[Git][ghc/ghc][wip/simplifier-tweaks] 2 commits: Wibbles

[Simon Peyton Jones (@simonpj)]

Simon Peyton Jones pushed to branch wip/simplifier-tweaks at Glasgow Haskell Compiler / GHC


Commits:
12609f12 by Simon Peyton Jones at 2023-10-10T16:26:33+01:00
Wibbles

- - - - -
d5d52467 by Simon Peyton Jones at 2023-10-10T16:26:42+01:00
Tickish comment

- - - - -


3 changed files:

- compiler/GHC/Core/Opt/OccurAnal.hs
- compiler/GHC/Tc/Solver/Equality.hs
- compiler/GHC/Types/Tickish.hs


Changes:

=====================================
compiler/GHC/Core/Opt/OccurAnal.hs
=====================================
@@ -2490,6 +2490,20 @@ But it is not necessary to gather CoVars from the types of other binders.
 
 * For case-alt binders, if the type mentions a CoVar, so will the scrutinee
   (since it has the same type)
+
+Note [Simplifying ticks]
+~~~~~~~~~~~~~~~~~~~~~~~~
+Here's a worry I tripped over, but have not actioned (#24078). Consider
+   f (Tick t e)
+where f is strict.  According to GHC.Core.Opt.Simplify.Iteraion.simplTick,
+if (t `tickishScopesLike` SoftScope) we will float the tick to the outside:
+   Tick t (f e)
+Fine for counting ticks, not for scoped ticks.
+
+Now suppose `f` is a join point `$j`. That should not lose the tail-call-ness.
+We care just counting t.  But here we zap tail calls precisely for
+   (t `tickishScopesLike` SoftScope)
+To me it looks as if we should zap with precisely the negation!
 -}
 
 occAnal env (Tick tickish body)
@@ -2499,7 +2513,7 @@ occAnal env (Tick tickish body)
                   -- If we drop a tick due to the issues described below it's
                   -- not the end of the world.
 
-  | tickish `tickishScopesLike` SoftScope
+  | tickish `tickishScopesLike` SoftScope  -- Worry: see Note [Simplifying ticks]
   = WUD (markAllNonTail usage) (Tick tickish body')
 
   | Breakpoint _ _ ids _ <- tickish
@@ -3827,7 +3841,8 @@ tagRecBinders lvl body_uds details_s
        , AlwaysTailCalled arity <- lookupTailCallInfo unadj_uds bndr
        = JoinPoint arity
        | otherwise
-       = assert (not will_be_joins) -- Should be AlwaysTailCalled if
+       = warnPprTrace (will_be_joins) "tagRecBinders" (ppr bndr $$ ppr bndrs) $
+                                    -- Should be AlwaysTailCalled if
          NotJoinPoint               -- we are making join points!
 
      -- 2. Adjust usage details of each RHS, taking into account the


=====================================
compiler/GHC/Tc/Solver/Equality.hs
=====================================
@@ -49,7 +49,6 @@ import GHC.Utils.Outputable
 import GHC.Utils.Panic
 import GHC.Utils.Misc
 import GHC.Utils.Monad
-import GHC.Utils.Constants( debugIsOn )
 
 import GHC.Data.Pair
 import GHC.Data.Bag
@@ -843,13 +842,6 @@ canTyConApp ev eq_rel both_generative (ty1,tc1,tys1) (ty2,tc2,tys2)
     then canEqHardFailure ev ty1 ty2
     else canEqSoftFailure ev eq_rel ty1 ty2
   where
-    -- Reconstruct the types for error messages. This would do
-    -- the wrong thing (from a pretty printing point of view)
-    -- for functions, because we've lost the FunTyFlag; but
-    -- in fact we never call canTyConApp on a saturated FunTyCon
-    ty1 = mkTyConApp tc1 tys1
-    ty2 = mkTyConApp tc2 tys2
-
      -- See Note [Decomposing TyConApp equalities]
      -- and Note [Decomposing newtype equalities]
     can_decompose inerts


=====================================
compiler/GHC/Types/Tickish.hs
=====================================
@@ -106,9 +106,11 @@ data GenTickish pass =
     -- the user.
     ProfNote {
       profNoteCC    :: CostCentre, -- ^ the cost centre
+
       profNoteCount :: !Bool,      -- ^ bump the entry count?
       profNoteScope :: !Bool       -- ^ scopes over the enclosed expression
                                    -- (i.e. not just a tick)
+      -- Invariant: the False/False case never happens
     }
 
   -- | A "tick" used by HPC to track the execution of each



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/735951ddd7b86a8c5979ffea394797d49314c0fc...d5d5246735d309504863bfc1066b40e2beda2a62

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/735951ddd7b86a8c5979ffea394797d49314c0fc...d5d5246735d309504863bfc1066b40e2beda2a62
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