[LLVMdev] Looking for advice on how to debug a problem with C++ style exception handling code that my compiler generates.
Christian Schafmeister
chris.schaf at verizon.net
Sat Apr 11 17:49:36 PDT 2015
Logan,
There may be some confusion here because I wasn’t completely clear about what was calling what and where “invoke” appeared to be necessary when I JIT the code and where I realize that “invoke” is absolutely necessary. As far as I understand the Itanium ABI I only need an “invoke” in function A when function B is invoked - the rest of the calls to C() and D() should be able to use “call".
Here’s an example in C++ (for illustration) and then the identical example in Common Lisp.
In Common Lisp I can compile individual expressions in the REPL.
Here is a more illustrative example. If I compile all of these functions ahead of time - everything works fine and D() and C() are called using LLVM “call”.
But if I JIT these functions one at a time it will fail. If I use llvm “invoke” to JIT the function C() - then the thrown exception will make it to A() and everything works.
// In C++
//
void D() {
throw MyException();
};
void C() {
D(); // This should be able to be called with “call”
// - but if I JIT this function it won’t work - I need “invoke” and a dummy landing pad
};
void B() {
C(); // This can be called with "call"
};
void A() {
try {
B(); // <<--- This needs to be called with "invoke"
} catch ( MyException& e) {
/* Do something */
};
};
In Common Lisp:
(defun d ()
(throw 'Exception nil))
(defun c ()
(d)) ; <-- This can be called with llvm “call” but if I JIT this function it will fail! I need to use “invoke”!
(defun b ()
(c)) ; <-- This can be called with llvm "call"
(defun a ()
(catch 'exception
(b))) ; <-- This needs to called with llvm “invoke"
On Apr 11, 2015, at 8:15 PM, Logan Chien <tzuhsiang.chien at gmail.com> wrote:
> Hi Christian,
>
> > I don’t see anything in the Itanium ABI that says I need to call the function that throws an exception with “invoke” to get exception handling to work!
>
> AFAICT, it is the design of LLVM IR and its implementation. To catch the exceptions thrown by the callee functions, we should use the invoke instruction along with the landingpad instruction. If you are calling a function with a call instruction, the LLVM backend will simply assumes frame unwinding when the callee throws an exception.
>
>
> FYI, according to LLVM reference manual, an invoke-instruction has following semantics:
> The ‘invoke‘ instruction causes control to transfer to a specified function, with the possibility of control flow transfer to either the ‘normal‘ label or the ‘exception‘ label. If the callee function returns with the “ret” instruction, control flow will return to the “normal” label. If the callee (or any indirect callees) returns via the “resume” instruction or other exception handling mechanism, control is interrupted and continued at the dynamically nearest “exception” label.
>
> OTOH, here's the description for call instruction:
> The ‘call‘ instruction represents a simple function call.
>
>
> Hope this is helpful.
>
> Best regards,
> Logan
>
> On Sun, Apr 12, 2015 at 6:33 AM, Christian Schafmeister <chris.schaf at verizon.net> wrote:
>
> It appears that I've found a very curious effect where if I JIT a function that throws an exception and I use "call" to call it the throw fails despite there being a "catch" clause on the stack. But if I use “invoke” it works fine!
>
> If I call the function (@cc_invoke) that throws a “core::CatchThrow” exception like this:
> call void @cc_invoke({ {}*, i64 }* %result-ptr, {}* %3, i64 2, {}* %4, {}* %5, {}* null, {}* null, {}* null)
> ;; Comment out the next two lines and
> ; to label %return0 unwind label %landing-pad1
> ;return0:
> ret void
>
> landing-pad1: ; No predecessors!
> %6 = landingpad { i8*, i32 } personality i32 (...)* @__gxx_personality_v0
> cleanup
> resume { i8*, i32 } %6
> }
>
> It fails with: libc++abi.dylib: terminating with uncaught exception of type core::CatchThrow
>
> If instead I convert the “call” into an “invoke” and hook in a dummy landing-pad like this:
>
> invoke void @cc_invoke({ {}*, i64 }* %result-ptr, {}* %3, i64 2, {}* %4, {}* %5, {}* null, {}* null, {}* null)
> to label %return0 unwind label %landing-pad1
> return0:
> ret void
>
> landing-pad1: ; No predecessors!
> %6 = landingpad { i8*, i32 } personality i32 (...)* @__gxx_personality_v0
> cleanup
> resume { i8*, i32 } %6
> }
>
> The CatchThrow exception is caught and everything works fine! I don’t need to call the caller or any outer function with “invoke” except for in the function that has the landing pad that recognizes core::CatchThrow.
