diff options
Diffstat (limited to 'compiler/GHC/Core/Opt/Arity.hs')
| -rw-r--r-- | compiler/GHC/Core/Opt/Arity.hs | 18 |
1 files changed, 4 insertions, 14 deletions
diff --git a/compiler/GHC/Core/Opt/Arity.hs b/compiler/GHC/Core/Opt/Arity.hs index c9142443c1..77df389dfb 100644 --- a/compiler/GHC/Core/Opt/Arity.hs +++ b/compiler/GHC/Core/Opt/Arity.hs @@ -509,9 +509,6 @@ this transformation. So we try to limit it as much as possible: Of course both (1) and (2) are readily defeated by disguising the bottoms. -Another place where -fpedantic-bottoms comes up is during eta-reduction. -See Note [Eta reduction soundness], the bit about -fpedantic-bottoms. - 4. Note [Newtype arity] ~~~~~~~~~~~~~~~~~~~~~~~~ Non-recursive newtypes are transparent, and should not get in the way. @@ -2382,20 +2379,13 @@ case where `e` is trivial): `e = \x y. bot`. Could we relax to "*At least one call in the same trace* is with n args"? - (NB: Strictness analysis can only answer this relaxed question, not the - original formulation.) - Consider what happens for + No. Consider what happens for ``g2 c = c True `seq` c False 42`` Here, `g2` will call `c` with 2 arguments (if there is a call at all). But it is unsound to eta-reduce the arg in `g2 (\x y. e x y)` to `g2 e` when `e = \x. if x then bot else id`, because the latter will diverge when - the former would not. - - On the other hand, with `-fno-pedantic-bottoms` , we will have eta-expanded - the definition of `e` and then eta-reduction is sound - (see Note [Dealing with bottom]). - Consequence: We have to check that `-fpedantic-bottoms` is off; otherwise - eta-reduction based on demands is in fact unsound. + the former would not. Fortunately, the strictness analyser will report + "Not always called with two arguments" for `g2` and we won't eta-expand. See Note [Eta reduction based on evaluation context] for the implementation details. This criterion is tested extensively in T21261. @@ -2541,7 +2531,7 @@ Then this is how the pieces are put together: Because it's irrelevant! When we simplify an expression, we do so under the assumption that it is currently under evaluation.) This sub-demand literally says "Whenever this expression is evaluated, it - is also called with two arguments, potentially multiple times". + is called with at least two arguments, potentially multiple times". * Then the simplifier takes apart the lambda and simplifies the lambda group and then calls 'tryEtaReduce' when rebuilding the lambda, passing the |