I’m pretty reluctant to change the Core Language to support one very dark
corner use-case. Then every Core consumer and producer must understand the
semantics of this new field, and respect that semantics.
As I understand it, our definition
keepAlive# k x s = case nodiv x s of (# s1, r #) ->
case touch# k s1 of (# s2, _ #) ->
(# s2, r #)
does just what we want, except perhaps efficiency, without changing the
language. If that’s true, let’s convince ourselves that
* The efficiency cannot be recovered
* The loss of efficiency matters a lot
before taking more drastic steps.
For completeness,
nodiv :: IO a -> IO a
behaves like the identity function except that (nodiv e) never says “guarantees
to diverges”
Simon
From: Sylvain Henry <[email protected]>
Sent: 26 May 2020 18:04
To: Ben Gamari <[email protected]>; Simon Peyton Jones <[email protected]>
Cc: GHC developers <[email protected]>
Subject: Re: keepAlive# primop
Hi ghc-devs,
After a discussion today about `keepAlive#`, I think Option E [1] is even more
appealing.
To recap the idea was to have keep-alive variable sets attached to
case-expressions in Core. E.g. `case {k} x of ...`
1. One of the issue was the semantics of `keepAlive#`. `keepAlive#` is very
intricate with evaluation. As Simon mentioned we want a semantics like:
keepAlive# k x ==> x `seq` touch# k `seq` x
with potentially a special `seq` to deal with diverging `x`.
With `case {k} x of ...` we have this semantics. Even if we throw the case
alternatives if `x` diverges, we don't throw the keep-alive set so we're good.
2. Simon wanted to push `keepAlive#` into case-expressions. With this approach
we should only have to fix case-of-case to take keep-alive sets into account.
case {k} (case {k2} x of .. -> a; ... -> b) of
C0 .. -> e1
C1 .. -> e2
===>
case {k,k2} x of
... -> case {k} a of { C0 .. -> e1; C1 ... -> e2 }
... -> case {k} b of { C0 .. -> e1; C1 ... -> e2 }
3. Compared to other approaches: we don't have to use stack frames (good for
performance) and we don't have to deal with a continuation (good for Core
optimizations, hence perf).
4. Implementing this approach is quite straightforward even if it modifies
Core. I did it last month in [2]. This patch doesn't fully work yet with `-O`
because some transformation (related to join points IIRC) doesn't take
keep-alive sets into account but it should be pretty easy to fix if we want to
use this approach.
Given how hard it is to come up with a good design/implementation of other
approaches, this one strikes me as probably the more principled we have and yet
it is relatively easy to implement. What do you think?
Cheers,
Sylvain
[1]
https://gitlab.haskell.org/ghc/ghc/-/wikis/proposal/with-combinator#option-e-tag-core-case-expression-with-kept-alive-variables<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitlab.haskell.org%2Fghc%2Fghc%2F-%2Fwikis%2Fproposal%2Fwith-combinator%23option-e-tag-core-case-expression-with-kept-alive-variables&data=02%7C01%7Csimonpj%40microsoft.com%7C048cbe5f73764660a89e08d80196cc57%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637261094481517008&sdata=FgsyLDDHU9uwfy3N2WERGxey1maas7o%2BKvlOYRRt1yo%3D&reserved=0>
[2]
https://gitlab.haskell.org/hsyl20/ghc/-/commits/hsyl20-keepalive<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitlab.haskell.org%2Fhsyl20%2Fghc%2F-%2Fcommits%2Fhsyl20-keepalive&data=02%7C01%7Csimonpj%40microsoft.com%7C048cbe5f73764660a89e08d80196cc57%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637261094481517008&sdata=8In4f2ghmFkHd0BK8iUUOoXT5L4L2smtaD4cNBNKfIg%3D&reserved=0>
On 13/04/2020 19:51, Ben Gamari wrote:
Ccing ghc-devs@ since this discussion is something of general interest
to the community.
Sylvain Henry <[email protected]><mailto:[email protected]> writes:
Simon, Ben,
I've been reading and thinking about `readRW#` issues which are very
related to issues we have with `keepAlive#` primop.
To recap, the problem is that we want some transformations (that Simon
has listed in [1]) to consider:
```
case runRW# f of ...
case keepAlive# k a of ...
```
as if they were really:
```
case f realWorld# of ...
case a of ...
```
BUT without breaking the semantics of runRW# and keepAlive#.
I have been thinking about a solution that I have described on the wiki:
https://gitlab.haskell.org/ghc/ghc/-/wikis/proposal/with-combinator#option-e-tag-core-case-expression-with-kept-alive-variables<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitlab.haskell.org%2Fghc%2Fghc%2F-%2Fwikis%2Fproposal%2Fwith-combinator%23option-e-tag-core-case-expression-with-kept-alive-variables&data=02%7C01%7Csimonpj%40microsoft.com%7C048cbe5f73764660a89e08d80196cc57%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637261094481527002&sdata=MVHNThzNSCyckXB0KajxAjOWPAl3kUpgjNWD4F6OWO0%3D&reserved=0>
The idea is to keep a set of variable names in each Core case-expression
that are kept alive during the evaluation of the scrutinee.
I think it would work very nicely with your `newState#` primop described
in [2], both for `runST` and for `unsafeDupablePerformIO` (details on
the wiki).
It requires a little more upfront work to adapt the code involving
case-expressions. But it will force us to review all transformations to
check if they are sound when keep-alive sets are not empty, which we
would have to do anyway if we implemented another option. We could start
by disabling transformations involving non-empty keep-alive sets and
iterate to enable the sound ones.
I would like your opinions on the approach. I may have totally missed
something.
Thanks for writing this down!
Indeed it is an interesting idea. However, as expressed on IRC, I
wonder whether this problem rises to the level where it warrants an
adaptation to our Core representation. It feels a bit like the
tail is wagging the dog here, especially given how the "tail" here
merely exists to support FFI.
That being said, this is one of the few options which remain on the
table that doesn't require changes to user code. Moreover, the
applicability to runRW# is quite intriguing.
Another (admittedly, more ad-hoc) option that would avoid modifying Core
would be to teach the simplifier about the class of
"continuation-passing" primops (e.g. `keepAlive#` and `runRW#`), allowing it
to push case analyses into the continuation argument. That is,
case keepAlive# x expr of pat -> rhs
~>
keepAlive# x (case expr of pat -> rhs)
Of course, doing this is a bit tricky since one must rewrite the
application of keepAlive# to ensure that the resulting application is
well-typed. Admittedly, this doesn't help the runRW# case (although this
could presumably be accommodated by touch#'ing the final state token in
the runRW# desugaring emitted by CorePrep).
On the whole, I'm not a fan of this ad-hoc option. It increases the
complexity of the simplifier all to support a single operation. By
comparison, the Core extension looks somewhat appealing.
Cheers,
- Ben
[1]
https://gitlab.haskell.org/ghc/ghc/-/wikis/proposal/with-combinator#option-e-tag-core-case-expression-with-kept-alive-variables<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitlab.haskell.org%2Fghc%2Fghc%2F-%2Fwikis%2Fproposal%2Fwith-combinator%23option-e-tag-core-case-expression-with-kept-alive-variables&data=02%7C01%7Csimonpj%40microsoft.com%7C048cbe5f73764660a89e08d80196cc57%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637261094481527002&sdata=MVHNThzNSCyckXB0KajxAjOWPAl3kUpgjNWD4F6OWO0%3D&reserved=0>
P.S. A minor note: the keepAlive# "pseudo-instruction" mentioned on the
Wiki [1] is precisely the touch# operation we have today.
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