diff options
| -rw-r--r-- | compiler/prelude/primops.txt.pp | 105 |
1 files changed, 52 insertions, 53 deletions
diff --git a/compiler/prelude/primops.txt.pp b/compiler/prelude/primops.txt.pp index 8b327ddbc4..1d879c438e 100644 --- a/compiler/prelude/primops.txt.pp +++ b/compiler/prelude/primops.txt.pp @@ -2992,59 +2992,58 @@ primop DataToTagOp "dataToTag#" GenPrimOp primop TagToEnumOp "tagToEnum#" GenPrimOp Int# -> a -{- Note [dataToTag#] -~~~~~~~~~~~~~~~~~~~~ -The primop dataToTag# is unusual because it evaluates its argument. -Only `SeqOp` shares that property. (Other primops do not do anything -as fancy as argument evaluation.) The special handling for dataToTag# -is: - -* CoreUtils.exprOkForSpeculation has a special case for DataToTagOp, - (actually in app_ok). Most primops with lifted arguments do not - evaluate those arguments, but DataToTagOp and SeqOp are two - exceptions. We say that they are /never/ ok-for-speculation, - regardless of the evaluated-ness of their argument. - See CoreUtils Note [PrimOps that evaluate their arguments] - -* There is a special case for DataToTagOp in StgCmmExpr.cgExpr, - that evaluates its argument and then extracts the tag from - the returned value. - -* An application like (dataToTag# (Just x)) is optimised by - dataToTagRule in PrelRules. - -* A case expression like - case (dataToTag# e) of <alts> - gets transformed t - case e of <transformed alts> - by PrelRules.caseRules; see Note [caseRules for dataToTag] - -See Trac #15696 for a long saga. - -Note [dataToTag# hack] -~~~~~~~~~~~~~~~~~~~~~~ -(This a temporary hack: see Trac #15696 commment:60.) - -dataToTag# evaluates its argument, so we don't want to float it out. -Consider: - - \z. case x of y -> let v = dataToTag# y in ... - -To improve floating, the FloatOut pass (deliberately) does a -binder-swap on the case, to give - - \z. case x of y -> let v = dataToTag# x in ... - -Now FloatOut might float that v-binding outside the \z - - let v = dataToTag# x in \z. case x of y -> ... - -But that is bad because that might mean x gets evaluated much too early! - -Solution: make dataToTag# into a can_fail primop. That will stop it floating -(see Note [PrimOp can_fail and has_side_effects] in PrimOp). It's a bit of -a hack but never mind. --} +-- Note [dataToTag#] +-- ~~~~~~~~~~~~~~~~~ +-- The primop dataToTag# is unusual because it evaluates its argument. +-- Only `SeqOp` shares that property. (Other primops do not do anything +-- as fancy as argument evaluation.) The special handling for dataToTag# +-- is: +-- +-- * CoreUtils.exprOkForSpeculation has a special case for DataToTagOp, +-- (actually in app_ok). Most primops with lifted arguments do not +-- evaluate those arguments, but DataToTagOp and SeqOp are two +-- exceptions. We say that they are /never/ ok-for-speculation, +-- regardless of the evaluated-ness of their argument. +-- See CoreUtils Note [PrimOps that evaluate their arguments] +-- +-- * There is a special case for DataToTagOp in StgCmmExpr.cgExpr, +-- that evaluates its argument and then extracts the tag from +-- the returned value. +-- +-- * An application like (dataToTag# (Just x)) is optimised by +-- dataToTagRule in PrelRules. +-- +-- * A case expression like +-- case (dataToTag# e) of <alts> +-- gets transformed t +-- case e of <transformed alts> +-- by PrelRules.caseRules; see Note [caseRules for dataToTag] +-- +-- See Trac #15696 for a long saga. +-- +-- Note [dataToTag# hack] +-- ~~~~~~~~~~~~~~~~~~~~~~ +-- (This a temporary hack: see Trac #15696 commment:60.) +-- +-- dataToTag# evaluates its argument, so we don't want to float it out. +-- Consider: +-- +-- \z. case x of y -> let v = dataToTag# y in ... +-- +-- To improve floating, the FloatOut pass (deliberately) does a +-- binder-swap on the case, to give +-- +-- \z. case x of y -> let v = dataToTag# x in ... +-- +-- Now FloatOut might float that v-binding outside the \z +-- +-- let v = dataToTag# x in \z. case x of y -> ... +-- +-- But that is bad because that might mean x gets evaluated much too early! +-- +-- Solution: make dataToTag# into a can_fail primop. That will stop it floating +-- (see Note [PrimOp can_fail and has_side_effects] in PrimOp). It's a bit of +-- a hack but never mind. ------------------------------------------------------------------------ section "Bytecode operations" |