Changes between Version 1 and Version 2 of Commentary/Compiler/SeqMagic
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 Jun 17, 2011 10:44:18 PM (6 years ago)
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Commentary/Compiler/SeqMagic
v1 v2 1 1 = Seq magic = 2 2 3 The innocentlooking `seq` operator causes all manner of mayhem in GHC. This page summarises the issues. 3 The innocentlooking `seq` operator causes all manner of mayhem in GHC. This page summarises the issues. See also discussion in Trac #5129. 4 4 5 5 == The baseline position == … … 17 17 }}} 18 18 19 == Problems == 19 But this approach has problems; see `Note [Deguaring seq]` in `DsUtils`. 20 20 21 Here are some of the problems that showed up. See `Note [Deguaring seq]` in `DsUtils`. 22 23 24 Trac #1031. Consider 21 === Problem 1 (Trac #1031) === 22 Consider 25 23 {{{ 26 24 f x y = x `seq` (y `seq` (# x,y #)) 27 25 }}} 28 The `[CoreSyn let/app invariant]` means that, other things being equal, because26 The `[CoreSyn let/app invariant]` (see `CoreSyn`) means that, other things being equal, because 29 27 the argument to the outer `seq` has an unlifted type, we'll use callbyvalue thus: 28 {{{ 29 f x y = case (y `seq` (# x,y #)) of v > x `seq` v 30 }}} 31 But that is bad for two reasons: 32 * we now evaluate `y` before `x`, and 33 * we can't bind `v` to an unboxed pair 30 34 31 f x y = case (y `seq` (# x,y #)) of v > x `seq` v 32 33 But that is bad for two reasons: 34 (a) we now evaluate y before x, and 35 (b) we can't bind v to an unboxed pair 36 37 Seq is very, very special! So we recognise it right here, and desugar to 35 Seq is very, very special! Treating it as a twoargument function, strict in 36 both arguments, doesn't work. We "fixed" this by treating `seq` as a language 37 construct, desugared by the desugarer, rather than as a function that may (or 38 may not) be inlined by the simplifier. So the above term is desugared to: 38 39 {{{ 39 40 case x of _ > case y of _ > (# x,y #) 40 41 }}} 41 Note [Desugaring seq (2)] cf Trac #2273 42 ~~~~~~~~~~~~~~~~~~~~~~~~~ 42 43 === Problem 2 (Trac #2273) === 44 43 45 Consider 46 {{{ 44 47 let chp = case b of { True > fst x; False > 0 } 45 48 in chp `seq` ...chp... 46 Here the seq is designed to plug the space leak of retaining (snd x) 49 }}} 50 Here the `seq` is designed to plug the space leak of retaining `(snd x)` 47 51 for too long. 48 52 49 If we rely on the ordinary inlining of seq, we'll get 53 If we rely on the ordinary inlining of `seq`, we'll get 54 {{{ 50 55 let chp = case b of { True > fst x; False > 0 } 51 56 case chp of _ { I# > ...chp... } 52 53 But since chpis cheap, and the case is an alluring contet, we'll54 inline chp into the case scrutinee. Now there is only one use of chp,57 }}} 58 But since `chp` is cheap, and the case is an alluring contet, we'll 59 inline `chp` into the case scrutinee. Now there is only one use of `chp`, 55 60 so we'll inline a second copy. Alas, we've now ruined the purpose of 56 61 the seq, by reintroducing the space leak: 62 {{{ 57 63 case (case b of {True > fst x; False > 0}) of 58 64 I# _ > ...case b of {True > fst x; False > 0}... 59 65 }}} 60 66 We can try to avoid doing this by ensuring that the binderswap in the 61 67 case happens, so we get his at an early stage: 68 {{{ 62 69 case chp of chp2 { I# > ...chp2... } 70 }}} 63 71 But this is fragile. The real culprit is the source program. Perhaps we 64 72 should have said explicitly 73 {{{ 65 74 let !chp2 = chp in ...chp2... 