diff options
Diffstat (limited to 'compiler/GHC/Core/Unfold.hs')
| -rw-r--r-- | compiler/GHC/Core/Unfold.hs | 18 |
1 files changed, 17 insertions, 1 deletions
diff --git a/compiler/GHC/Core/Unfold.hs b/compiler/GHC/Core/Unfold.hs index e1195be42a..b189064995 100644 --- a/compiler/GHC/Core/Unfold.hs +++ b/compiler/GHC/Core/Unfold.hs @@ -412,6 +412,8 @@ inlineBoringOk e , exprIsTrivial a = go (credit-1) f go credit (Tick _ e) = go credit e -- dubious go credit (Cast e _) = go credit e + go credit (Case scrut _ _ [(_,_,rhs)]) -- See Note [Inline unsafeCoerce] + | isUnsafeEqualityProof scrut = go credit rhs go _ (Var {}) = boringCxtOk go _ _ = boringCxtNotOk @@ -459,7 +461,21 @@ calcUnfoldingGuidance dflags is_top_bottoming expr | otherwise = (+) -- See Note [Function and non-function discounts] -{- +{- Note [Inline unsafeCoerce] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +We really want to inline unsafeCoerce, even when applied to boring +arguments. It doesn't look as if its RHS is smaller than the call + unsafeCoerce x = case unsafeEqualityProof @a @b of UnsafeRefl -> x +but that case is discarded -- see Note [Implementing unsafeCoerce] +in base:Unsafe.Coerce. + +Moreover, if we /don't/ inline it, we may be left with + f (unsafeCoerce x) +which will build a thunk -- bad, bad, bad. + +Conclusion: we really want inlineBoringOk to be True of the RHS of +unsafeCoerce. This is (U4a) in Note [Implementing unsafeCoerce]. + Note [Computing the size of an expression] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The basic idea of sizeExpr is obvious enough: count nodes. But getting the |