diff options
Diffstat (limited to 'compiler/GHC/Core/Unfold.hs')
| -rw-r--r-- | compiler/GHC/Core/Unfold.hs | 35 |
1 files changed, 20 insertions, 15 deletions
diff --git a/compiler/GHC/Core/Unfold.hs b/compiler/GHC/Core/Unfold.hs index 199f4bfca4..a8e4c03b95 100644 --- a/compiler/GHC/Core/Unfold.hs +++ b/compiler/GHC/Core/Unfold.hs @@ -1002,10 +1002,6 @@ ufUseThreshold At a call site, if the unfolding, less discounts, is smaller than this, then it's small enough inline -ufKeenessFactor - Factor by which the discounts are multiplied before - subtracting from size - ufDictDiscount The discount for each occurrence of a dictionary argument as an argument of a class method. Should be pretty small @@ -1024,6 +1020,22 @@ ufVeryAggressive loop breakers. +Historical Note: Before April 2020 we had another factor, +ufKeenessFactor, which would scale the discounts before they were subtracted +from the size. This was justified with the following comment: + + -- We multiply the raw discounts (args_discount and result_discount) + -- ty opt_UnfoldingKeenessFactor because the former have to do with + -- *size* whereas the discounts imply that there's some extra + -- *efficiency* to be gained (e.g. beta reductions, case reductions) + -- by inlining. + +However, this is highly suspect since it means that we subtract a *scaled* size +from an absolute size, resulting in crazy (e.g. negative) scores in some cases +(#15304). We consequently killed off ufKeenessFactor and bumped up the +ufUseThreshold to compensate. + + Note [Function applications] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ In a function application (f a b) @@ -1307,8 +1319,7 @@ tryUnfolding dflags id lone_variable extra_doc = text "discounted size =" <+> int discounted_size discounted_size = size - discount small_enough = discounted_size <= ufUseThreshold dflags - discount = computeDiscount dflags arg_discounts - res_discount arg_infos cont_info + discount = computeDiscount arg_discounts res_discount arg_infos cont_info where mk_doc some_benefit extra_doc yes_or_no @@ -1553,14 +1564,9 @@ which Roman did. -} -computeDiscount :: DynFlags -> [Int] -> Int -> [ArgSummary] -> CallCtxt +computeDiscount :: [Int] -> Int -> [ArgSummary] -> CallCtxt -> Int -computeDiscount dflags arg_discounts res_discount arg_infos cont_info - -- We multiple the raw discounts (args_discount and result_discount) - -- ty opt_UnfoldingKeenessFactor because the former have to do with - -- *size* whereas the discounts imply that there's some extra - -- *efficiency* to be gained (e.g. beta reductions, case reductions) - -- by inlining. +computeDiscount arg_discounts res_discount arg_infos cont_info = 10 -- Discount of 10 because the result replaces the call -- so we count 10 for the function itself @@ -1569,8 +1575,7 @@ computeDiscount dflags arg_discounts res_discount arg_infos cont_info -- Discount of 10 for each arg supplied, -- because the result replaces the call - + round (ufKeenessFactor dflags * - fromIntegral (total_arg_discount + res_discount')) + + total_arg_discount + res_discount' where actual_arg_discounts = zipWith mk_arg_discount arg_discounts arg_infos total_arg_discount = sum actual_arg_discounts |