66 67 But that's painful. So the code here does a little hack to make seq68 more robust: a saturated application of 'seq' is turned *directly*into75 }}} 76 But that's painful. So the desugarer does a little hack to make `seq` 77 more robust: a saturated application of `seq` is turned '''directly''' into 69 78 the case expression, thus: 79 {{{ 70 80 x `seq` e2 ==> case x of x > e2  Note shadowing! 71 81 e1 `seq` e2 ==> case x of _ > e2 72 82 }}} 73 83 So we desugar our example to: 84 {{{ 74 85 let chp = case b of { True > fst x; False > 0 } 75 86 case chp of chp { I# > ...chp... } 87 }}} 76 88 And now all is well. 77 89 78 The reason it's a hack is because if you define mySeq=seq, the hack 79 won't work on mySeq. 90 Be careful not to desugar 91 {{{ 92 True `seq` e ==> case True of True { ... } 93 }}} 94 which stupidly tries to bind the datacon 'True'. This is easily avoided. 80 95 81 Note [Desugaring seq (3)] cf Trac #2409 82 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 83 The isLocalId ensures that we don't turn 84 True `seq` e 85 into 86 case True of True { ... } 87 which stupidly tries to bind the datacon 'True'. 96 The whole thing is a hack though; if you define `mySeq=seq`, the hack 97 won't work on `mySeq`. 88 98 89 === The need for special rules === 99 === Problem 3 (Trac #5262) === 100 101 Consider 102 {{{ 103 f x = x `seq` (\y.y) 104 }}} 105 With the above desugaring we get 106 {{{ 107 f x = case x of x { _ > \y.y } 108 }}} 109 and now ete expansion gives 110 {{{ 111 f x y = case x of x { _ > y } 112 }}} 113 Now suppose that we have 114 {{{ 115 f (length xs) `seq` 3 116 }}} 117 Plainly `(length xs)` should be evaluated... but it isn't because `f` has arity 2. 118 (Without O this doesn't happen.) 119 120 === Problem 4: seq in the IO monad == 121 122 See the extensive discussion in Trac #5129. 123 124 === Problem 5: the need for special rules === 90 125 91 126 Roman found situations where he had … … 102 137 enough support that you can do this using a rewrite rule: 103 138 {{{ 104 RULE "f/seq" forall n . seq (f n) e = seq n e139 RULE "f/seq" forall n e. seq (f n) e = seq n e 105 140 }}} 106 141 You write that rule. When GHC sees a case expression that discards … … 112 147 into a case expression on the LHS of a rule. 113 148 114 To increase applicability of these userdefined rules, we also have the following builtin rule for `seq` 149 To increase applicability of these userdefined rules, we also 150 have the following builtin rule for `seq` 115 151 {{{ 116 152 seq (x > co) y = seq x y … … 124 160 125 161 162 = A better way = 163 164 Here's our new plan. 165 * Introduce a new primop `seq# :: a > State# s > (# a, State# s #)` 166 * An application of the primop is not considered cheap. 167 * Desugar `seq` thus: 168 {{{ 169 x `seq` e2 ==> case seq# x RW of (# x, _ #) > e2  Note shadowing! 170 e1 `seq` e2 ==> case seq# x RW of (# _, _ #) > e2 171 }}} 172 * Define `evaluate` thus 173 {{{ 174 evaluate :: a > IO () 175 evaluate x = IO (\s > case seq# x s of 176 (# _, s' #) > (# (), s' #) 177 }}} 178 179 All the same equations hold as with the old defn for `seq`, but the problems 180 go away: 181 * Problem 1: (seq x y) is elaborated in the desugarer 182 * Problem 2: problem largely unaffected 183 * Problem 3: if we regard `(seq# a b)` as expensive, we won't eta expand. 184 * Problem 4: unchanged