[LLVMdev] Can simplifycfg kill llvm.lifetime intrinsics?

[Rafael Espíndola]

> Ok, suppose you have the following code:
> BB1:
>   llvm.lifetime.start(%a)
>   store to %a
>   llvm.lifetime.end(%a)
>   br %BB2
>
> BB2:
>   <some code>
>   br %BB1
>
> If you remove the first "llvm.lifetime.start", then when you enter
> %BB1 for the second time, your "store to %a" can be considered invalid,
> as you've already called llvm.lifetime.end for this variable.

Oh, I was reading "precedes/following" as having static (dominance)
meaning. That is, in the above example you could not delete the store
since it is not true that
llvm.lifetime.end dominates it.

Nick, is this what you had in mind? If not, then we must delete a
matching llvm.lifetime.end, but it is not clear how we define
"matching". In the following code some executions will hit the first
llvm.lifetime.end and others will hit the second one.


BB0:
...
llvm.lifetime.start(%a)
...
br i1 %foo, label %BB1, label %BB2

BB1:
...
llvm.lifetime.end(%a)
...
br label %bar

BB2:
...
llvm.lifetime.end(%a)
...
br label %zed


Cheers,
Rafael