Mir: remove Panic terminator and instead just call suitable lang item

[nikomatsakis]

Panic in MIR is unimplemented. It should call into the panic lang item.

[wesleywiser]

Hi, I'd like to give this a shot.

[nikomatsakis]

@wesleywiser great! If you want to chat about it, the easiest way is to privmsg me on IRC (nmatsakis) or send me an email (nmatsakis@mozilla.com). I will note though that it may be that we should reshape panics to just be calls, in which case this task is more about handling calls than about panics per se, which is maybe a bigger job (but a very important one).

[nikomatsakis]

I now think we should refactor panics to simply call into the appropriate lang item and not have an explicit Panic terminator.

[nagisa]

Replacing Panic terminator with explicit call to language item that’s followed by a Diverge (I assume Diverge has the same semantics as intrinsics::unreachable) terminator sounds good to me.

[nagisa]

On the other hand, it would make implementing certain codegen features such as -C no-landing-pads near-impossible? Then probably doesn’t sound as good as it did before 😃

[nikomatsakis]

@nagisa I don't see why it makes that any harder. In such a mode we presumably would not emit a Panic either, and/or we'd change the lang item to abort (and strip away the landing pads path so that the call block has no successors).

[nikomatsakis]

@nagisa also, the semantics of diverge are not unreachable. The diverge
terminator continues unwinding. Perhaps it should have been called "resume".

On Fri, Nov 13, 2015 at 3:00 PM, Simonas Kazlauskas <
notifications@github.com> wrote:

On the other hand, it would make implementing certain codegen features
such as -C no-landing-pads near-impossible? Then probably doesn’t sounds
as good as it did before 😃

—
Reply to this email directly or view it on GitHub
#29573 (comment).

[nikomatsakis]

Though, we could say that if landing pads are disabled, then the Diverge
terminator just traps, and then we can keep directing all panic pathways
directly into the Diverge block. Not sure, plenty of ok options I guess.

On Fri, Nov 13, 2015 at 3:07 PM, Nicholas Matsakis nmatsakis@mozilla.com
wrote:

@nagisa also, the semantics of diverge are not unreachable. The diverge
terminator continues unwinding. Perhaps it should have been called "resume".

On Fri, Nov 13, 2015 at 3:00 PM, Simonas Kazlauskas <
notifications@github.com> wrote:

On the other hand, it would make implementing certain codegen features
such as -C no-landing-pads near-impossible? Then probably doesn’t sounds
as good as it did before 😃

—
Reply to this email directly or view it on GitHub
#29573 (comment).