[GHC] #8195: Different floating point results with -msse2 on 32bit Linux

29 Aug 2013

      #8195: Different floating point results with -msse2 on 32bit Linux
--------------------------+------------------------------------------------
       Reporter:          |             Owner:
  jstolarek               |            Status:  new
           Type:  bug     |         Milestone:
       Priority:  normal  |           Version:  7.7
      Component:          |  Operating System:  Linux
  Compiler (NCG)          |   Type of failure:  None/Unknown
       Keywords:          |         Test Case:  perf/should_run/Conversions
   Architecture:  x86     |          Blocking:
     Difficulty:          |
  Unknown                 |
     Blocked By:          |
Related Tickets:          |
--------------------------+------------------------------------------------
 I noticed strange behaviour of `Conversions` test on 32bit machines.
 Here's the code of this test:

 {{{
 {-# LANGUAGE BangPatterns #-}

 -- | Tests that conversions between various primitive types (e.g.
 -- Word, Double, etc) doesn't allocate.
 module Main (main) where

 import Data.Word

 -- Repeatedly convert Words to Doubles
 loop :: Floating a => Word -> a
 loop n = go 0 0.0
   where
     go i !acc | i < n = go (i+1) (acc + fromIntegral i)
               | otherwise = acc
 {-# SPECIALISE loop :: Word -> Float #-}
 {-# SPECIALISE loop :: Word -> Double #-}

 main :: IO ()
 main = do
     print (loop 1000000 :: Float)
     print (loop 1000000 :: Double)
 }}}
 This test is expected to produce:
 {{{
 [t-jastol@cam-05-unx : /5playpen/t-jastol/ghc-validate] inplace/bin/ghc-
 stage2 -O -fforce-recomp testsuite/tests/perf/should_run/Conversions.hs
 [1 of 1] Compiling Main             (
 testsuite/tests/perf/should_run/Conversions.hs,
 testsuite/tests/perf/should_run/Conversions.o )
 Linking testsuite/tests/perf/should_run/Conversions ...
 [t-jastol@cam-05-unx : /5playpen/t-jastol/ghc-validate]
 ./testsuite/tests/perf/should_run/Conversions
 4.9994036e11
 4.999995e11
 }}}
 My optimization of self-recursive tail loops
 (d61c3ac186c94021c851f7a2a6d20631e35fc1ba) broke that and now both `Float`
 and `Double` results are identical:
 {{{
 4.999995e11
 4.999995e11
 }}}
 I believe my optimisation shouldn't affect this, but clearly it does.

 Now here's the interesting part: if I generate SSE assembly by adding
 `-msse2` flag, then I get expected results:
 {{{
 4.9994036e11
 4.999995e11
 }}}
 I'm not sure if we should get different results for different instruction
 sets. I'm putting it up as a ticket so perhaps we could investigate this
 more one day if we consider it relevant.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8195
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