Improve SpecConstr for free variables
This ticket records a suggestion for improving SpecConstr
, so we don't lose it. The original SpecConstr
transformation is described in "Call pattern specialisation for Haskell". Consider this program
f x = let g y = ...case x of { z:zs -> e1; [] -> e2 } ...
in
...case x of
z:zs -> if ... then g 3 else g 4
[] -> ...
Here 'x' is free in 'g', but x's value is known at g's call sites. It's not enough just to know "x is a cons" inside g; we must also have access to z,zs. So perhaps the thing to do is to imagine lambda-lifting 'g' to become 'gl' thus:
gl x y = ...case x of { z:zs -> e1; [] -> e2 } ...
f x = ...case x of
z:zs -> if ... then gl x 3 else gl x 4
[] -> ...
Now the SpecConstr
transformation will apply nicely. And it's arguably a bad shortcoming that currently the mere act of lambda lifting can affect how effective SpecConstr
is.
Of course, we only want to lambda-lift wrt the parameters that'll be specialised, so this transformation probably wants to be done at the same time as the rest of SpecConstr
. I don't have much idea of how hard that'd be, but it's a nice idea.
Trac metadata
Trac field | Value |
---|---|
Version | 6.8.2 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | Unknown |
Architecture | Unknown |