{-# RULE "op1@C[a]" forall a, d:C a.
op1 [a] (df_i d) = op1_i a d #-}
-* The dictionary function itself is inlined as vigorously as we
+* We want to inline the dictionary function itself as vigorously as we
possibly can, so that we expose that dictionary constructor to
- selectors as much as poss. That is why the op_i stuff is in
- *separate* bindings, so that the df_i binding is small enough
- to inline. See Note [Inline dfuns unconditionally].
+ selectors as much as poss. We don't actually inline it; rather, we
+ use a Builtin RULE for the ClassOps (see MkId.mkDictSelId) to short
+ circuit such applications. But the RULE only applies if it can "see"
+ the dfun's DFunUnfolding.
* Note that df_i may be mutually recursive with both op1_i and op2_i.
It's crucial that df_i is not chosen as the loop breaker, even
though op1_i has a (user-specified) INLINE pragma.
- Not even once! Else op1_i, op2_i may be inlined into df_i.
* Instead the idea is to inline df_i into op1_i, which may then select
methods from the MkC record, and thereby break the recursion with
* If op1_i is marked INLINE by the user there's a danger that we won't
inline df_i in it, and that in turn means that (since it'll be a
loop-breaker because df_i isn't), op1_i will ironically never be
- inlined. We need to fix this somehow -- perhaps allowing inlining
- of INLINE functions inside other INLINE functions.
+ inlined. But this is OK: the recursion breaking happens by way of
+ a RULE (the magic ClassOp rule above), and RULES work inside InlineRule
+ unfoldings. See Note [RULEs enabled in SimplGently] in SimplUtils
+
Note [Subtle interaction of recursion and overlap]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<dm-rhs> and for <rhs>, but that doesn't matter: the *renamer* will have
complained if 'b' is mentioned in <rhs>.
-Note [Inline dfuns unconditionally]
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-The code above unconditionally inlines dict funs. Here's why.
-Consider this program:
-
- test :: Int -> Int -> Bool
- test x y = (x,y) == (y,x) || test y x
- -- Recursive to avoid making it inline.
-
-This needs the (Eq (Int,Int)) instance. If we inline that dfun
-the code we end up with is good:
-
- Test.$wtest =
- \r -> case ==# [ww ww1] of wild {
- PrelBase.False -> Test.$wtest ww1 ww;
- PrelBase.True ->
- case ==# [ww1 ww] of wild1 {
- PrelBase.False -> Test.$wtest ww1 ww;
- PrelBase.True -> PrelBase.True [];
- };
- };
- Test.test = \r [w w1]
- case w of w2 {
- PrelBase.I# ww ->
- case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
- };
-
-If we don't inline the dfun, the code is not nearly as good:
-
- (==) = case PrelTup.$fEq(,) PrelBase.$fEqInt PrelBase.$fEqInt of tpl {
- PrelBase.:DEq tpl1 tpl2 -> tpl2;
- };
-
- Test.$wtest =
- \r [ww ww1]
- let { y = PrelBase.I#! [ww1]; } in
- let { x = PrelBase.I#! [ww]; } in
- let { sat_slx = PrelTup.(,)! [y x]; } in
- let { sat_sly = PrelTup.(,)! [x y];
- } in
- case == sat_sly sat_slx of wild {
- PrelBase.False -> Test.$wtest ww1 ww;
- PrelBase.True -> PrelBase.True [];
- };
-
- Test.test =
- \r [w w1]
- case w of w2 {
- PrelBase.I# ww ->
- case w1 of w3 { PrelBase.I# ww1 -> Test.$wtest ww ww1; };
- };
-
-Why didn't GHC inline $fEq in those days? Because it looked big:
-
- PrelTup.zdfEqZ1T{-rcX-}
- = \ @ a{-reT-} :: * @ b{-reS-} :: *
- zddEq{-rf6-} _Ks :: {PrelBase.Eq{-23-} a{-reT-}}
- zddEq1{-rf7-} _Ks :: {PrelBase.Eq{-23-} b{-reS-}} ->
- let {
- zeze{-rf0-} _Kl :: (b{-reS-} -> b{-reS-} -> PrelBase.Bool{-3c-})
- zeze{-rf0-} = PrelBase.zeze{-01L-}@ b{-reS-} zddEq1{-rf7-} } in
- let {
- zeze1{-rf3-} _Kl :: (a{-reT-} -> a{-reT-} -> PrelBase.Bool{-3c-})
- zeze1{-rf3-} = PrelBase.zeze{-01L-} @ a{-reT-} zddEq{-rf6-} } in
- let {
- zeze2{-reN-} :: ((a{-reT-}, b{-reS-}) -> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
- zeze2{-reN-} = \ ds{-rf5-} _Ks :: (a{-reT-}, b{-reS-})
- ds1{-rf4-} _Ks :: (a{-reT-}, b{-reS-}) ->
- case ds{-rf5-}
- of wild{-reW-} _Kd { (a1{-rf2-} _Ks, a2{-reZ-} _Ks) ->
- case ds1{-rf4-}
- of wild1{-reX-} _Kd { (b1{-rf1-} _Ks, b2{-reY-} _Ks) ->
- PrelBase.zaza{-r4e-}
- (zeze1{-rf3-} a1{-rf2-} b1{-rf1-})
- (zeze{-rf0-} a2{-reZ-} b2{-reY-})
- }
- } } in
- let {
- a1{-reR-} :: ((a{-reT-}, b{-reS-})-> (a{-reT-}, b{-reS-})-> PrelBase.Bool{-3c-})
- a1{-reR-} = \ a2{-reV-} _Ks :: (a{-reT-}, b{-reS-})
- b1{-reU-} _Ks :: (a{-reT-}, b{-reS-}) ->
- PrelBase.not{-r6I-} (zeze2{-reN-} a2{-reV-} b1{-reU-})
- } in
- PrelBase.zdwZCDEq{-r8J-} @ (a{-reT-}, b{-reS-}) a1{-reR-} zeze2{-reN-})
-
-and it's not as bad as it seems, because it's further dramatically
-simplified: only zeze2 is extracted and its body is simplified.
%************************************************************************
projects / ghc-hetmet.git / blobdiff