use UniqSupply in FastString?

[Nicolas Frisby]

Thanks for the feedback.

On Fri, Jul 5, 2013 at 6:03 PM, Simon Peyton-Jones <simonpj at microsoft.com>wrote:

>  A UniqSupply has a single shared Int behind the scenes, so it’s really
> no different. ****
>
> **
>

A UniqSupply also has the splitting "magic"; that's what's makes it an
attractive for this use case.


>  **
>
> Simon Marlow may want to comment on your proposed solutions. Personally I
> think Option 1 is most attractive. Yes, the API changes, but in a decent
> way and one that may be useful for other things.  ****
>
> **
>
I wasn't thinking beyond this trouble with the two libHSghc images; the
idea of encasing the entire plugin in a call does sound "useful for other
things". Indeed, we may want to enforce this with some abstract types and
functions, `mkPlugin` and `mkPluginPass`, for future-proofness.

Even so, Option 1 suffers from the same laziness issues as Option 2. Our
synchronization steps happen when control passes back and forth between the
compiler and the plugin under the assumption that the global variables of
the "inactive" one won't be changing.  If a plugin pass forces a compiler's
thunk (or vice versa) and that thunk allocates a FastString, then that
breaks our assumption (…there go the missiles).  Avoiding this sort of
incoherency-due-to-unsafePerfomIO-and-laziness seems very delicate and is
why I'm favoring Options 3, 5, and 6.

> **
>
> Now I think of it, why can’t ‘install’ do the workAroundGloblals call?
> Then clients would not need to.  Maybe I’m not thinking straight****
>
> **
>
I'm not sure what you mean here. It is key that 'reinitializeGlobals` is
called from the plugin — that's how we access the dynamically loaded
libHSghc's global variables.

I almost said "or else we couldn't access…" but then I thought that perhaps
there is a way we can. If so, we could fully hide the reinitializeGlobals
from the plugin author. The host compiler loads the plugin dynamically and
then extracts the 'MyPluging.plugin` symbol. For reasons I don't
understand, this also loads a second copy of libHSghc into memory. Can we
use the same dynamic loading mechanisms to extract that second image's
global variable symbols directly? Instead of requiring the plugin to alter
those variables by calling 'reinitializeGlobals`?

In other words, we always "load" a tiny special plugin that does the
'reinitializeGlobals` call and is otherwise a noop. Since all plugins share
the some dynamically loaded library, we just need 'reinitializeGlobals` to
be called once, no matter how many plugins are loaded. Then the actual
plugins wouldn't need to know about the whole fiasco.

I'd have to dig into the dynamic loading stuff to better estimate this… can
anyone chime in?

Thanks for the brain food!

>  **
>
> Simon ****
>
> ** **
>
> *From:* ghc-devs-bounces at haskell.org [mailto:ghc-devs-bounces at haskell.org]
> *On Behalf Of *Nicolas Frisby
> *Sent:* 05 July 2013 18:14
> *To:* ghc-devs at haskell.org
> *Subject:* use UniqSupply in FastString?****
>
> ** **
>
> Does it sound reasonable to change the FastString module to use a
> UniqSupply instead of using that Int for generating uniques?****
>
> ** **
>
> I've been trying to let a statically-linked compiler shares its FastString
> table with plugins. Status, background info, options here: ****
>
> ** **
>
>   http://hackage.haskell.org/trac/ghc/wiki/Plugins/ReinitializeGlobals****
>
> ** **
>
> (I got a little ahead of myself with Option 2…)****
>
> ** **
>
> Uniques for FastStrings are currently allocated linearly using a global
> Int variable. Because unsafePerformIO is used, it's difficult to keep the
> two global Ints in synch (one for the compiler, the other for the
> plugins). The danger is that the compiler and a plugin might allocate the
> same unique for distinct FastStrings — that'd break a major invariant. If
> we used UniqSupply, we'd avoid that danger, just about for free.****
>
> ** **
>
> I'm not sure how robust/speedy UniqSupply is though. Considering its
> widespread use, I figured it'd be good enough by a pretty wide margin;
> FastString *creation* seems relatively infrequent.****
>
> ** **
>
> Thanks for your input.****
>
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