[Git][ghc/ghc][wip/enable-poly-spec] compiler: Enable polymorphic-specialisation

11 Aug 2025


      Ben Gamari pushed to branch wip/enable-poly-spec at Glasgow Haskell Compiler / GHC


Commits:
aeaae131 by Ben Gamari at 2025-08-11T18:32:31-04:00
compiler: Enable polymorphic-specialisation

The remaining hazards to soundness should be addressed
by #23109.

- - - - -


4 changed files:

- compiler/GHC/Driver/DynFlags.hs
- compiler/GHC/Driver/Flags.hs
- docs/users_guide/using-optimisation.rst
- testsuite/tests/simplCore/should_compile/T8331.stderr


Changes:

=====================================
compiler/GHC/Driver/DynFlags.hs
=====================================
@@ -1250,6 +1250,7 @@ optLevelFlags -- see Note [Documenting optimisation flags]
     , ([1,2],   Opt_CfgBlocklayout)      -- Experimental
 
     , ([1,2],   Opt_Specialise)
+    , ([1,2],   Opt_PolymorphicSpecialisation)
     , ([1,2],   Opt_CrossModuleSpecialise)
     , ([1,2],   Opt_InlineGenerics)
     , ([1,2],   Opt_Strictness)


=====================================
compiler/GHC/Driver/Flags.hs
=====================================
@@ -907,6 +907,7 @@ optimisationFlags = EnumSet.fromList
    , Opt_SpecialiseAggressively
    , Opt_CrossModuleSpecialise
    , Opt_StaticArgumentTransformation
+   , Opt_PolymorphicSpecialisation
    , Opt_CSE
    , Opt_StgCSE
    , Opt_StgLiftLams


=====================================
docs/users_guide/using-optimisation.rst
=====================================
@@ -1325,10 +1325,7 @@ as such you shouldn't need to set any of them explicitly. A flag
     :reverse: -fno-polymorphic-specialisation
     :category:
 
-    :default: off
-
-    Warning, this feature is highly experimental and may lead to incorrect runtime
-    results. Use at your own risk (:ghc-ticket:`23469`, :ghc-ticket:`23109`, :ghc-ticket:`21229`, :ghc-ticket:`23445`).
+    :default: on
 
     Enable specialisation of function calls to known dictionaries with free type variables.
     The created specialisation will abstract over the type variables free in the dictionary.


=====================================
testsuite/tests/simplCore/should_compile/T8331.stderr
=====================================
@@ -1,5 +1,148 @@
 
