id,summary,reporter,owner,description,type,status,priority,milestone,component,version,resolution,keywords,cc,os,architecture,failure,difficulty,testcase,blockedby,blocking,related
4495,GHC fails to inline methods of single-method classes,diatchki,igloo,"Consider the following module:
{{{
module A where

class C a where
  f :: a

g :: C a => (a,a)
g = (f,f)
}}}

GHC has a nice optimization where the dictionaries for classes with
just a single method (and no super-classes) are represented with a newtype.
This means that, essentially, the ""dictionary"" is the method itself,
and the code that ""extracts"" a method from a dictionary is just the identity
function.  For this reason, we can completely eliminate the extraction code
by always inlining the method.

This works as expected in GHC 6.12.2 but something seems to have changed in
the current HEAD, so that the ""extraction code"" (i.e., the method ""f"") does
not seem to get eliminated:


[GHC 6.12.2]
{{{
f :: forall a. (C a) => a
GblId[ClassOp]

f = \ (@ a) (tpl [ALWAYS Once Nothing] :: C a) -> tpl `cast` ...

g :: forall a. (C a) => (a, a)
GblId

g = __inline_me (\ (@ a) (d :: C a) -> (d `cast` ..., d `cast` ...)
}}}

[HEAD as of Nov 14 2010]
{{{
f [InlPrag=[NEVER]] :: forall a. C a => a
[GblId[ClassOp],
 Arity=1,
 Unf=Unf{Src=<vanilla>, TopLvl=True, Arity=1, Value=True,
         ConLike=True, Cheap=True, Expandable=True,
         Guidance=ALWAYS_IF(unsat_ok=True,boring_ok=True)},
 RULES: Built in rule for f: ""Class op f""]
f = \ (@ a) (tpl [Occ=Once] :: C a) -> tpl `cast` ...


g :: forall a. C a => (a, a)
[GblId,
 Arity=1,
 Unf=Unf{Src=InlineStable, TopLvl=True, Arity=1, Value=True,
         ConLike=True, Cheap=True, Expandable=True,
         Guidance=ALWAYS_IF(unsat_ok=True,boring_ok=False)
         Tmpl= \ (@ a) (d :: C a) -> (f @ a d, f @ a d)}]

g = \ (@ a) (d :: C a) -> (f @ a d, f @ a d)
}}}
",bug,closed,normal,7.0.2,Compiler,7.1,fixed,,,Unknown/Multiple,Unknown/Multiple,Runtime performance bug,,,,,