[LLVMdev] Is LLVM expressive enough to represent asynchronous exceptions?

[Duncan Sands]

Hi Chris, I've CC'd Eric Botcazou in the hope that he will clear up just what
the Ada front-end needs from the rest of the compiler as far as asynchronous
exceptions are concerned.

>> gcc Ada turns signals into exceptions.  As far as I know it does this
>> completely asynchronously, and the fact that LLVM doesn't support this
>> is rather bad as far as Ada is concerned.  That said, the Ada front-end
>
> You're saying that it turns asynch signals like SIGHUP (which can occur on any machine instruction) into signals?

AFAIK, on posix systems only SIGFPE, SIGILL, SIGSEGV and SIGBUS are turned into
exceptions.  For other signals, if there is no signal handler then the program
is terminated; if the user installed a signal handler then it is run, and as far
as I know there is no way for a user installed handler to raise an exception at
the original point of execution without going deep into nasal demon territory.
So SIGFPE, SIGILL, SIGSEGV and SIGBUS are the only ones that need to be thought
about.

I'm going to ignore the possibility that the user uses "kill" or somesuch to hit
an Ada program with one of these signals.  I think that falls out of the scope
of the language standard, so any behaviour would probably be correct as far as
the standard is concerned.

For SIGILL, SIGSEGV and SIGBUS, I think the program has to be erroneous [*] to
get one of these in the first place (note that null pointer dereferences and
other things corresponding to "bounded errors" in Ada parlance [**] do not cause 
a segfault because the compiler inserts code to check for a null pointer
before every dereference and explicitly raises an exception if the pointer is
null), so I suspect that the compiler tries to do something (raise an
exception) because it is helpful (quality of implementation) rather than
because the language requires it.  I wouldn't bet my life on this though :)
As for SIGFPE, you will also never get this in normal usage (things like
dividing INT_MIN by -1 never happen because the compiler inserts code to check
for this before every division) and, based on the fact that the runtime doesn't
even bother to attach a helpful message to the exception raised on SIGFPE, I'm
not sure why the compiler even bothers with this one.

In short, I think if LLVM ignores the possibility of signals being turned into
exceptions then programs will probably still conform to what the language
standard requires, but there might be some users complaining that stack
overflow detection and recovery and other neat compiler features don't work
when using LLVM as the GCC backend.

But I don't know enough about this to be sure.

Ciao, Duncan.

[*] In C parlance, the program has undefined behaviour.
[**] These are errors for which the consequences are carefully circumscribed
by the language standard.