 ==================== Tidy Core rules ====================
+"SPEC $c*> @(ST s) @_"
+    forall (@s) (@r) ($dApplicative :: Applicative (ST s)).
+      $fApplicativeReaderT_$c*> @(ST s) @r $dApplicative
+      = ($fApplicativeReaderT2 @s @r)
+        `cast` (forall (a :: <*>_N) (b :: <*>_N).
+                <ReaderT r (ST s) a>_R
+                %<'Many>_N ->_R <ReaderT r (ST s) b>_R
+                %<'Many>_N ->_R <r>_R %<'Many>_N ->_R Sym (N:ST <s>_N <b>_R)
+                                ; Sym (N:ReaderT <*>_N <r>_R <ST s>_R <b>_N)
+                :: Coercible
+                     (forall a b.
+                      ReaderT r (ST s) a -> ReaderT r (ST s) b -> r -> STRep s b)
+                     (forall a b.
+                      ReaderT r (ST s) a -> ReaderT r (ST s) b -> ReaderT r (ST s) b))
+"SPEC $c<$ @(ST s) @_"
+    forall (@s) (@r) ($dFunctor :: Functor (ST s)).
+      $fFunctorReaderT_$c<$ @(ST s) @r $dFunctor
+      = ($fApplicativeReaderT6 @s @r)
+        `cast` (forall (a :: <*>_N) (b :: <*>_N).
+                <a>_R
+                %<'Many>_N ->_R <ReaderT r (ST s) b>_R
+                %<'Many>_N ->_R <r>_R %<'Many>_N ->_R Sym (N:ST <s>_N <a>_R)
+                                ; Sym (N:ReaderT <*>_N <r>_R <ST s>_R <a>_N)
+                :: Coercible
+                     (forall a b. a -> ReaderT r (ST s) b -> r -> STRep s a)
+                     (forall a b. a -> ReaderT r (ST s) b -> ReaderT r (ST s) a))
+"SPEC $c<* @(ST s) @_"
+    forall (@s) (@r) ($dApplicative :: Applicative (ST s)).
+      $fApplicativeReaderT_$c<* @(ST s) @r $dApplicative
+      = ($fApplicativeReaderT1 @s @r)
+        `cast` (forall (a :: <*>_N) (b :: <*>_N).
+                <ReaderT r (ST s) a>_R
+                %<'Many>_N ->_R <ReaderT r (ST s) b>_R
+                %<'Many>_N ->_R <r>_R %<'Many>_N ->_R Sym (N:ST <s>_N <a>_R)
+                                ; Sym (N:ReaderT <*>_N <r>_R <ST s>_R <a>_N)
+                :: Coercible
+                     (forall a b.
+                      ReaderT r (ST s) a -> ReaderT r (ST s) b -> r -> STRep s a)
+                     (forall a b.
+                      ReaderT r (ST s) a -> ReaderT r (ST s) b -> ReaderT r (ST s) a))
+"SPEC $c<*> @(ST s) @_"
+    forall (@s) (@r) ($dApplicative :: Applicative (ST s)).
+      $fApplicativeReaderT9 @(ST s) @r $dApplicative
+      = ($fApplicativeReaderT4 @s @r)
+        `cast` (forall (a :: <*>_N) (b :: <*>_N).
+                <ReaderT r (ST s) (a -> b)>_R
+                %<'Many>_N ->_R <ReaderT r (ST s) a>_R
+                %<'Many>_N ->_R <r>_R
+                %<'Many>_N ->_R Sym (N:ST <s>_N <b>_R)
+                :: Coercible
+                     (forall a b.
+                      ReaderT r (ST s) (a -> b) -> ReaderT r (ST s) a -> r -> STRep s b)
+                     (forall a b.
+                      ReaderT r (ST s) (a -> b) -> ReaderT r (ST s) a -> r -> ST s b))
+"SPEC $c>> @(ST s) @_"
+    forall (@s) (@r) ($dMonad :: Monad (ST s)).
+      $fMonadReaderT1 @(ST s) @r $dMonad
+      = $fMonadAbstractIOSTReaderT_$s$c>> @s @r
+"SPEC $c>>= @(ST s) @_"
+    forall (@s) (@r) ($dMonad :: Monad (ST s)).
+      $fMonadReaderT2 @(ST s) @r $dMonad
+      = ($fMonadAbstractIOSTReaderT2 @s @r)
+        `cast` (forall (a :: <*>_N) (b :: <*>_N).
+                <ReaderT r (ST s) a>_R
+                %<'Many>_N ->_R <a -> ReaderT r (ST s) b>_R
+                %<'Many>_N ->_R <r>_R
+                %<'Many>_N ->_R Sym (N:ST <s>_N <b>_R)
+                :: Coercible
+                     (forall a b.
+                      ReaderT r (ST s) a -> (a -> ReaderT r (ST s) b) -> r -> STRep s b)
+                     (forall a b.
+                      ReaderT r (ST s) a -> (a -> ReaderT r (ST s) b) -> r -> ST s b))
+"SPEC $cfmap @(ST s) @_"
+    forall (@s) (@r) ($dFunctor :: Functor (ST s)).
+      $fFunctorReaderT_$cfmap @(ST s) @r $dFunctor
+      = ($fApplicativeReaderT7 @s @r)
