#2625: Unexpected -ddump-simpl output for derived Ord instance and UNPACKed
fields
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Reporter: aslatter | Owner:
Type: bug | Status: new
Priority: low | Milestone: _|_
Component: Compiler | Version: 6.8.3
Severity: normal | Resolution:
Keywords: | Difficulty: Unknown
Testcase: | Architecture: x86_64 (amd64)
Os: Unknown/Multiple |
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Changes (by simonpj):
* priority: normal => low
* milestone: 6.10 branch => _|_
Comment:
Here's what is happening. The compare functions for A and B look like
this (initially):
{{{
compareA :: A -> A -> Ordering
compareA (A x1 .. x8) (A y1 .. y8)
= compare (B x1 .. x8) (B y1 .. y8)
compareB :: B -> B -> Ordering
compareB (B x1 .. x8) (B y1 .. y8)
= if x1>y1 then GT else ... blah blah blah...
}}}
Note that the signature of `compareB` '''must''' be `B -> B -> Ordering`,
so `compareA` is obliged to re-box the B argument. That always
potentially happens when you use `{-# UNPACK #-}`: if you match on the
constructor you may need to construct a value of the unpacked type to pass
on to some other function.
OK after strictness analysis you might hope that `compareB` gets 16
arguments. But GHC is wary about giving workers an arbitrarily large
number of arguments, so it has an arbitrary threshold of 10. As a result,
the worker for `compareB` looks like this:
{{{
$wcompareB :: Int# -> ... -> Int# -> B -> Ordering
$wcompareB x1 .. x8 (B y1 .. y8) = <blah>
}}}
The first argument is unpacked but not the second. And that is what is
happening to you.
I don't know a good solution in general. `{-# UNPACK #-}` can (and must)
lead to the possibility of reboxing. (In this case `compareB` is strict,
so its wrapper may do unboxing ,up to the threshold, but in general the
called function may be lazy.)
There is a workaround here: the flag `-fmax-worker-args=N` will change the
threshold
to N, and that fixes the problem. But it's not a ''satisfying'' fix.
I'll leave this open at low priority. By all means contribute
suggestions!
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2625#comment:3>
GHC <http://www.haskell.org/ghc/>
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