[commit: random] master: Adding the simple speed test from ticket #427. (dfeff47)

[Ian Lynagh]

Repository : ssh://darcs.haskell.org//srv/darcs/packages/random

On branch  : master

http://hackage.haskell.org/trac/ghc/changeset/dfeff473ceefcc7e8d4b202fe154e8f869f12e67

>---------------------------------------------------------------

commit dfeff473ceefcc7e8d4b202fe154e8f869f12e67
Author: Ryan Newton <rrnewton at gmail.com>
Date:   Tue Jun 28 14:25:45 2011 -0400

    Adding the simple speed test from ticket #427.

>---------------------------------------------------------------

 tests/slowness.hs |   55 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 55 insertions(+), 0 deletions(-)

diff --git a/tests/slowness.hs b/tests/slowness.hs
new file mode 100644
index 0000000..162a4cf
--- /dev/null
+++ b/tests/slowness.hs
@@ -0,0 +1,55 @@
+
+-- http://hackage.haskell.org/trac/ghc/ticket/427
+
+-- Two (performance) problems in one:
+
+{-# OPTIONS -fffi #-}
+module Main (main) where
+
+import Control.Monad
+import System.Random
+
+foreign import ccall unsafe "random" _crandom :: IO Int
+foreign import ccall unsafe "stdlib.hs" rand :: IO Int
+
+randomInt :: (Int, Int) -> IO Int
+randomInt (min,max) = do
+--    n <- _crandom
+    n <- rand
+    return $ min + n `rem` range
+    where
+        range = max - min + 1
+
+main = replicateM_ (5*10^6) $ do
+    x <- randomRIO (0::Int,1000) :: IO Int
+--    x <- randomInt (0::Int,1000) :: IO Int
+    x `seq` return ()
+    return ()
+
+-- First, without the "seq" at the end, hardly anything is
+-- evaluated and we're building huge amounts of thunks.
+-- Three ideas about this one:
+-- - Blame the user :)
+-- - data StdGen = StdGen !Int !Int
+--   Use strict fields in StdGen. Doesn't actually help
+-- (at least in this example).
+-- - Force evaluation of the StdGen in getStdRandom.
+--   Does help in this example, but also changes behaviour
+-- of the library:
+--   x <- randomRIO undefined
+--   currently dies only when x (or the result of a later
+-- randomRIO) is evaluated. This change causes it to die
+-- immediately.
+
+-- Second, even _with_ the "seq", replacing "randomRIO" by
+-- "randomInt" speeds the thing up with a factor of about
+-- 30. (2 to 3.6, in a "real world" university practicum
+-- exercise of 900 lines of code)
+-- Even given the fact that they're not really doing the
+-- same thing, this seems rather much :(
+
+--------------------------------------------------------------------------------
+
+-- [2011.06.28] RRN:
+-- I'm currently seeing 1425 ms vs 43 ms for the above.  33X
+-- difference. If I use rand() instead it's about 52ms.