+        `cast` (forall (a :: <*>_N) (b :: <*>_N).
+                <a -> b>_R
+                %<'Many>_N ->_R <ReaderT r (ST s) a>_R
+                %<'Many>_N ->_R <r>_R %<'Many>_N ->_R Sym (N:ST <s>_N <b>_R)
+                                ; Sym (N:ReaderT <*>_N <r>_R <ST s>_R <b>_N)
+                :: Coercible
+                     (forall a b. (a -> b) -> ReaderT r (ST s) a -> r -> STRep s b)
+                     (forall a b. (a -> b) -> ReaderT r (ST s) a -> ReaderT r (ST s) b))
+"SPEC $cliftA2 @(ST s) @_"
+    forall (@s) (@r) ($dApplicative :: Applicative (ST s)).
+      $fApplicativeReaderT_$cliftA2 @(ST s) @r $dApplicative
+      = ($fApplicativeReaderT3 @s @r)
+        `cast` (forall (a :: <*>_N) (b :: <*>_N) (c :: <*>_N).
+                <a -> b -> c>_R
+                %<'Many>_N ->_R <ReaderT r (ST s) a>_R
+                %<'Many>_N ->_R <ReaderT r (ST s) b>_R
+                %<'Many>_N ->_R <r>_R %<'Many>_N ->_R Sym (N:ST <s>_N <c>_R)
+                                ; Sym (N:ReaderT <*>_N <r>_R <ST s>_R <c>_N)
+                :: Coercible
+                     (forall a b c.
+                      (a -> b -> c)
+                      -> ReaderT r (ST s) a -> ReaderT r (ST s) b -> r -> STRep s c)
+                     (forall a b c.
+                      (a -> b -> c)
+                      -> ReaderT r (ST s) a -> ReaderT r (ST s) b -> ReaderT r (ST s) c))
+"SPEC $cp1Applicative @(ST s) @_"
+    forall (@s) (@r) ($dApplicative :: Applicative (ST s)).
+      $fApplicativeReaderT_$cp1Applicative @(ST s) @r $dApplicative
+      = $fApplicativeReaderT_$s$fFunctorReaderT @s @r
+"SPEC $cp1Monad @(ST s) @_"
+    forall (@s) (@r) ($dMonad :: Monad (ST s)).
+      $fMonadReaderT_$cp1Monad @(ST s) @r $dMonad
+      = $fApplicativeReaderT_$s$fApplicativeReaderT @s @r
+"SPEC $cpure @(ST s) @_"
+    forall (@s) (@r) ($dApplicative :: Applicative (ST s)).
+      $fApplicativeReaderT_$cpure @(ST s) @r $dApplicative
+      = ($fApplicativeReaderT5 @s @r)
+        `cast` (forall (a :: <*>_N).
+                <a>_R
+                %<'Many>_N ->_R <r>_R %<'Many>_N ->_R Sym (N:ST <s>_N <a>_R)
+                                ; Sym (N:ReaderT <*>_N <r>_R <ST s>_R <a>_N)
+                :: Coercible
+                     (forall a. a -> r -> STRep s a)
+                     (forall a. a -> ReaderT r (ST s) a))
+"SPEC $creturn @(ST s) @_"
+    forall (@s) (@r) ($dMonad :: Monad (ST s)).
+      $fMonadReaderT_$creturn @(ST s) @r $dMonad
+      = ($fApplicativeReaderT5 @s @r)
+        `cast` (forall (a :: <*>_N).
+                <a>_R
+                %<'Many>_N ->_R <r>_R %<'Many>_N ->_R Sym (N:ST <s>_N <a>_R)
+                                ; Sym (N:ReaderT <*>_N <r>_R <ST s>_R <a>_N)
+                :: Coercible
+                     (forall a. a -> r -> STRep s a)
+                     (forall a. a -> ReaderT r (ST s) a))
+"SPEC $fApplicativeReaderT @(ST s) @_"
+    forall (@s) (@r) ($dApplicative :: Applicative (ST s)).
+      $fApplicativeReaderT @(ST s) @r $dApplicative
+      = $fApplicativeReaderT_$s$fApplicativeReaderT @s @r
+"SPEC $fFunctorReaderT @(ST s) @_"
+    forall (@s) (@r) ($dFunctor :: Functor (ST s)).
+      $fFunctorReaderT @(ST s) @r $dFunctor
+      = $fApplicativeReaderT_$s$fFunctorReaderT @s @r
+"SPEC $fMonadReaderT @(ST s) @_"
+    forall (@s) (@r) ($dMonad :: Monad (ST s)).
+      $fMonadReaderT @(ST s) @r $dMonad
+      = $fMonadAbstractIOSTReaderT_$s$fMonadReaderT @s @r
 "USPEC useAbstractMonad @(ReaderT Int (ST s))"
     forall (@s)
            ($dMonadAbstractIOST :: MonadAbstractIOST (ReaderT Int (ST s))).



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/aeaae131605ad98f24ab666df471cd59...

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View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/aeaae131605ad98f24ab666df471cd59...
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Ben Gamari (＠bgamari)

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