#8329: dmdTransformDictSelSig panic
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Reporter: monoidal | Owner: simonpj
Type: bug | Status: new
Priority: highest | Milestone:
Component: Compiler | Version: 7.7
Resolution: | Keywords:
Operating System: Unknown/Multiple | Architecture:
Type of failure: Compile-time crash | Unknown/Multiple
Test Case: | Difficulty: Unknown
Blocking: | Blocked By:
| Related Tickets:
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Comment (by Simon Peyton Jones ):
In [changeset:9bd366642f495fe26db261317dd416de1ff854bc/ghc]:
{{{
#!CommitTicketReference repository="ghc"
revision="9bd366642f495fe26db261317dd416de1ff854bc"
Lift an unnecessary assumption in the demand analyser (fix Trac #8329)
Here's the Note about the (simple) fix. Apparently #8329 prevented all
23 packages of the Snap framework from compiling.
Note [Demand transformer for a ditionary selector]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If we evaluate (op dict-expr) under demand 'd', then we can push the
demand 'd'
into the appropriate field of the dictionary. What *is* the appropriate
field?
We just look at the strictness signature of the class op, which will be
something like: U(AAASAAAAA). Then replace the 'S' by the demand 'd'.
For single-method classes, which are represented by newtypes the signature
of 'op' won't look like U(...), so the splitProdDmd_maybe will fail.
That's fine: if we are doing strictness analysis we are also doing inling,
so we'll have inlined 'op' into a cast. So we can bale out in a
conservative
way, returning topDmdType.
It is (just.. Trac #8329) possible to be running strictness analysis
*without*
having inlined class ops from single-method classes. Suppose you are
using
ghc --make; and the first module has a local -O0 flag. So you may load a
class
without interface pragmas, ie (currently) without an unfolding for the
class
ops. Now if a subsequent module in the --make sweep has a local -O flag
you might do strictness analysis, but there is no inlining for the class
op.
This is wierd so I'm not worried about whether this optimises brilliantly;
but
it should not fall over.
}}}
--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8329#comment:3
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