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Diffstat (limited to 'compiler/GHC/Core/SimpleOpt.hs')
| -rw-r--r-- | compiler/GHC/Core/SimpleOpt.hs | 27 |
1 files changed, 27 insertions, 0 deletions
diff --git a/compiler/GHC/Core/SimpleOpt.hs b/compiler/GHC/Core/SimpleOpt.hs index b14b421e49..afb51d1219 100644 --- a/compiler/GHC/Core/SimpleOpt.hs +++ b/compiler/GHC/Core/SimpleOpt.hs @@ -957,6 +957,31 @@ will happen the next time either. See test T16254, which checks the behavior of newtypes. +Note [Don't float join points] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +exprIsConApp_maybe should succeed on + let v = e in Just v +returning [x=e] as one of the [FloatBind]. But it must +NOT succeed on + join j x = rhs in Just v +because join-points can't be gaily floated. Consider + case (join j x = rhs in Just) of + K p q -> blah +We absolutely must not "simplify" this to + join j x = rhs + in blah +because j's return type is (Maybe t), quite different to blah's. + +You might think this could never happen, because j can't be +tail-called in the body if the body returns a constructor. But +in !3113 we had a /dead/ join point (which is not illegal), +and its return type was wonky. + +The simple thing is not to float a join point. The next iteration +of the simplifier will sort everything out. And it there is +a join point, the chances are that the body is not a constructor +application, so failing faster is good. + Note [exprIsConApp_maybe for data-con wrappers: tricky corner] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Generally speaking @@ -1065,6 +1090,8 @@ exprIsConApp_maybe (in_scope, id_unf) expr in go subst' (float:floats) body (CC args co) go subst floats (Let (NonRec bndr rhs) expr) cont + | not (isJoinId bndr) + -- Crucial guard! See Note [Don't float join points] = let rhs' = subst_expr subst rhs (subst', bndr') = subst_bndr subst bndr float = FloatLet (NonRec bndr' rhs') |