diff options
Diffstat (limited to 'compiler/GHC/Core.hs')
| -rw-r--r-- | compiler/GHC/Core.hs | 67 |
1 files changed, 66 insertions, 1 deletions
diff --git a/compiler/GHC/Core.hs b/compiler/GHC/Core.hs index a92252a61c..fa93f545fe 100644 --- a/compiler/GHC/Core.hs +++ b/compiler/GHC/Core.hs @@ -42,7 +42,7 @@ module GHC.Core ( foldBindersOfBindStrict, foldBindersOfBindsStrict, collectBinders, collectTyBinders, collectTyAndValBinders, collectNBinders, collectNValBinders_maybe, - collectArgs, stripNArgs, collectArgsTicks, flattenBinds, + collectArgs, collectValArgs, stripNArgs, collectArgsTicks, flattenBinds, collectFunSimple, exprToType, @@ -994,6 +994,60 @@ tail position: A cast changes the type, but the type must be the same. But operationally, casts are vacuous, so this is a bit unfortunate! See #14610 for ideas how to fix this. +Note [Strict fields in Core] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Evaluating a data constructor worker evaluates its strict fields. + +In other words, if `MkT` is strict in its first field and `xs` reduces to +`error "boom"`, then `MkT xs b` will throw that error. +Conversely, it is sound to seq the field before the call to the constructor, +e.g., with `case xs of xs' { __DEFAULT -> MkT xs' b }`. +Let's call this transformation "field seq insertion". + +Note in particular that the data constructor application `MkT xs b` above is +*not* a value, unless `xs` is! + +This has pervasive effect on the Core pipeline: + + * `exprIsHNF`/`exprIsConLike`/`exprOkForSpeculation` need to assert that the + strict arguments of a DataCon worker are values/ok-for-spec themselves. + + * `exprIsConApp_maybe` inserts field seqs in the `FloatBind`s it returns, so + that the Simplifier, Constant-folding, the pattern-match checker, etc. + all see the insert field seqs when they match on strict workers. Often this + is just to emphasise strict semantics, but for case-of-known constructor + and case-to-let field insertion is *vital*, otherwise these transformations + would lose field seqs. + + * The demand signature of a data constructor is strict in strict field + position, whereas is it's normally lazy. Likewise the demand *transformer* + of a DataCon worker can add stricten up demands on strict field args. + See Note [Demand transformer for data constructors]. + + * In the absence of `-fpedantic-bottoms`, it is still possible that some seqs + are ultimately dropped or delayed due to eta-expansion. + See Note [Dealing with bottom]. + +Strict field semantics is exploited in STG by Note [Tag Inference]: +It performs field seq insertion to statically guarantee *taggedness* of strict +fields, establishing the Note [STG Strict Field Invariant]. (Happily, most +of those seqs are immediately detected as redundant by tag inference and are +omitted.) From then on, DataCon worker semantics are actually lazy, hence it is +important that STG passes maintain the Strict Field Invariant. + + +Historical Note: +The delightfully simple description of strict field semantics is the result of +a long saga (#20749, the bits about strict data constructors in #21497, #22475), +where we tried a more lenient (but actually not) semantics first that would +allow both strict and lazy implementations of DataCon workers. This was favoured +because the "pervasive effect" throughout the compiler was deemed too large +(when it really turned out to be very modest). +Alas, this semantics would require us to implement `exprIsHNF` in *exactly* the +same way as above, otherwise the analysis would not be conservative wrt. the +lenient semantics (which includes the strict one). It is also much harder to +explain and maintain, as it turned out. + ************************************************************************ * * In/Out type synonyms @@ -2081,6 +2135,17 @@ collectArgs expr go e as = (e, as) -- | Takes a nested application expression and returns the function +-- being applied and the arguments to which it is applied +collectValArgs :: Expr b -> (Expr b, [Arg b]) +collectValArgs expr + = go expr [] + where + go (App f a) as + | isValArg a = go f (a:as) + | otherwise = go f as + go e as = (e, as) + +-- | Takes a nested application expression and returns the function -- being applied. Looking through casts and ticks to find it. collectFunSimple :: Expr b -> Expr b collectFunSimple expr |