Make last a good consumer
authorJoachim Breitner <mail@joachim-breitner.de>
Tue, 22 Jul 2014 09:21:01 +0000 (11:21 +0200)
committerJoachim Breitner <mail@joachim-breitner.de>
Tue, 22 Jul 2014 09:21:01 +0000 (11:21 +0200)
Summary:
Make last a good consumer simply by implementing it as foldl. This fixes Trac: #9339.
Thanks to David Feuer for bringing it up.

Test Plan: perf/should_run/T9339 + general validation

Reviewers: austin

Reviewed By: austin

Subscribers: phaskell, simonmar, relrod, carter

Differential Revision: https://phabricator.haskell.org/D86

Trac Issues: #9339

libraries/base/GHC/List.lhs
testsuite/tests/perf/should_run/T9339.hs [new file with mode: 0644]
testsuite/tests/perf/should_run/T9339.stdout [new file with mode: 0644]
testsuite/tests/perf/should_run/all.T

index e004ded..9b6cc2e 100644 (file)
@@ -83,11 +83,8 @@ last [x]                =  x
 last (_:xs)             =  last xs
 last []                 =  errorEmptyList "last"
 #else
--- eliminate repeated cases
-last []                 =  errorEmptyList "last"
-last (x:xs)             =  last' x xs
-  where last' y []     = y
-        last' _ (y:ys) = last' y ys
+-- use foldl to allow fusion
+last = foldl (\_ x -> x) (errorEmptyList "last")
 #endif
 
 -- | Return all the elements of a list except the last one.
diff --git a/testsuite/tests/perf/should_run/T9339.hs b/testsuite/tests/perf/should_run/T9339.hs
new file mode 100644 (file)
index 0000000..96f5f72
--- /dev/null
@@ -0,0 +1,4 @@
+-- Tests that `last` successfully fuses.
+
+main :: IO ()
+main = print $ last $ filter odd $ [1::Int ..10000000]
diff --git a/testsuite/tests/perf/should_run/T9339.stdout b/testsuite/tests/perf/should_run/T9339.stdout
new file mode 100644 (file)
index 0000000..e161ae3
--- /dev/null
@@ -0,0 +1 @@
+9999999
index a9d7c03..8b8547e 100644 (file)
@@ -379,3 +379,12 @@ test('T9203',
       only_ways(['normal'])],
      compile_and_run,
      ['-O2'])
+
+test('T9339',
+     [stats_num_field('bytes allocated',
+                      [ (wordsize(64),    80050760, 5) ]),
+                      # w/o fusing last: 320005080
+                      # 2014-07-22:       80050760
+      only_ways(['normal'])],
+     compile_and_run,
+     ['-O2'])