Simon Peyton Jones pushed to branch wip/T23559 at Glasgow Haskell Compiler / GHC

Commits:

• 89a3718e
  by Simon Peyton Jones at 2025-11-26T14:59:23+00:00

  Switch off specialisation in ExactPrint
  
  In !15057 (where we re-introduced -fpolymoprhic-specialisation) we found
  that ExactPrint's compile time blew up by a factor of 5.  It turned out
  to be caused by bazillions of specialisations of `markAnnotated`.
  
  Since ExactPrint isn't perf-critical, it does not seem worth taking
  the performance hit, so this patch switches off specialisation in
  this one module.
  

• 85b1977f
  by Simon Peyton Jones at 2025-11-26T14:59:32+00:00

  Switch -fpolymorphic-specialisation on by default
  
  This patch addresses #23559.
  
  Now that !10479 has landed and #26329 is fixed, we can switch on
  polymorphic specialisation by default, addressing a bunch of other
  tickets listed in #23559.
  
  Metric changes:
    * CoOpt_Singleton: +4% compiler allocations: we just get more
      specialisations
  
    * info_table_map_perf: -20% decrease in compiler allocations.
      This is caused by using -fno-specialise in ExactPrint.hs
      Without that change we get a 4x blow-up in compile time;
      see !15058 for details
  
  Metric Decrease:
      info_table_map_perf
  Metric Increase:
      CoOpt_Singletons
  

4 changed files:

• compiler/GHC/Driver/DynFlags.hs
• compiler/GHC/Driver/Flags.hs
• docs/users_guide/using-optimisation.rst
• utils/check-exact/ExactPrint.hs

Changes:

     , ([1,2],   Opt_CfgBlocklayout)      -- Experimental

     , ([1,2],   Opt_Specialise)

+    , ([1,2],   Opt_PolymorphicSpecialisation)  -- Now on by default (#23559)

     , ([1,2],   Opt_CrossModuleSpecialise)

     , ([1,2],   Opt_InlineGenerics)

     , ([1,2],   Opt_Strictness)

    , Opt_SpecialiseAggressively

    , Opt_CrossModuleSpecialise

    , Opt_StaticArgumentTransformation

+   , Opt_PolymorphicSpecialisation

    , Opt_CSE

    , Opt_StgCSE

    , Opt_StgLiftLams

     :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.

 {-# LANGUAGE UndecidableInstances  #-} -- For the (StmtLR GhcPs GhcPs (LocatedA (body GhcPs))) ExactPrint instance

 {-# OPTIONS_GHC -Wno-incomplete-uni-patterns -Wno-incomplete-record-updates #-}

+-- We switch off specialisation in this module. Otherwise we get lots of functions

+-- specialised on lots of (GHC syntax tree) data types.  Compilation time allocation

+-- (at least with -fpolymorphic-specialisation; see !15058) blows up from 17G to 108G.

+-- Bad! ExactPrint is not a performance-critical module so it's not worth taking the

+-- largely-fruitless hit in compile time.

+{-# OPTIONS_GHC -fno-specialise #-}

+

 module ExactPrint

   (

     ExactPrint(..)