diff options
Diffstat (limited to 'compiler/GHC/Core/Opt/DmdAnal.hs')
| -rw-r--r-- | compiler/GHC/Core/Opt/DmdAnal.hs | 63 |
1 files changed, 59 insertions, 4 deletions
diff --git a/compiler/GHC/Core/Opt/DmdAnal.hs b/compiler/GHC/Core/Opt/DmdAnal.hs index 36c512d656..1263792d05 100644 --- a/compiler/GHC/Core/Opt/DmdAnal.hs +++ b/compiler/GHC/Core/Opt/DmdAnal.hs @@ -59,9 +59,18 @@ _ = pprTrace -- Tired of commenting out the import all the time -- | Options for the demand analysis data DmdAnalOpts = DmdAnalOpts - { dmd_strict_dicts :: !Bool -- ^ Use strict dictionaries - , dmd_unbox_width :: !Int -- ^ Use strict dictionaries + { dmd_strict_dicts :: !Bool + -- ^ Value of `-fdicts-strict` (on by default). + -- When set, all functons are implicitly strict in dictionary args. + , dmd_do_boxity :: !Bool + -- ^ Governs whether the analysis should update boxity signatures. + -- See Note [Don't change boxity without worker/wrapper]. + , dmd_unbox_width :: !Int + -- ^ Value of `-fdmd-unbox-width`. + -- See Note [Unboxed demand on function bodies returning small products] , dmd_max_worker_args :: !Int + -- ^ Value of `-fmax-worker-args`. + -- Don't unbox anything if we end up with more than this many args. } -- This is a strict alternative to (,) @@ -146,6 +155,40 @@ unforced thunks in demand or strictness information; and it is the most memory-intensive part of the compilation process, so this added seqBinds makes a big difference in peak memory usage. +Note [Don't change boxity without worker/wrapper] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Consider (T21754) + f n = n+1 + {-# NOINLINE f #-} +With `-fno-worker-wrapper`, we should not give `f` a boxity signature that says +that it unboxes its argument! Client modules would never be able to cancel away +the box for n. Likewise we shouldn't give `f` the CPR property. + +Similarly, in the last run of DmdAnal before codegen (which does not have a +worker/wrapper phase) we should not change boxity in any way. Remember: an +earlier result of the demand analyser, complete with worker/wrapper, has aleady +given a demand signature (with boxity info) to the function. +(The "last run" is mainly there to attach demanded-once info to let-bindings.) + +In general, we should not run Note [Boxity analysis] unless worker/wrapper +follows to exploit the boxity and make sure that calling modules can observe the +reported boxity. + +Hence DmdAnal is configured by a flag `dmd_do_boxity` that is True only +if worker/wrapper follows after DmdAnal. If it is not set, and the signature +is not subject to Note [Boxity for bottoming functions], DmdAnal tries +to transfer over the previous boxity to the new demand signature, in +`setIdDmdAndBoxSig`. + +Why isn't CprAnal configured with a similar flag? Because if we aren't going to +do worker/wrapper we don't run CPR analysis at all. (see GHC.Core.Opt.Pipeline) + +It might be surprising that we only try to preserve *arg* boxity, not boxity on +FVs. But FV demands won't make it into interface files anyway, so it's a waste +of energy. +Besides, W/W zaps the `DmdEnv` portion of a signature, so we don't know the old +boxity to begin with; see Note [Zapping DmdEnv after Demand Analyzer]. + Note [Analysing top-level bindings] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider a CoreProgram like @@ -257,6 +300,16 @@ setBindIdDemandInfo top_lvl id dmd = setIdDemandInfo id $ case top_lvl of TopLevel | not (isInterestingTopLevelFn id) -> topDmd _ -> dmd +-- | Update the demand signature, but be careful not to change boxity info if +-- `dmd_do_boxity` is True or if the signature is bottom. +-- See Note [Don't change boxity without worker/wrapper] +-- and Note [Boxity for bottoming functions]. +setIdDmdAndBoxSig :: DmdAnalOpts -> Id -> DmdSig -> Id +setIdDmdAndBoxSig opts id sig = setIdDmdSig id $ + if dmd_do_boxity opts || isBottomingSig sig + then sig + else transferArgBoxityDmdSig (idDmdSig id) sig + -- | Let bindings can be processed in two ways: -- Down (RHS before body) or Up (body before RHS). -- This function handles the up variant. @@ -1018,7 +1071,8 @@ dmdAnalRhsSig top_lvl rec_flag env let_dmd id rhs sig = mkDmdSigForArity threshold_arity (DmdType sig_fv final_rhs_dmds rhs_div) - final_id = id `setIdDmdSig` sig + opts = ae_opts env + final_id = setIdDmdAndBoxSig opts id sig !final_env = extendAnalEnv top_lvl env final_id sig -- See Note [Aggregated demand for cardinality] @@ -1858,8 +1912,9 @@ dmdFix :: TopLevelFlag dmdFix top_lvl env let_dmd orig_pairs = loop 1 initial_pairs where + opts = ae_opts env -- See Note [Initialising strictness] - initial_pairs | ae_virgin env = [(setIdDmdSig id botSig, rhs) | (id, rhs) <- orig_pairs ] + initial_pairs | ae_virgin env = [(setIdDmdAndBoxSig opts id botSig, rhs) | (id, rhs) <- orig_pairs ] | otherwise = orig_pairs -- If fixed-point iteration does not yield a result we use this instead |