Static data and RULES

[Simon Peyton Jones]

I’m sorry I still don’t understand the problem.  Can you give an example?  It all works fine today; what will change in the proposed new scheme.  Indeed what IS the proposed new scheme?

I’m lost

Simon

From: David Feuer [mailto:david.feuer at gmail.com]
Sent: 16 February 2017 23:51
To: Simon Peyton Jones <simonpj at microsoft.com>
Cc: ghc-devs <ghc-devs at haskell.org>; Reid Barton <rwbarton at gmail.com>; Ben Gamari <bgamari at gmail.com>
Subject: RE: Static data and RULES

Sorry; guess I should have given more background on that. This goes back to the performance problems Ben encountered in Typeable. The goal is to avoid trying to optimize something over and over that's never ever going to change. If we know that a term is made only of static data, we can skip it altogether in simplification. Suppose we have

foo = Just (Right [1])

Then no amount of optimization will ever be useful. But what about RULES? If the outermost pattern in a rule matches on a data constructor, then it's not static anymore! We may be replacing it with something else. So we need a finer mechanism. We *also* need a finer mechanism for strict constructors in general. We need to avoid inlining those too early if they're mentioned in any position in RULES. Trying to make this work right automagically looks a bit tricky in the face of orphan rules and such.

On Feb 16, 2017 6:35 PM, "Simon Peyton Jones" <simonpj at microsoft.com<mailto:simonpj at microsoft.com>> wrote:
I don’t understand any of this.

However, RULES are allowed to match on data constructors and it would be nice to let that keep happening.

Why won’t it keep happening?  What is the problem you are trying to solve?  Why does the fast-path make it harder?

Maybe open a ticket?

Simon

From: ghc-devs [mailto:ghc-devs-bounces at haskell.org<mailto:ghc-devs-bounces at haskell.org>] On Behalf Of David Feuer
Sent: 16 February 2017 22:13
To: Ben Gamari <bgamari at gmail.com<mailto:bgamari at gmail.com>>; Reid Barton <rwbarton at gmail.com<mailto:rwbarton at gmail.com>>
Cc: ghc-devs <ghc-devs at haskell.org<mailto:ghc-devs at haskell.org>>
Subject: Static data and RULES

Ben Gamari and Reid Barton are interested in making it cheaper for static data to pass through simplification. The basic idea is that if a term is already made entirely of data constructors and literals, then there's nothing left to optimize.

However, RULES are allowed to match on data constructors and it would be nice to let that keep happening. But on the other hand, RULES are apparently (according to Duncan Coutts) already broken for strict data constructors, because they have workers and wrappers.

My thought: let's allow phased INLINE and NOINLINE pragmas for data constructors. The default would be INLINE. The ~ phase choice would not be available: once inline, always inline.

Semantics
~~~~~~~~~~

For all constructors:

If a constructor is allowed by pragmas to inline in a certain phase, then in that phase terms built from it can be considered static. Once static, always static.

If a constructor is not allowed to inline in a certain phase, terms built from it will be considered non-static.

After demand analysis and worker/wrapper, all constructors are considered inline.

For strict constructors:

A strict constructor wrapper prohibited from inlining in a certain phase simply will not.

Strict constructor wrappers will all be allowed to inline after demand analysis and worker/wrapper. This matches the way we now handle wrappers actually created in that phase.

Syntax:

For GADT syntax, this is easy:

data Foo ... where
  {-# INLINE [1] Bar #-}
  Bar :: ...

For traditional syntax, I think it's probably best to pull the pragmas to the top:

{-# NOINLINE Quux #-}
data Baz ... = Quux ... | ...
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