Deprecate -fdmd-tx-dict-sel.

It's behaviour is now unconditionally enabled as
it's slightly beneficial.

There are almost no benchmarks which benefit from
disabling it, so it's not worth the keep this
configurable.

This fixes #18429.

4 files changed, 9 insertions, 7 deletions

diff --git a/compiler/GHC/Core/Opt/DmdAnal.hs b/compiler/GHC/Core/Opt/DmdAnal.hs
index 7f4327591e..e11d220773 100644
--- a/compiler/GHC/Core/Opt/DmdAnal.hs
+++ b/compiler/GHC/Core/Opt/DmdAnal.hs
@@ -486,8 +486,9 @@ dmdTransform env var dmd
   | isDataConWorkId var
   = dmdTransformDataConSig (idArity var) dmd
   -- Dictionary component selectors
-  | gopt Opt_DmdTxDictSel (ae_dflags env),
-    Just _ <- isClassOpId_maybe var
+  -- Used to be controlled by a flag.
+  -- See #18429 for some perf measurements.
+  | Just _ <- isClassOpId_maybe var
   = dmdTransformDictSelSig (idStrictness var) dmd
   -- Imported functions
   | isGlobalId var
diff --git a/compiler/GHC/Driver/Flags.hs b/compiler/GHC/Driver/Flags.hs
index f5f642ce46..1b437c3235 100644
--- a/compiler/GHC/Driver/Flags.hs
+++ b/compiler/GHC/Driver/Flags.hs
@@ -188,7 +188,8 @@ data GeneralFlag
    | Opt_OmitYields
    | Opt_FunToThunk               -- allow GHC.Core.Opt.WorkWrap.Utils.mkWorkerArgs to remove all value lambdas
    | Opt_DictsStrict                     -- be strict in argument dictionaries
-   | Opt_DmdTxDictSel              -- use a special demand transformer for dictionary selectors
+   | Opt_DmdTxDictSel              -- ^ deprecated, no effect and behaviour is now default.
+                                   -- Allowed switching of a special demand transformer for dictionary selectors
    | Opt_Loopification                  -- See Note [Self-recursive tail calls]
    | Opt_CfgBlocklayout             -- ^ Use the cfg based block layout algorithm.
    | Opt_WeightlessBlocklayout         -- ^ Layout based on last instruction per block.
diff --git a/compiler/GHC/Driver/Session.hs b/compiler/GHC/Driver/Session.hs
index 82be1ab02e..c7fd59e1b3 100644
--- a/compiler/GHC/Driver/Session.hs
+++ b/compiler/GHC/Driver/Session.hs
@@ -3472,7 +3472,8 @@ fFlagsDeps = [
   flagSpec "diagnostics-show-caret"           Opt_DiagnosticsShowCaret,
   flagSpec "dicts-cheap"                      Opt_DictsCheap,
   flagSpec "dicts-strict"                     Opt_DictsStrict,
-  flagSpec "dmd-tx-dict-sel"                  Opt_DmdTxDictSel,
+  depFlagSpec "dmd-tx-dict-sel"
+      Opt_DmdTxDictSel "effect is now unconditionally enabled",
   flagSpec "do-eta-reduction"                 Opt_DoEtaReduction,
   flagSpec "do-lambda-eta-expansion"          Opt_DoLambdaEtaExpansion,
   flagSpec "eager-blackholing"                Opt_EagerBlackHoling,
@@ -4004,7 +4005,6 @@ optLevelFlags :: [([Int], GeneralFlag)]
 optLevelFlags -- see Note [Documenting optimisation flags]
   = [ ([0,1,2], Opt_DoLambdaEtaExpansion)
     , ([0,1,2], Opt_DoEtaReduction)       -- See Note [Eta-reduction in -O0]
-    , ([0,1,2], Opt_DmdTxDictSel)
     , ([0,1,2], Opt_LlvmTBAA)
 
     , ([0],     Opt_IgnoreInterfacePragmas)
diff --git a/docs/users_guide/using-optimisation.rst b/docs/users_guide/using-optimisation.rst
index 438560c481..4c44509f21 100644
--- a/docs/users_guide/using-optimisation.rst
+++ b/docs/users_guide/using-optimisation.rst
@@ -353,8 +353,7 @@ by saying ``-fno-wombat``.
     Make dictionaries strict.
 
 .. ghc-flag:: -fdmd-tx-dict-sel
-    :shortdesc: Use a special demand transformer for dictionary selectors.
-        Always enabled by default.
+    :shortdesc: *(deprecated)* Use a special demand transformer for dictionary selectors.
     :type: dynamic
     :reverse: -fno-dmd-tx-dict-sel
     :category:
@@ -362,6 +361,7 @@ by saying ``-fno-wombat``.
     :default: on
 
     Use a special demand transformer for dictionary selectors.
+    Behaviour is unconditionally enabled starting with 8.14
 
 .. ghc-flag:: -fdo-eta-reduction
     :shortdesc: Enable eta-reduction. Implied by :ghc-flag:`-O`.