the selector.</p> perform the check against the selector.</p>
<p>Finally, the entry and exit of catch code is bracketed with calls <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> to <tt>__cxa_begin_catch</tt> and <tt>__cxa_end_catch</tt>.</p>
@@ -318,16 +305,14 @@
<div> <div>
<p>A cleanup is extra code which needs to be run as part of unwinding <p>A cleanup is extra code which needs to be run as part of unwinding a scope.
a scope. C++ destructors are a prominent example, but other C++ destructors are a prominent example, but other languages and language
languages and language extensions provide a variety of different extensions provide a variety of different kinds of cleanup. In general, a
kinds of cleanup. In general, a landing pad may need to run landing pad may need to run arbitrary amounts of cleanup code before actually
arbitrary amounts of cleanup code before actually entering a catch entering a catch block. To indicate the presence of cleanups, a
block. To indicate the presence of cleanups, a landing pad's call <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a>
to <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> should should have a <i>cleanup</i> clause. Otherwise, the unwinder will not stop at
end with the argument <tt>i32 0</tt>; otherwise, the unwinder will the landing pad if there are no catches or filters that require it to.</p>
not stop at the landing pad if there are no catches or filters that
require it to.</p>
<p>Do not allow a new exception to propagate out of the execution of a <p>Do not allow a new exception to propagate out of the execution of a
cleanup. This can corrupt the internal state of the unwinder. cleanup. This can corrupt the internal state of the unwinder.
@@ -337,9 +322,9 @@
<p>When all cleanups have completed, if the exception is not handled <p>When all cleanups have completed, if the exception is not handled
by the current function, resume unwinding by calling the by the current function, resume unwinding by calling the
<a href="#llvm_eh_resume"><tt>llvm.eh.resume</tt></a> intrinsic, <a href="LangRef.html#i_resume"><tt>resume</tt> instruction</a>, passing in
passing in the results of <tt>llvm.eh.exception</tt> and the results of the <tt>landingpad</tt> instruction for the original landing
<tt>llvm.eh.selector</tt> for the original landing pad.</p> pad.</p>
</div> </div>
@@ -352,21 +337,19 @@
<p>C++ allows the specification of which exception types can be thrown from a <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 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 invalid types. To express this in LLVM code the
call <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a>. The <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a> will
arguments are a reference to the exception structure, a reference to the have a filter clause. The clause consists of an array of type infos.
personality function, the length of the filter expression (the number of type <tt>landingpad</tt> will return a negative value if the exception does not
infos plus one), followed by the type infos themselves. match any of the type infos. If no match is found then a call
<a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> will return a to <tt>__cxa_call_unexpected</tt> should be made, otherwise
negative value if the exception does not match any of the type infos. If no <tt>_Unwind_Resume</tt>. Each of these functions requires a reference to the
match is found then a call to <tt>__cxa_call_unexpected</tt> should be made, exception structure. Note that the most general form of a
otherwise <tt>_Unwind_Resume</tt>. Each of these functions requires a <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a> can
reference to the exception structure. Note that the most general form of an have any number of catch, cleanup, and filter clauses (though having more
<a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> call can contain than one cleanup is pointless). The LLVM C++ front-end can generate such
any number of type infos, filter expressions and cleanups (though having more <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instructions</a> due
than one cleanup is pointless). The LLVM C++ front-end can generate such to inlining creating nested exception handling scopes.</p>
<a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> calls due to
inlining creating nested exception handling scopes.</p>
</div> </div>
@@ -377,29 +360,27 @@
<div> <div>
<p>The unwinder delegates the decision of whether to stop in a call <p>The unwinder delegates the decision of whether to stop in a call frame to
frame to that call frame's language-specific personality function. that call frame's language-specific personality function. Not all
Not all personalities functions guarantee that they will stop to personalities functions guarantee that they will stop to perform
perform cleanups: for example, the GNU C++ personality doesn't do cleanups. For example, the GNU C++ personality doesn't do so unless the
so unless the exception is actually caught somewhere further up the exception is actually caught somewhere further up the stack. When using this
stack. When using this personality to implement EH for a language personality to implement EH for a language that guarantees that cleanups will
that guarantees that cleanups will always be run, be sure to always be run, be sure to indicate a catch-all in the
indicate a catch-all in the <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a>
<a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> call
rather than just cleanups.</p> rather than just cleanups.</p>
<p>In order for inlining to behave correctly, landing pads must be <p>In order for inlining to behave correctly, landing pads must be prepared to
prepared to handle selector results that they did not originally handle selector results that they did not originally advertise. Suppose that
advertise. Suppose that a function catches exceptions of a function catches exceptions of type <tt>A</tt>, and it's inlined into a
type <tt>A</tt>, and it's inlined into a function that catches function that catches exceptions of type <tt>B</tt>. The inliner will update
exceptions of type <tt>B</tt>. The inliner will update the the <tt>landingpad</tt> instruction for the inlined landing pad to include
selector for the inlined landing pad to include the fact the fact that <tt>B</tt> is caught. If that landing pad assumes that it will
that <tt>B</tt> is caught. If that landing pad assumes that it only be entered to catch an <tt>A</tt>, it's in for a rude surprise.
will only be entered to catch an <tt>A</tt>, it's in for a rude Consequently, landing pads must test for the selector results they understand
surprise. Consequently, landing pads must test for the selector and then resume exception propagation with the
results they understand and then resume exception propagation <a href="LangRef.html#i_resume"><tt>resume</tt> instruction</a> if none of
with the <a href="#llvm_eh_resume"><tt>llvm.eh.resume</tt></a> the conditions match.</p>
intrinsic if none of the conditions match.</p>
</div> </div>
@@ -412,67 +393,13 @@
<div> <div>
<p>LLVM uses several intrinsic functions (name prefixed with "llvm.eh") to <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 "<tt>llvm.eh</tt>") to
provide exception handling information at various points in generated provide exception handling information at various points in generated
code.</p> code.</p>
<!-- ======================================================================= -->
<h4>
<a name="llvm_eh_exception">llvm.eh.exception</a>
</h4>
<div>
<pre>
i8* %<a href="#llvm_eh_exception">llvm.eh.exception</a>()
</pre>
<p>This intrinsic returns a pointer to the exception structure.</p>
</div>
<!-- ======================================================================= -->
<h4>
<a name="llvm_eh_selector">llvm.eh.selector</a>
</h4>
<div>
<pre>
i32 %<a href="#llvm_eh_selector">llvm.eh.selector</a>(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, or if only a cleanup is matched, different
personality functions may or may not cause control to stop at the
landing pad; see <a href="#restrictions">the restrictions</a> for
more information. 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>If a landing pad containing a call to <tt>llvm.eh.selector</tt> is
inlined into an <tt>invoke</tt> instruction, the selector arguments
for the outer landing pad are appended to those of the inlined
landing pad. Consequently, landing pads must be written to ignore
selector values that they did not originally advertise.</p>
</div>
<!-- ======================================================================= --> <!-- ======================================================================= -->
<h4> <h4>
<a name="llvm_eh_typeid_for">llvm.eh.typeid.for</a> <a name="llvm_eh_typeid_for">llvm.eh.typeid.for</a>
@@ -491,33 +418,6 @@
</div> </div>
<!-- ======================================================================= -->
<h4>
<a name="llvm_eh_resume">llvm.eh.resume</a>
</h4>
<div>
<pre>
void %<a href="#llvm_eh_resume">llvm.eh.resume</a>(i8*, i32) noreturn
</pre>
<p>This intrinsic is used to resume propagation of an exception after
landing at a landing pad. The first argument should be the result
of <a href="#llvm_eh_exception">llvm.eh.exception</a> for that
landing pad, and the second argument should be the result of
<a href="#llvm_eh_selector">llvm.eh.selector</a>. When a call to
this intrinsic is inlined into an invoke, the call is transformed
into a branch to the invoke's unwind destination, using its
arguments in place of the calls
to <a href="#llvm_eh_exception">llvm.eh.exception</a> and
<a href="#llvm_eh_selector">llvm.eh.selector</a> there.</p>
<p>This intrinsic is not implicitly <tt>nounwind</tt>; calls to it
will always throw. It may not be invoked.</p>
</div>
<!-- ======================================================================= --> <!-- ======================================================================= -->
<h4> <h4>
<a name="llvm_eh_sjlj_setjmp">llvm.eh.sjlj.setjmp</a> <a name="llvm_eh_sjlj_setjmp">llvm.eh.sjlj.setjmp</a>