[LLVMdev] Dependence Analysis [was: Flow-Sensitive AA]

[Daniel Berlin]

On Fri, Aug 22, 2008 at 8:50 PM, Daniel Berlin <dberlin at dberlin.org> wrote:
> On Fri, Aug 22, 2008 at 5:49 PM, John Regehr <regehr at cs.utah.edu> wrote:
>>> Thanks!  This is all very interesting, and tells me that LLVM has a
>>> way to go to fully support all of these capabilities (if that is the
>>> right thing to do, which isn't clear).  OTOH, it looks like a lot of
>>> real-world software that is using LLVM already doesn't seem to be
>>> affected by the lack of them.
>>
>> LLVM's current choice is safe for all applications.  The trapping behavior
>> would be a really nice addition, though.
>>
>> Has anyone quantified the optimizations afforded by undefined signed
>> overflow?  I'd expect that the benefits are minimal for most codes.  On
>> the other hand I've seen reports that gcc's -fwrapv hurts performance of
>> gcc output significantly.  I'm not sure if that is true.  Also, it could
>> be the case that -fwrapv is implemented poorly.
> No, it's not implemented badly.  We've quantified it's performance
> before and it hurts about 3-5 without high level loop
> opts/vectorization on.

3-5%, sorry.