wiki:Hoopl/Cleanup

Version 1 (modified by jstolarek, 8 months ago) (diff)

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Hoopl cleanup

This page was created in August 2013 as a temporary place to store proposals about cleaning up Hoopl library. After these changes are implemented we should replace this page with either another page or a Note in the source code that would explain current design of Hoopl.

Me (Jan Stolarek, JS) and Simon PJ had some discussion recently about cleaning up Hoopl to make its interface more consistent and less confusing

Can we be explicit about which join situations can occur inside hoopl?

A personal note by Jan Stolarek

On my first contact with Hoopl I was very confused by some of its behaviour. Here's a question I mailed to ghc-devs on 13th July 2013:

In my algorithm I need to initialize all of the blocks in a graph with bottom element of a lattice, except for the entry block, which needs some other initial values. I've written something like this:

cmmCopyPropagation dflags graph = do
    let entry_blk = g_entry graph
    g' <- dataflowPassFwd graph [(entry_blk, (Top , Top))] $
            analRewFwd cpLattice cpTransfer cpRewrite
    return . fst $ g'

cpLattice = DataflowLattice "copy propagation" (Bottom, Bottom) cpJoin

However, it seems that Bottom values passed to cpLattice are ignored - I could replace them with undefined and the code would still run without causing an error. Is there something obviously wrong in the way I pass initial fact values to dataflowPassFwd, or should I look for the problem in other parts of my code?

I think this question resulted from Hoopl's current behaviour where it sometimes ignores bottom passed in b y the user and sometimes does not.

When I did copy propagation pass I had data type that looked like this:

data Facts = Bottom | Const (M.Map a CPFact)

and I wrote join funstion which analyzed all four possible cases of joining facts:

join (Const f) (Const f) = ...
join (Const f) Bottom    = ...
join Bottom    (Const f) = ...
join Bottom    Bottom    = ...

But only one of them was ever used actually. While I expected this for the last two ones and replaced them with compiler panic, I certainly did not expect that join (Const f) Bottom will not be used. Only after some tiresome debugging and analyzing the source code did I realize that Hoopl optimizes away this kind of join. I think that being explicit about the redundance of bottom in forward analysis will make Hoopl easier to use for newcommers.