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Diffstat (limited to 'compiler/GHC/Core/Utils.hs')
| -rw-r--r-- | compiler/GHC/Core/Utils.hs | 13 |
1 files changed, 10 insertions, 3 deletions
diff --git a/compiler/GHC/Core/Utils.hs b/compiler/GHC/Core/Utils.hs index eea81d1502..b4c736bcdc 100644 --- a/compiler/GHC/Core/Utils.hs +++ b/compiler/GHC/Core/Utils.hs @@ -2409,13 +2409,20 @@ case where `e` is trivial): like `g (\x y z. e x y z)` to `g e`, because that diverges when `e = \x y. bot`. - Could we relax to "At least *one call in the same trace* is with n args"? + 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 ``g2 c = c True `seq` c False 42`` - Here, `g2` will call `c` with 2 two arguments (if there is a call at all). - But it is unsafe to eta-reduce the arg in `g2 (\x y. e x y)` to `g2 e` + 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-pendantic-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. See Note [Eta reduction based on evaluation context] for the implementation details. This criterion is tested extensively in T21261. |