[Git][ghc/ghc][wip/T18502] Add two bangs to improve perf of flattening

[Simon Peyton Jones]

Simon Peyton Jones pushed to branch wip/T18502 at Glasgow Haskell Compiler / GHC


Commits:
65ef562c by Simon Peyton Jones at 2020-07-27T14:05:53+01:00
Add two bangs to improve perf of flattening

This tiny patch improves the compile time of flatten-heavy
programs by 1-2%, by adding two bangs.

Addresses (somewhat) #18502

This reduces allocation by
   T9872b   -1.1%
   T9872d   -3.3%

   T5321Fun -0.2%
   T5631    -0.2%
   T5837    +0.1%
   T6048    +0.1%

Metric Decrease:
    T9872b
    T9872d

- - - - -


1 changed file:

- compiler/GHC/Core/Coercion.hs


Changes:

=====================================
compiler/GHC/Core/Coercion.hs
=====================================
@@ -1891,7 +1891,9 @@ substForAllCoBndrUsingLC sym sco (LC subst lc_env) tv co
 --
 --   For the inverse operation, see 'liftCoMatch'
 ty_co_subst :: LiftingContext -> Role -> Type -> Coercion
-ty_co_subst lc role ty
+ty_co_subst !lc role ty
+    -- !lc: making this function strict in lc allows callers to
+    -- pass its two components separately, rather than boxing them
   = go role ty
   where
     go :: Role -> Type -> Coercion
@@ -2864,9 +2866,9 @@ simplifyArgsWorker orig_ki_binders orig_inner_ki orig_fvs
          -- need a coercion (kind_co :: old_kind ~ new_kind).
          --
          -- The bangs here have been observed to improve performance
-         -- significantly in optimized builds.
-         let kind_co = mkSymCo $
-               liftCoSubst Nominal lc (tyCoBinderType binder)
+         -- significantly in optimized builds; see #18502
+         let !kind_co = mkSymCo $
+                        liftCoSubst Nominal lc (tyCoBinderType binder)
              !casted_xi = xi `mkCastTy` kind_co
              casted_co =  mkCoherenceLeftCo role xi kind_co co
 



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/65ef562cf75fe22b2437af994ec0c8610a85e34b

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/65ef562cf75fe22b2437af994ec0c8610a85e34b
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