[llvm-commits] [llvm] r94046 - /llvm/trunk/lib/CodeGen/DwarfEHPrepare.cpp

[Jim Grosbach]

On Jan 21, 2010, at 9:25 AM, Duncan Sands wrote:

> Hi Jim,
> 
>> I totally agree it's a bit cringe-inducing. It's necessitated by the self-described hack in SelectionDAGISel::SelectAllBasicBlocks()  which tries to deal with eh.selectors that end up in blocks not an immediate unwind successor to an invoke. A hack has begat another hack, essentially.
>> I spent a good while trying to think of better options, but didn't come up with anything better. The best alternative I came up with was to remove the invokes, selectors, et.al. completely in the SjLj pass, but that would require a re-implementation all of the EH mechanisms that come afterwards to support the exception tables. While I'm not sure that wouldn't be a good thing in the long term, it's also a much larger project.
> 
> one thing I don't understand is why SjLj lowering is run before DwarfEHPrepare,
> and not after.  The role of DwarfEHPrepare is to normalize and simplify eh stuff
> so life is easy for the code generators that come after.  Surely this simplified
> input is what should be fed to SjLj lowering (I confess I never looked at what
> the SjLj lowering stuff does...).
> 

Things get even trickier if the order is reversed, and another aspect of the same issue. Specifically, SjLj needs to split critical edges at the invokes. When a landing pad is shared between multiple invokes and also has a normal edge coming in (from inlining or whatever), the combination of the edge splitting and normalization causes the selector to be more than a single block away from the landing pad, and SelectAllBasicBlocks() can't handle that.

I've considered combining the two EH passes and conditionalizing the sjlj-only portions. That would allow finer granularity of ordering and likely avoid having to do as many of these sorts of gyrations.

> PS: DwarfEHPrepare currently doesn't simplify selectors (by making sure that
> they are always in a landing pad), but it should, and will one day when I get
> round to it*
> 
> * As the song says "Tomorrow, tommorow, it's only a day away!" :)

I know the feeling all too well. :)