Major improvements to the specialiserwip/T17966

This patch is joint work of Alexis King and Simon PJ.  It does some
significant refactoring of the type-class specialiser.  Main highlights:

* We can specialise functions with types like
     f :: Eq a => a -> Ord b => b => blah
  where the classes aren't all at the front (#16473).  Here we can
  correctly specialise 'f' based on a call like
     f @Int @Bool dEqInt x dOrdBool
  This change really happened in an earlier patch
     commit 2d0cf6252957b8980d89481ecd0b79891da4b14b
     Author: Sandy Maguire <sandy@sandymaguire.me>
     Date:   Thu May 16 12:12:10 2019 -0400
  work that this new patch builds directly on that work, and refactors
  it a bit.

* We can specialise functions with implicit parameters (#17930)
     g :: (?foo :: Bool, Show a) => a -> String
  Previously we could not, but now they behave just like a non-class
  argument as in 'f' above.

* We can specialise under-saturated calls, where some (but not all of
  the dictionary arguments are provided (#17966).  For example, we can
  specialise the above 'f' based on a call
     map (f @Int dEqInt) xs
  even though we don't (and can't) give Ord dictionary.

  This may sound exotic, but #17966 is a program from the wild, and
  showed significant perf loss for functions like f, if you need
  saturation of all dictionaries.

* We fix a buglet in which a floated dictionary had a bogus demand
  (#17810), by using zapIdDemandInfo in the NonRec case of specBind.

* A tiny side benefit: we can drop dead arguments to specialised
  functions; see Note [Drop dead args from specialisations]

* Fixed a bug in deciding what dictionaries are "interesting"; see
  Note [Keep the old dictionaries interesting]

This is all achieved by by building on Sandy Macguire's work in
defining SpecArg, which mkCallUDs uses to describe the arguments of
the call. Main changes:

* Main work is in specHeader, which marched down the [InBndr] from the
  function definition and the [SpecArg] from the call site, together.

* specCalls no longer has an arity check; the entire mechanism now
  handles unders-saturated calls fine.

* mkCallUDs decides on an argument-by-argument basis whether to
  specialise a particular dictionary argument; this is new.
  See mk_spec_arg in mkCallUDs.

It looks as if there are many more lines of code, but I think that
all the extra lines are comments!

1 files changed, 6 insertions, 5 deletions

diff --git a/compiler/GHC/Core/Unfold.hs b/compiler/GHC/Core/Unfold.hs
index 411a954428..4b7cb80daf 100644
--- a/compiler/GHC/Core/Unfold.hs
+++ b/compiler/GHC/Core/Unfold.hs
@@ -172,15 +172,16 @@ mkInlinableUnfolding dflags expr
   where
     expr' = simpleOptExpr dflags expr
 
-specUnfolding :: DynFlags -> [Var] -> (CoreExpr -> CoreExpr) -> Arity
+specUnfolding :: DynFlags -> Id -> [Var] -> (CoreExpr -> CoreExpr) -> Arity
               -> Unfolding -> Unfolding
 -- See Note [Specialising unfoldings]
 -- specUnfolding spec_bndrs spec_app arity_decrease unf
 --   = \spec_bndrs. spec_app( unf )
 --
-specUnfolding dflags spec_bndrs spec_app arity_decrease
+specUnfolding dflags fn spec_bndrs spec_app arity_decrease
               df@(DFunUnfolding { df_bndrs = old_bndrs, df_con = con, df_args = args })
-  = ASSERT2( arity_decrease == count isId old_bndrs - count isId spec_bndrs, ppr df )
+  = ASSERT2( arity_decrease == count isId old_bndrs - count isId spec_bndrs
+           , ppr df $$ ppr spec_bndrs $$ ppr (spec_app (Var fn)) $$ ppr arity_decrease )
     mkDFunUnfolding spec_bndrs con (map spec_arg args)
       -- There is a hard-to-check assumption here that the spec_app has
       -- enough applications to exactly saturate the old_bndrs
@@ -194,7 +195,7 @@ specUnfolding dflags spec_bndrs spec_app arity_decrease
                    -- The beta-redexes created by spec_app will be
                    -- simplified away by simplOptExpr
 
-specUnfolding dflags spec_bndrs spec_app arity_decrease
+specUnfolding dflags _ spec_bndrs spec_app arity_decrease
               (CoreUnfolding { uf_src = src, uf_tmpl = tmpl
                              , uf_is_top = top_lvl
                              , uf_guidance = old_guidance })
@@ -211,7 +212,7 @@ specUnfolding dflags spec_bndrs spec_app arity_decrease
 
    in mkCoreUnfolding src top_lvl new_tmpl guidance
 
-specUnfolding _ _ _ _ _ = noUnfolding
+specUnfolding _ _ _ _ _ _ = noUnfolding
 
 {- Note [Specialising unfoldings]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~