Re: Run-time absence info

Date: Wed, 13 Apr 2022 08:59:06 +0200 From: Andreas Klebinger To: ghc-devs@haskell.org Subject: Re: Absence info at run-time Message-ID: Content-Type: text/plain; charset=UTF-8; format=flowed

W/W should transform such a function into one who takes one less argument removing any runtime overhead at least for fully applied functions.

Fully applied functions are definitely not what I'm talking about. I suppose your suggestion is then if we an expression`f x` where bar

takes multiple arguments, but doesn't use the current argument then GHC should:

* Inspect f, check if the first argument to f is used * If we can determine it isn't used instead of creating a PAP capturing `f` and `x` instead only capture `f` and record this in the PAP closure somehow. * Once the PAP is fully applied pass a dummy argument instead of `x` to f.

Yes, that's the idea. The "somehow" is described below. If f is a known call that seems doable, although adding a bitmap to paps

might require us to increase the size of all PAP closures, making this optimization less useful.

Every PAP has to carry its arity. I just looked it up: typedef struct { StgHeader header; StgHalfWord arity; // number of arguments left to apply, if zero this is an AP closure StgHalfWord n_args; // number of applied arguments StgClosure *fun; // guaranteed to point to a FUN closure StgClosure *payload[]; } StgPAP; On a 64-bit machine, we can combine arity and absence info for functions of up to 31 arguments into half a word. Specifically, set the lowest bit to indicate whether the first argument is used, etc. Then bitwise-or that with 1 `shiftL` arity. Each partial application performed shifts right by the number of arguments applied; if the result is 1, we know it's fully applied. (Any idea why we need the number of arguments that have been applied in the PAP? If that's actually only need for AP, then we might be able to make things a little more compact.)

If `f` is a unknown function there is currently no way to get absent/used info for it's arguments at runtime. And changing that would be a major change which seems unlikely to pay off.

I described a mechanism for encoding this concisely above. The same encoding could be used for functions that haven't been partially applied. I don't know where their arities are stashed. In their info tables?

So I think this would be theoretically possible, but it would rarely pay off.

I'm not sure it's so rare. We end up having to work around this issue in libraries, with varying levels of effort and success.

Also do you have an example where `(const a) b` leads to stupid thunks? It seems to me const should always be inlined in such a case, avoiding a PAP allocation.

There won't be a PAP allocation if const inlines. The stupid thunk is a thunk to apply (\ _ -> a) to b. Suppose we define `fmap` for plain arrays (I know; probably not the best example), and let <$ take its default implementation: (<$) a = fmap (const a) If we calculate x <$ arr, that will fill an array with thunks, each of them retaining an element of the original array which will never be used.

Am 12/04/2022 um 23:02 schrieb David Feuer:

...

Suppose `f` doesn't use its first argument. When forming the thunk (or partial application) `f a`, we don't need to record `a`. What if instead of arity, we store a bitmap used/absent arguments, terminated by a 1 bit? Could we then get rid of "stupid thunks" like `(const a) b`?

_______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs

------------------------------

Message: 3 Date: Wed, 13 Apr 2022 08:21:25 +0000 From: "Sebastian Graf" To: "GHC Devs" Subject: Markup language/convention for Notes? Message-ID: Content-Type: text/plain; charset="utf-8"; Format="flowed"

Hi Devs,

When writing Notes, I find myself using markdown-inspired or haddock-inspired features. The reason is that I keep telling myself

...

In 5 years time, we'll surely have an automated tool that renders Notes referenced under the cursor in a popup in our IDE

And I might not be completely wrong about that, after all the strong conventions about Note declaration syntax allow me to do jump-to-definition on Note links in my IDE already (thanks to a shell script written by Zubin!). Still, over the years I kept drifting between markdown and haddock syntax, sometimes used `backticked inline code` or haddock 'ticks' to refer to functions in the compiler (sometimes even 'GHC.Fully.Qualified.ticks') and for code blocks I used all of the following forms:

Haddock "code quote"

...

id :: a -> a id x = x

Markdown triple backticks

```hs id :: a -> a id x = x ```

Indentation by spaces

id :: a -> a id x = x

And so on.

I know that at least Simon was thrown off in the past about my use of "tool-aware markup", perhaps also because I kept switching the targetted tool. I don't like that either. So I wonder Do you think it is worth optimising Notes for post-processing by an external tool?I think it's only reasonable if we decide for a target syntax. Which syntax should it be? Cheers, Sebastian