[PATCH] SimplifyCFG: Avoid miscompilations due to removed lifetime intrinsics

[Nick Lewycky]

Reid Kleckner wrote:
> On Wed, Jul 30, 2014 at 12:46 AM, Nick Lewycky <nicholas at mxc.ca
> <mailto:nicholas at mxc.ca>> wrote:
>
>     Rafael Ávila de Espíndola wrote:
>
>         This is fine by me.
>
>         I agree with Reid that we need to figure out what to do with
>           the lifetime intrinsics, but lets not miscompile code while we
>         do that.
>
>
>     My proposal is that we should combine lifetime.start and
>     lifetime.end into a single intrinsic that means "store undef to
>     through this pointer" but doesn't get optimized away like store to
>     undef does. The additional optz'n possible from having distinct
>     start and end isn't worth it.
>
>     There's an obvious question of whether storing to undef should just
>     work that way on its own. Anyone wanting to implement that should be
>     aware that historically we've used store to undef as a way for
>     passes which can't modify the CFG to indicate unreachable, so we'll
>     need an answer for that.
>
>
> Hang on, store to undef or store undef to the pointer?  I think you mean
> storing undef to / through the pointer, in which case, I don't think we
> use that for unreachable.

Oh. Right. The intrinsic should mean store of undef. The unreachable is 
store to undef (also of undef, but that matters less).