[Git][ghc/ghc][master] Add a RULE to make lookup fuse

[Marge Bot (@marge-bot)]

Marge Bot pushed to branch master at Glasgow Haskell Compiler / GHC


Commits:
74a882dc by MorrowM at 2023-08-02T06:00:03-04:00
Add a RULE to make lookup fuse

See https://github.com/haskell/core-libraries-committee/issues/175

Metric Increase:
    T18282

- - - - -


5 changed files:

- libraries/base/GHC/List.hs
- libraries/base/changelog.md
- + testsuite/tests/perf/compiler/LookupFusion.hs
- + testsuite/tests/perf/compiler/LookupFusion.stdout
- testsuite/tests/perf/compiler/all.T


Changes:

=====================================
libraries/base/GHC/List.hs
=====================================
@@ -1405,6 +1405,11 @@ lookup _key []          =  Nothing
 lookup  key ((x,y):xys)
     | key == x           =  Just y
     | otherwise         =  lookup key xys
+{-# NOINLINE [1] lookup #-} -- see Note [Fusion for lookup]
+{-# RULES
+"lookup/build" forall x (g :: forall b. ((k, a) -> b -> b) -> b -> b).
+  lookup x (build g) = g (\(k, v) r -> if x == k then Just v else r) Nothing
+#-}
 
 -- | Map a function returning a list over a list and concatenate the results.
 -- 'concatMap' can be seen as the composition of 'concat' and 'map'.
@@ -1609,6 +1614,15 @@ NB: Zips for larger tuples are in the List module.
   happens in phase 1.
 
   Ditto rule "zipWithList".
+
+Note [Fusion for lookup]
+~~~~~~~~~~~~~~~~~~~~~~~~
+Implementing lookup with foldr has the potential to cause code duplication
+if fusion doesn't occur, so we use RULES instead so that lookup can participate
+in list fusion.
+The NONINLINE pragma gives the RULE a chance to fire.
+It's recursive, so won't inline anyway, but saying so is more explicit.
+See the discussion in https://gitlab.haskell.org/ghc/ghc/-/merge_requests/10715/
 -}
 
 ----------------------------------------------


=====================================
libraries/base/changelog.md
=====================================
@@ -1,5 +1,8 @@
 # Changelog for [`base` package](http://hackage.haskell.org/package/base)
 
+## 4.20.0.0 *TBA*
+  * Add a `RULE` to `Prelude.lookup`, allowing it to participate in list fusion ([CLC proposal #174](https://github.com/haskell/core-libraries-committee/issues/175))
+
 ## 4.19.0.0 *TBA*
   * Add `{-# WARNING in "x-partial" #-}` to `Data.List.{head,tail}`.
     Use `{-# OPTIONS_GHC -Wno-x-partial #-}` to disable it.


=====================================
testsuite/tests/perf/compiler/LookupFusion.hs
=====================================
@@ -0,0 +1,6 @@
+module Main where
+
+import Data.List (iterate')
+
+main :: IO ()
+main = print $ lookup (2^20) $ iterate' (\(!k,!v) -> (k + 1, v + 2)) (0 :: Int, 0 :: Int)


=====================================
testsuite/tests/perf/compiler/LookupFusion.stdout
=====================================
@@ -0,0 +1 @@
+Just 2097152
\ No newline at end of file


=====================================
testsuite/tests/perf/compiler/all.T
=====================================
@@ -690,3 +690,8 @@ test('T22744',
      ],
      multimod_compile,
      ['T22744', '-v0'])
+
+test ('LookupFusion',
+      [collect_stats('bytes allocated',2), when(wordsize(32), skip)],
+      compile_and_run,
+      ['-O2 -package base'])



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/74a882dc0b7cd1cd28634baedd0b2b933f418a9b

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/74a882dc0b7cd1cd28634baedd0b2b933f418a9b
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