[GHC] #2156: compilation math/truncate bug with optimization enabled

[GHC]

#2156: compilation math/truncate bug with optimization enabled
-------------------------+--------------------------------------------------
    Reporter:  trevorm   |        Owner:       
        Type:  bug       |       Status:  new  
    Priority:  normal    |    Milestone:       
   Component:  Compiler  |      Version:  6.8.2
    Severity:  critical  |   Resolution:       
    Keywords:            |     Testcase:       
Architecture:  x86       |           Os:  Linux
-------------------------+--------------------------------------------------
Comment (by trevorm):

 On investigating further I see that the optimized code generated is
 {{{
 subl $8, %esp ; fnstcw 4(%esp)
 ffree %st(7) ; fld %st(0)
 movzwl 4(%esp), %eax ; orl $0xC00, %eax
 movl %eax, 0(%esp) ; fldcw 0(%esp)
 fistpl 0(%esp)
 fldcw 4(%esp) ; movl 0(%esp), %eax
 }}}
 which sets rounding mode to chop and then converts to integer. So the
 value 2 is correct according to the definition of truncate.

 So the optimized code behaves as if the values are Double precision, while
 the unoptimized code behaves as if they have better (infinite?) precision

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2156#comment:4>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler