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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
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 3 specification at
<a href="http://www.eagercon.com/dwarf/dwarf3std.htm">DWARF 3 Standard</a>.
Frames</a>, with details of the DWARF 4 specification at
<a href="http://dwarfstd.org/Dwarf4Std.php">DWARF 4 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>
@ -116,10 +116,10 @@
<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>For each function which does exception processing &mdash; be
it <tt>try</tt>/<tt>catch</tt> blocks or cleanups &mdash; that function
registers itself on a global frame list. When exceptions are unwinding, 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
@ -134,6 +134,7 @@
exceptions are thrown. As exceptions are, by their nature, intended for
uncommon code paths, DWARF exception handling is generally preferred to
SJLJ.</p>
</div>
<!-- ======================================================================= -->
@ -176,8 +177,8 @@
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 <i>catch</i> portion of
a <i>try</i>/<i>catch</i> sequence. When execution resumes at a landing
<p>A landing pad corresponds to the code found in the <tt>catch</tt> portion of
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
the <i>type</i> of exception thrown. The selector is then used to determine
which <i>catch</i> should actually process the exception.</p>
@ -193,11 +194,8 @@
<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
<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
C++ examples.</p>
@ -210,16 +208,19 @@
<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>
breaks down into two steps.</p>
<ol>
<li>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.</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
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 structure.</p>
<p>In C++, the allocation of the exception structure is done by then
<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 structure.</p>
</div>
@ -244,50 +245,36 @@
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 to convey information about the landing
pad to the back end.</p>
<p>The LLVM <a href="LangRef.html#i_landingpad"><tt>landingpad</tt>
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>
<li><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 <tt>invoke</tt> has branched
to a landing pad. Due to code generation limitations, it must currently
be called in the landing pad itself.</li>
<li><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 <tt>try</tt>/<tt>catch</tt> sequence. Each of the
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>The <tt>landingpad</tt> instruction takes a reference to the personality
function to be used for this <tt>try</tt>/<tt>catch</tt> sequence. The
remainder of the instruction is a list of <i>catch</i> and <i>filter</i>
clauses. The exception is tested against the clauses sequentially from first
to last. The selector value 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>
<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
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 check
against the selector.</p>
<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 the <a href="LangRef.html#i_landingpad"><tt>landingpad</tt>
instruction</a>, then neither the last catch nor <i>catch all</i> need to
perform 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>.</p>
@ -318,16 +305,14 @@
<div>
<p>A cleanup is extra code which needs to be run as part of unwinding
a scope. C++ destructors are a prominent example, but other
languages and language extensions provide a variety of different
kinds of cleanup. In general, a landing pad may need to run
arbitrary amounts of cleanup code before actually entering a catch
block. To indicate the presence of cleanups, a landing pad's call
to <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> should
end with the argument <tt>i32 0</tt>; otherwise, the unwinder will
not stop at the landing pad if there are no catches or filters that
require it to.</p>
<p>A cleanup is extra code which needs to be run as part of unwinding a scope.
C++ destructors are a prominent example, but other languages and language
extensions provide a variety of different kinds of cleanup. In general, a
landing pad may need to run arbitrary amounts of cleanup code before actually
entering a catch block. To indicate the presence of cleanups, a
<a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a>
should have a <i>cleanup</i> clause. Otherwise, the unwinder will 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
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
by the current function, resume unwinding by calling the
<a href="#llvm_eh_resume"><tt>llvm.eh.resume</tt></a> intrinsic,
passing in the results of <tt>llvm.eh.exception</tt> and
<tt>llvm.eh.selector</tt> for the original landing pad.</p>
<a href="LangRef.html#i_resume"><tt>resume</tt> instruction</a>, passing in
the results of the <tt>landingpad</tt> instruction for the original landing
pad.</p>
</div>
@ -352,21 +337,19 @@
<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>
invalid types. To express this in LLVM code the
<a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a> will
have a filter clause. The clause consists of an array of type infos.
<tt>landingpad</tt> 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 a
<a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a> can
have any number of catch, cleanup, and filter clauses (though having more
than one cleanup is pointless). The LLVM C++ front-end can generate such
<a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instructions</a> due
to inlining creating nested exception handling scopes.</p>
</div>
@ -377,29 +360,27 @@
<div>
<p>The unwinder delegates the decision of whether to stop in a call
frame to that call frame's language-specific personality function.
Not all personalities functions guarantee that they will stop to
perform cleanups: for example, the GNU C++ personality doesn't do
so unless the exception is actually caught somewhere further up the
stack. When using this personality to implement EH for a language
that guarantees that cleanups will always be run, be sure to
indicate a catch-all in the
<a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> call
<p>The unwinder delegates the decision of whether to stop in a call frame to
that call frame's language-specific personality function. Not all
personalities functions guarantee that they will stop to perform
cleanups. For example, the GNU C++ personality doesn't do so unless the
exception is actually caught somewhere further up the stack. When using this
personality to implement EH for a language that guarantees that cleanups will
always be run, be sure to indicate a catch-all in the
<a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a>
rather than just cleanups.</p>
<p>In order for inlining to behave correctly, landing pads must be
prepared to handle selector results that they did not originally
advertise. Suppose that a function catches exceptions of
type <tt>A</tt>, and it's inlined into a function that catches
exceptions of type <tt>B</tt>. The inliner will update the
selector for the inlined landing pad to include the fact
that <tt>B</tt> is caught. If that landing pad assumes that it
will only be entered to catch an <tt>A</tt>, it's in for a rude
surprise. Consequently, landing pads must test for the selector
results they understand and then resume exception propagation
with the <a href="#llvm_eh_resume"><tt>llvm.eh.resume</tt></a>
intrinsic if none of the conditions match.</p>
<p>In order for inlining to behave correctly, landing pads must be prepared to
handle selector results that they did not originally advertise. Suppose that
a function catches exceptions of type <tt>A</tt>, and it's inlined into a
function that catches exceptions of type <tt>B</tt>. The inliner will update
the <tt>landingpad</tt> instruction for the inlined landing pad to include
the fact that <tt>B</tt> is caught. If that landing pad assumes that it will
only be entered to catch an <tt>A</tt>, it's in for a rude surprise.
Consequently, landing pads must test for the selector results they understand
and then resume exception propagation with the
<a href="LangRef.html#i_resume"><tt>resume</tt> instruction</a> if none of
the conditions match.</p>
</div>
@ -412,67 +393,13 @@
<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
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>
<a name="llvm_eh_typeid_for">llvm.eh.typeid.for</a>
@ -491,33 +418,6 @@
</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>
<a name="llvm_eh_sjlj_setjmp">llvm.eh.sjlj.setjmp</a>