[GHC] #5165: GHC doesn't optimize FP excess precision properly

[GHC]

#5165: GHC doesn't optimize FP excess precision properly
-----------------------+----------------------------------------------------
    Reporter:  ezyang  |       Owner:                             
        Type:  bug     |      Status:  new                        
    Priority:  normal  |   Component:  Compiler                   
     Version:  7.1     |    Keywords:                             
    Testcase:          |   Blockedby:                             
          Os:  Linux   |    Blocking:                             
Architecture:  x86     |     Failure:  Incorrect result at runtime
-----------------------+----------------------------------------------------
 jmcarthur helped in constructing the test case. Consider the following
 program:

 {{{
 module Main where

 {-# NOINLINE x #-}
 {-# NOINLINE y #-}
 x, y :: Double
 x = 3
 y = 7

 main = print $ x/y == (3/7 :: Double)
 }}}

 When run on 32-bit Linux, we get the following results:

 {{{
 ezyang at javelin:~$ ghc --version
 The Glorious Glasgow Haskell Compilation System, version 7.0.3
 ezyang at javelin:~$ ghc --make Bar.hs -o Bar -O0 -fforce-recomp && ./Bar
 [1 of 1] Compiling Main             ( Bar.hs, Bar.o )
 Linking Bar ...
 True
 ezyang at javelin:~$ ghc --make Bar.hs -o Bar -O -fforce-recomp && ./Bar
 [1 of 1] Compiling Main             ( Bar.hs, Bar.o )
 Linking Bar ...
 False
 }}}

 The second result should be True (and only false if `-fexcess-precision`
 is specified). I also tested on a devel2 build of GHC HEAD and got similar
 results.

 This appears to be a regression from 6.12.1:

 {{{
 ezyang at javelin:~$ /usr/bin/ghc --version
 The Glorious Glasgow Haskell Compilation System, version 6.12.1
 ezyang at javelin:~$ /usr/bin/ghc --make Bar.hs -o Bar -O && ./Bar
 True
 }}}

 I also kind of suspect `-fexcess-precision` doesn't do anything for the
 native codegen (based on source code surfing), but it's hard to tell with
 this current bug.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5165>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler