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Simon Peyton Jones pushed to branch wip/spj-try-opt-coercion at Glasgow Haskell Compiler / GHC

Commits:

82a09545
by Simon Peyton Jones at 2025-12-31T16:20:30+00:00
```
Wibble
```

8d188235

by Simon Peyton Jones at 2025-12-31T16:20:42+00:00

Try harder to keep the substitution empty

See Note [Keeping the substitution empty]

3 changed files:

compiler/GHC/Core/Opt/Simplify/Iteration.hs
compiler/GHC/Core/Subst.hs
compiler/GHC/Core/TyCo/Subst.hs

Changes:

compiler/GHC/Core/Opt/Simplify/Iteration.hs

@@ -1657,7 +1657,6 @@ simplCast env body co0 cont0
          addCoerce co1 (CastIt { sc_co = co2, sc_cont = cont })  -- See Note [Optimising reflexivity]
            = addCoerce (mkTransCo co1 co2) cont
                        -- False: (mkTransCo co1 co2) is not fully optimised
 -                      -- See Note [Avoid re-simplifying coercions]
          addCoerce co (ApplyToTy { sc_arg_ty = arg_ty, sc_cont = tail })
            | Just (arg_ty', m_co') <- pushCoTyArg co arg_ty

compiler/GHC/Core/Subst.hs

@@ -380,8 +380,10 @@ substIdBndr _doc rec_subst subst@(Subst in_scope env tvs cvs) old_id
      old_ty = idType old_id
      old_w = idMult old_id
 -    no_type_change = (isEmptyVarEnv tvs && isEmptyVarEnv cvs) ||
 +    no_type_change = isEmptyTCvSubst subst ||
                       (noFreeVarsOfType old_ty && noFreeVarsOfType old_w)
 +                     -- isEmptyTCvSubst: see Note [Keeping the substitution empty]
 +                     --                  in GHC.Core.TyCo.Subst
          -- new_id has the right IdInfo
          -- The lazy-set is because we're in a loop here, with

compiler/GHC/Core/TyCo/Subst.hs

@@ -960,7 +960,8 @@ substTyVarBndrUsing subst_fn subst@(Subst in_scope idenv tenv cenv) old_var
      -- Assertion check that we are not capturing something in the substitution
      old_ki = tyVarKind old_var
 -    no_kind_change = noFreeVarsOfType old_ki -- verify that kind is closed
 +    no_kind_change = isEmptyTCvSubst subst || noFreeVarsOfType old_ki
 +                     -- isEmptyTCvSubst: see Note [Keeping the substitution empty]
      no_change = no_kind_change && (new_var == old_var)
          -- no_change means that the new_var is identical in
          -- all respects to the old_var (same unique, same kind)
@@ -988,7 +989,8 @@ substCoVarBndrUsing subst_fn subst@(Subst in_scope idenv tenv cenv) old_var
      (Subst (in_scope `extendInScopeSet` new_var) idenv tenv new_cenv, new_var)
    where
      new_co         = mkCoVarCo new_var
 -    no_kind_change = noFreeVarsOfTypes [t1, t2]
 +    no_kind_change = isEmptyTCvSubst subst || noFreeVarsOfTypes [t1, t2]
 +                     -- isEmptyTCvSubst: see Note [Keeping the substitution empty]
      no_change      = new_var == old_var && no_kind_change
      new_cenv | no_change = delVarEnv cenv old_var
@@ -1034,3 +1036,22 @@ substTyCoBndr subst (Anon ty af)          = (subst, Anon (substScaledTy subst ty
  substTyCoBndr subst (Named (Bndr tv vis)) = (subst', Named (Bndr tv' vis))
                                            where
                                              (subst', tv') = substVarBndr subst tv
++
 +{- Note [Keeping the substitution empty]
 +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 +A very common situation is where we run over a term doing no cloning,
 +no substitution, nothing.  In that case the TCvSubst till be empty, and
 +it is /very/ valuable to /keep/ it empty:
++
 +* It's wasted effort to build up an identity substitution mapping
 +  [x:->x, y:->y].
++
 +* When we come to a binder, if the incoming substitution is empty,
 +  we can avoid substituting its type; and that in turn may mean that
 +  the binder itself does not change and we don't need to extend the
 +  substitution.
++
 +* In the Simplifier we substitute over both types and coercions.
 +  If the substitution is empty, this is a no-op -- but only if it
 +  is empty!
 +-}