[LLVMdev] Getting exceptions to work?

[Duncan Sands]

Talin wrote:
> Nicolas Geoffray wrote:
>> Hi Talin,
>>
>> You're not using the llvm intrinsics for exception handling, so your 
>> code won't work. Using _Unwind_RaiseException should be OK, but your 
>> main function must at least use llvm.eh.exception, 
>> llvm.eh.selector.i32/64 and probably __cxa_begin_catch and __cxa_end_catch.
>>   
> Let me ask a follow-up question then - if the llvm.eh.* intrinsics are 
> required to define the landing pad, then what is the role of the 
> "unwind" target in the invoke instruction?

They are not required to define a landing pad.  They are currently
required by the code generators.  The interpreter works fine without
them for example.  The current design is pretty horrible: the dwarf
eh intrinsics were tacked on to the pre-existing invoke-unwind scheme,
and it shows.  Anyway, I plan to teach the code generators that if no
personality function was specified then it should use the C personality
function (yes, there is one).  This is probably good enough for simple
cases such as running cleanups.

According to the docs, the
> unwind target defines where execution will jump to when the called 
> function returns with an exception, but clearly that isn't true -  the
> flow of control passes directly to the personality function, bypassing 
> the unwind target completely (as far as I can tell).

The language description describes how it is supposed to work.  It
simply hasn't been implemented yet in the code generator.
Obviously some code is necessarily executed by the run-time when an
exception is thrown, to unwind the stack etc.  You should think of the
personality function as part of the runtime.

> By using the llvm.eh* intrinsics, I have managed to get the code to call 
> my custom personality function without crashing. However, what I don't 
> yet understand is how to get the result from the personality function 
> back to the landing pad. For example, I could use the function 
> _Unwind_SetIP to tell it where to jump to, except that I can't take the 
> address of a label in LLVM. I could call _Unwind_SetGR to set an index 
> and then have a switch in the landing pad that uses that index - except 
> that I would have no way to determine which register is safe to set in a 
> platform-independent way. I could even just have the personality 
> function do nothing at all, and have the landing pad do all the work - 
> except that if my personality function doesn't do anything, then the 
> landing pad is skipped entirely, and control resumes at the non-unwind 
> target of the original invoke instruction, as if no exception had been 
> thrown at all.

I would just use the C personality function, __gcc_personality_v0, if
I were you.  It should know where to jump to because the code generators
record the invoke unwind target in the dwarf exception handling info in
the object file.

Ciao,

Duncan.

>> Nicolas
>>
>>
>> Talin wrote:
>>   
>>> I've been struggling for several days, trying to get native exceptions 
>>> to work in my code. I managed to boil down the IR to the simplest 
>>> possible example that I can think of.
>>>
>>> If anyone on this list can tell me what changes I need to make to the 
>>> following code to get it to work (i.e. return 0 instead of a bus 
>>> error), it would make my life immensely better.
>>>
>>>    ; ModuleID = 'ExceptionTest'
>>>        %Object = type {}
>>>        %UnwindInfo = type { i64, void (i32, %UnwindInfo *)*, i16, i16 }
>>>        %Throwable = type { %Object, %UnwindInfo }
>>>
>>>    define i32 @main(i32, i8**) nounwind {
>>>    entry:
>>>        invoke fastcc void @throwSomething() to label %nounwind unwind
>>>    label %catch
>>>
>>>    catch:
>>>        ret i32 0
>>>
>>>    nounwind:
>>>        ret i32 -1
>>>    }
>>>
>>>    define internal fastcc void @throwSomething() noreturn {
>>>    entry:
>>>        %throwable = malloc %Throwable
>>>        call fastcc void @Throwable.construct(%Throwable* %throwable)
>>>        %unwindInfo = getelementptr %Throwable* %throwable, i32 0, i32 1
>>>        %throw = call i32 @_Unwind_RaiseException(%UnwindInfo* 
>>> %unwindInfo)
>>>        unreachable
>>>    }
>>>
>>>    define internal fastcc void @Throwable.construct(%Throwable* %self)
>>>    nounwind {
>>>    entry:
>>>        %exceptionClass = getelementptr %Throwable* %self, i32 0, i32 1,
>>>    i32 0
>>>        store i64 0, i64* %exceptionClass
>>>        %exceptionCleanup = getelementptr %Throwable* %self, i32 0, i32
>>>    1, i32 1
>>>        store void (i32, %UnwindInfo *)* @exceptionCleanupFn, void (i32,
>>>    %UnwindInfo *)** %exceptionCleanup
>>>        %private1 = getelementptr %Throwable* %self, i32 0, i32 1, i32 2
>>>        store i16 0, i16* %private1
>>>        %private2 = getelementptr %Throwable* %self, i32 0, i32 1, i32 3
>>>        store i16 0, i16* %private2
>>>        ret void
>>>    }
>>>
>>>    define internal void @exceptionCleanupFn(i32 %reason, %UnwindInfo *
>>>    %exc) {
>>>      ret void
>>>    }
>>>
>>>    declare i32 @_Unwind_RaiseException(%UnwindInfo*) noreturn
>>>
>>> -- Talin
>>>
>>> ------------------------------------------------------------------------
>>>
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