[LLVMdev] RFC: Exception Handling Proposal II

[Bill Wendling]

On Nov 28, 2010, at 3:47 PM, John McCall wrote:

> This is well-formed SSA;  the alloca instruction %x is in the entry block and thus dominates both the store in %try and the load in %catch.  mem2reg wants to eliminate %x and replace the load in %catch with a fixed value.  This involves looking at the value stored in %x at all predecessor points, which is a challenge because one of those predecessors is an arbitrary point within the block %try, and %x actually has two values over that extent.  So mem2reg would have to split %try and make a phi in %lp, like so:
> 
> try1: unwinds to %lp
>  %count = load i32* @count
>  br label %try2
> try2: unwinds to %lp
>  call void foo()
>  br label %return
> lp:
>  %t = phi i32 [ i32 0, label %try1 ], [ i32 %count, label %try2 ]
>  #etc.
> 
> That's a lot of added complexity for mem2reg (and other transformations that use it as a subroutine), but it's pretty much inherent in any design where edges don't always come from terminators.
> 
> Now, in this specific case, mem2reg could say "hey, %count can't throw, I don't need the phi at all".  But it would still need to split %lp to make the IR well-formed;  it's just that %lp1 wouldn't have an "unwinds" clause.  And in the general case, it needs a phi.

Thanks. Your initial statement was confusing. :-) Yes, this is one scenario (and possibly the only one??) for how things would work with adorning BBs with "unwinds to" attributes.

-bw