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If either side diverges, both do!

See Note [Trivial right-hand sides]

We now look inside one layer of recursive types, thanks to
checkRecTc.  It does mean we need an additional environment
field, ae_rec_tc.

I also commented out the apparently over-conservative test
at coercions.  I'm not 100% sure I'm right here, but I can't
see why the simpler code will go wrong, so I'm going to suck
it and see.

This just formalises an abstraction we've been using anyway,
namely to expand "recursive" TyCons until we see them twice.
We weren't doing this in typeArity, and that inconsistency
was leading to a subsequent ASSERT failure, when compiling
Stream.hs, which has a newtype like this

   newtype Stream m a b = Stream (m (Either b (a, Stream m a b)))

This major patch implements the cardinality analysis described
in our paper "Higher order cardinality analysis". It is joint
work with Ilya Sergey and Dimitrios Vytiniotis.

The basic is augment the absence-analysis part of the demand
analyser so that it can tell when something is used
	 never
	 at most once
 	 some other way

The "at most once" information is used
    a) to enable transformations, and
       in particular to identify one-shot lambdas
    b) to allow updates on thunks to be omitted.

There are two new flags, mainly there so you can do performance
comparisons:
    -fkill-absence   stops GHC doing absence analysis at all
    -fkill-one-shot  stops GHC spotting one-shot lambdas
                     and single-entry thunks

The big changes are:

* The Demand type is substantially refactored.  In particular
  the UseDmd is factored as follows
      data UseDmd
        = UCall Count UseDmd
        | UProd [MaybeUsed]
        | UHead
        | Used

      data MaybeUsed = Abs | Use Count UseDmd

      data Count = One | Many

  Notice that UCall recurses straight to UseDmd, whereas
  UProd goes via MaybeUsed.

  The "Count" embodies the "at most once" or "many" idea.

* The demand analyser itself was refactored a lot

* The previously ad-hoc stuff in the occurrence analyser for foldr and
  build goes away entirely.  Before if we had build (\cn -> ...x... )
  then the "\cn" was hackily made one-shot (by spotting 'build' as
  special.  That's essential to allow x to be inlined.  Now the
  occurrence analyser propagates info gotten from 'build's stricness
  signature (so build isn't special); and that strictness sig is
  in turn derived entirely automatically.  Much nicer!

* The ticky stuff is improved to count single-entry thunks separately.

One shortcoming is that there is no DEBUG way to spot if an
allegedly-single-entry thunk is acually entered more than once.  It
would not be hard to generate a bit of code to check for this, and it
would be reassuring.  But it's fiddly and I have not done it.

Despite all this fuss, the performance numbers are rather under-whelming.
See the paper for more discussion.

       nucleic2          -0.8%    -10.9%      0.10      0.10     +0.0%
         sphere          -0.7%     -1.5%      0.08      0.08     +0.0%
--------------------------------------------------------------------------------
            Min          -4.7%    -10.9%     -9.3%     -9.3%    -50.0%
            Max          -0.4%     +0.5%     +2.2%     +2.3%     +7.4%
 Geometric Mean          -0.8%     -0.2%     -1.3%     -1.3%     -1.8%

I don't quite know how much credence to place in the runtime changes,
but movement seems generally in the right direction.

usrbincc authored May 20, 2013 and ian@well-typed.com committed Jun 04, 2013

- Remove unused property `def_prompt`.

Make expands things even in comments

The LLVM back end does not yet work when building dynamically.

kgardas authored Apr 04, 2013 and ian@well-typed.com committed May 30, 2013

This patch disables shared libs support on arm-unknown-linux platform. It
unbreaks ghc-stage2 on this platform after recent Ian's changes
in dynamic/shared libs domain. The reason why ghc-stage2 fails when linked
with shared libs is still unknown so this is just a workaround at the moment,
but it at least recovers previous "correct" behavior of ghc-stage2
on ARM/Linux

where the invariant didn't hold, leading to
subsequent chaos. Happily an ASSERT caught it.

Somtimes we need (xs ++ ys) in situations where ys is
almost always empty.  Utils.chkAppend checks for that
case first.

Should help to track down cache-out-of-date problems

The details of this are described in Note [magicSingIId magic] in basicTypes/MkId.lhs

This long-overdue fix arranges to eta-reduce the axiom for a
data/newtype instance in the same way that we have longq
eta-reduced the axiom for a newtype.

See Note [Eta reduction for data family axioms] in TcInstDcls
and Note [Deriving, type families, and partial applications] in TcDeriv.

Fixes Trac #7809

Finally (I hope) fixes Trac #7903.

See Note [Zonking inside the knot] in TcHsSyn

See Note [Type determines value] in Specialise.

This is a long-standing regression (Trac #7797), which meant that in
particular the Eq [Char] instance does not get specialised.
(The *methods* do, but the dictionary itself doesn't.)  So when you
call a function
     f :: Eq a => blah
on a string type (ie a=[Char]), 7.6 passes a dictionary of un-specialised
methods.

This only matters when calling an overloaded function from a
specialised context, but that does matter in some programs.  I
remember (though I cannot find the details) that Nick Frisby discovered
this to be the source of some pretty solid performanc regresisons.

Anyway it works now. The key change is that a DFunUnfolding now takes
a form that is both simpler than before (the DFunArg type is eliminated)
and more general:

data Unfolding
  = ...
  | DFunUnfolding {     -- The Unfolding of a DFunId
    			-- See Note [DFun unfoldings]
      		  	--     df = /\a1..am. \d1..dn. MkD t1 .. tk
                        --                                 (op1 a1..am d1..dn)
     		      	--     	    	      	       	   (op2 a1..am d1..dn)
        df_bndrs :: [Var],      -- The bound variables [a1..m],[d1..dn]
        df_con   :: DataCon,    -- The dictionary data constructor (never a newtype datacon)
        df_args  :: [CoreExpr]  -- Args of the data con: types, superclasses and methods,
    }                           -- in positional order

That in turn allowed me to re-enable the DFunUnfolding specialisation in
DsBinds.  Lots of details here in TcInstDcls:
	  Note [SPECIALISE instance pragmas]

I also did some refactoring, in particular to pass the InScopeSet to
exprIsConApp_maybe (which in turn means it has to go to a RuleFun).

NB: Interface file format has changed!