New magic function for applying realWorld#

Test Plan: validate

Reviewers: goldfire, erikd, rwbarton, simonpj, austin, simonmar, hvr

Reviewed By: simonpj

Subscribers: simonmar, thomie

Differential Revision: https://phabricator.haskell.org/D1103

GHC Trac Issues: #10678

2 files changed, 31 insertions, 0 deletions

diff --git a/testsuite/tests/primops/should_run/T10678.hs b/testsuite/tests/primops/should_run/T10678.hs
new file mode 100644
index 0000000000..9019ab6345
--- /dev/null
+++ b/testsuite/tests/primops/should_run/T10678.hs
@@ -0,0 +1,22 @@
+{-# LANGUAGE MagicHash #-}
+
+import GHC.Prim
+
+main :: IO ()
+main = go 1000000# 10 (2^100)
+
+go :: Int# -> Integer -> Integer -> IO ()
+go 0# _ _ = return ()
+go n# a b = (a + b) `seq` go (n# -# 1#) a b
+{-# NOINLINE go #-}
+
+{-
+This test is based on a strategy from rwbarton relying on the inefficiency
+of `Integer` addition as defined by `integer-gmp` without `runRW#`.
+
+    When I was testing the patch interactively, I measured allocations for,
+    say, a million (large Integer) + (small Integer) additions.  If that
+    addition allocates, say, 6 words, then one can fairly reliably write the
+    program so that it will allocate between 6 million and 7 million words,
+    total.
+-}
diff --git a/testsuite/tests/primops/should_run/all.T b/testsuite/tests/primops/should_run/all.T
index b21279aa99..b0001d6d32 100644
--- a/testsuite/tests/primops/should_run/all.T
+++ b/testsuite/tests/primops/should_run/all.T
@@ -3,3 +3,12 @@ test('T7689', normal, compile_and_run, [''])
 # The test is using unboxed tuples, so omit ghci
 test('T9430', omit_ways(['ghci']), compile_and_run, [''])
 test('T10481', exit_code(1), compile_and_run, [''])
+test('T10678',
+     [stats_num_field('bytes allocated',
+                      [(wordsize(64), 88041768, 5)
+                       # 2015-11-04: 88041768 +/- 5%  (before runRW#)
+                       # 2015-11-04: 64004171         (after runRW#)
+                      ]),
+      only_ways('normal')
+     ],
+     compile_and_run, ['-O'])