>
> I don’t see anything in the Itanium ABI that says I need to call the function that throws an exception with “invoke” to get exception handling to work!
>
> Does anyone have any ideas what I might be doing wrong?
>
>
>
>
>
>
>
> On Apr 9, 2015, at 6:59 PM, Lang Hames <lhames at gmail.com> wrote:
>
>> Hi Christian,
>>
>> Andy's already covered the major points, but you could consider filing a bug at http://llvm.org/bugs too, especially if you've got a small test-case that demonstrates the issue. Exception handling hasn't been a priority in the past, but as more people adopt LLVM's JIT APIs I suspect it will get more attention, and bug reports will help us figure out what needs doing.
>>
>> Cheers,
>> Lang.
>>
>> On Thu, Apr 9, 2015 at 11:43 AM, Kaylor, Andrew <andrew.kaylor at intel.com> wrote:
>> Hi Christian,
>>
>>
>>
>> I suspect that at least some of the details depend on what platform you’re working on. I believe that MCJIT attempts to register eh frame information for either MachO or ELF objects (though for some ELF platforms nothing actually happens). What happens to it after that is a darker area, at least for me.
>>
>>
>>
>> Apparently there was a GDB command that did just what you want -- “info catch” -- but I had never used it and it has been removed. It’s too bad because it sounds like a nice feature. It was supposed to dump a list of catch handlers for whatever frame you’re looking at. I suspect, however, that it would have just confirmed that your catch handler isn’t properly hooked up without being helpful in figuring out why.
>>
>>
>>
>> You could try debugging the RuntimeDyld code that registers eh frames and see if that looks right. RuntimeDyld::registerEHFrames() might be a helpful starting point.
>>
>>
>>
>> -Andy
>>
>>
>>
>>
>>
>> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On Behalf Of Christian Schafmeister
>> Sent: Wednesday, April 08, 2015 10:09 PM
>> To: LLVM Developers Mailing List
>> Subject: [LLVMdev] Looking for advice on how to debug a problem with C++ style exception handling code that my compiler generates.
>>
>>
>>
>>
>>
>> Hi,
>>
>>
>>
>> I’m looking for advice on how to debug a problem with my exception handling code.
>>
>> I’m asking a specific question here - but general advice on how to debug something like this would be greatly appreciated.
>>
>>
>>
>> Is there any way to get a list of landing pad clauses that are active at a particular point in a program?
>> I'd like to get something like a backtrace but listing all active landing pad clauses. The typeids of the C++ types
>> I'm trying to debug a problem where an exception that I'm throwing is not being caught.
>>
>> I'm generating JITed code with LLVM and landing pads and I've got shared libraries - lots of things going on that could potentially be going wrong.
>>
>>
>>
>> A list of the pointer values like @_ZTIN4core9DynamicGoE is what I’m looking for. Then I could compare that to the typeids that I know should be in that list.
>>
>>
>>
>> "(TRY-0).landing-pad": ; preds = %"(TRY-0).normal-dest14", %"(TRY-0).tagbody-B-1", %"(TRY-0).normal-dest10", %"(TRY-0).normal-dest9", %"(TRY-0).normal-dest8", %"(TRY-0).normal-dest", %"(TRY-0).tagbody-#:G1621-0"
>>
>> %14 = landingpad { i8*, i32 } personality i32 (...)* @__gxx_personality_v0
>>
>> catch i8* @_ZTIN4core9DynamicGoE
>>
>> catch i8* @_ZTIN4core10ReturnFromE, !dbg !26
>>
>> %15 = extractvalue { i8*, i32 } %14, 0, !dbg !26
>>
>> %16 = extractvalue { i8*, i32 } %14, 1, !dbg !26
>>
>> %17 = call i32 @llvm.eh.typeid.for(i8* @_ZTIN4core9DynamicGoE), !dbg !26
>>
>> %18 = icmp eq i32 %16, %17, !dbg !26
>>
>> br i1 %18, label %"(TRY-0).handler-block14470", label %"(TRY-0).dispatch-header19", !dbg !26
>>
>>
>>
>> I’m getting this error when I throw a core::Unwind exception and I’m certain that there is a landing pad with that clause.
>>
>>
>>
>> libc++abi.dylib: terminating with uncaught exception of type core::Unwind
>>
>> ../../src/gctools/memoryManagement.cc:75 Trapped SIGABRT - starting debugger
>>
>> ABORT was called!!!!!!!!!!!!
>>
>>
>>
>>
>>
>> I’ve written a Common Lisp compiler that uses LLVM as the backend and it interoperates with C++ code and I use C++ exception handling for non-local exits.
>>
>>
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>>
>>
>
>
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