Distinct closure types vs. known infotables for stack frames

[Simon Peyton Jones]

Thanks for a clear writeup, Alexis.

My instinct is to do it all with closure types, not pointer comparison.
-
>
> Con: Adding more closure types unnecessarily pollutes code that branches
> on closure types, like the garbage collector.
>
-
>
> Con: Adding more closure types unnecessarily pollutes code that branches
> on closure types, like the garbage collector.
>
- I don't get this.  Different stack frames might have different layouts
(e.g a catch frame has a fixed size with with exactly two (or whatever)
pointers, etc).   This isn't pollution.  Just as every heap closure has a
closure type that describes its layout, so does every stack frame.

Perhaps you mean that in fact all stack frames share a single layout, with
a bitmap to describe it?  That seems sub-optimal for these special frames
where we statically know the entire shape!

Perhaps you mean that many stack frames share a common layout, so that the
case analysis on closure type might have many cases all pointing to the
same GC code.  But if so, isn't that true for heap closures too? If we are
concerned about that, we could have two "type" fields in the info table,
one exclusively concerned with layout, so that the GC could just branch on
that and only have a few cases to consider, and one with a finer
granularity.

In short, why are the design considerations for stack frames different to
heap objects?  I think of a stack frame simply as a heap object that
happens to be allocated on the stack

Simon


On Mon, 26 Jun 2023 at 21:21, Alexis King <lexi.lambda at gmail.com> wrote:

> Hello all,
>
> I am tinkering with the RTS again while trying to fix #23513
> <https://gitlab.haskell.org/ghc/ghc/-/issues/23513>, and every time I
> touch the exceptions/continuations code, I find myself waffling about
> whether to introduce more closure types. I’d like to get a second opinion
> so I can stop changing my mind!
>
> Currently, we have distinct closure types for certain special stack
> frames, like CATCH_FRAME, ATOMICALLY_FRAME, and UNDERFLOW_FRAME. However,
> there are other special stack frames that *don’t* get their own closure
> types: there is no MASK_ASYNC_EXCEPTIONS_FRAME or PROMPT_FRAME. Instead,
> when code needs to recognize these frames on the stack, it just looks for a
> known infotable pointer. That is, instead of writing
>
> if (frame->header.info->i.type == PROMPT_FRAME) { ... }
>
> we write
>
> if (*frame == &stg_prompt_frame_info) { ... }
>
> which works out because there’s only one info table that’s used for all
> prompt frames.
>
> There are a handful of stack frames that are recognized in this way by
> some part of the RTS, but the criteria used to determine which frames get
> their own types and which don’t is not particularly clear. For some frames,
> like UPDATE_FRAME, the closure type is necessary because it is shared
> between several infotables. But other types, like CATCH_FRAME and
> UNDERFLOW_FRAME, are only ever used by precisely one infotable.
>
> I think one can make the following arguments for/against using separate
> closure types for these stack frames:
>
>    -
>
>    Pro: It’s helpful to have separate types for particularly special
>    frames like UNDERFLOW_FRAME because it makes it easier to remember
>    which special cases to handle when walking the stack.
>    -
>
>    Pro: Branching on stack frame closure types using switch is easier to
>    read than comparing infotable pointers.
>    -
>
>    Con: Adding more closure types unnecessarily pollutes code that
>    branches on closure types, like the garbage collector.
>    -
>
>    Con: Using special closure types for these frames might make it seem
>    like they have some special layout when in fact they are just ordinary
>    stack frames.
>
> Does anyone have any opinions about this? I’m personally okay with the
> status quo, but the inconsistency makes me constantly second-guess whether
> someone else might feel strongly that I ought to be doing things the other
> way!
>
> Thanks,
> Alexis
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