[LLVMdev] Re: Newbie questions

Chris Lattner sabre at nondot.org
Wed Apr 26 21:02:36 PDT 2006

On Wed, 26 Apr 2006, Vikram Adve wrote:
> On Apr 26, 2006, at 3:23 PM, Chris Lattner wrote:
>> On Wed, 26 Apr 2006, Vikram Adve wrote:
>>>> On some platforms we generate explicit checks for null pointer
>>>> dereferences and for division by zero (division is a bit different in
>>>> that we also have to handle java's special rule for dividing -1 by the
>>>> minimum integer -- I think most platforms actually call the helper
>>>> function).
>>> Specifically, I don't think either of these approaches will be sufficient 
>>> when using LLVM as a JIT in libgcj.  In either case, an LLVM optimization 
>>> could still reorder trapping instructions.  In the first approach, this is 
>>> less likely to occur but is possible.  The second approach of course 
>>> wouldn't work because LLVM doesn't have a notion of implicitly trapping 
>>> instructions and could merrily reorder such operations.
>> Vikram, I'm confused.  How do you think LLVM will break this?
> By reordering two instructions that could throw exceptions.

One of us is confused.  The approach above never has instructions that 
throw exceptions.

> I'm talking about non-fatal conditions like divide-by-0 or a null reference 
> that are caught by exception handlers.

With the approach above, no exceptions are non-fatal.  There is no "catch 
a signal and turn that into an exception" of any sort.



More information about the llvm-dev mailing list