[GHC] #1819: x86 native codegen implements float2int# incorrectly

[GHC]

#1819: x86 native codegen implements float2int# incorrectly
-------------------------------+--------------------------------------------
    Reporter:  simonmar        |       Owner:                            
        Type:  bug             |      Status:  new                       
    Priority:  normal          |   Milestone:  6.8 branch                
   Component:  Compiler (NCG)  |     Version:  6.8.1                     
    Severity:  normal          |    Keywords:                            
  Difficulty:  Unknown         |    Testcase:  arith005 (see description)
Architecture:  x86             |          Os:  Unknown                   
-------------------------------+--------------------------------------------
 `float2Int#` is supposed to truncate towards zero, but the x86 native
 codegen implements it as rounding (or rather, whatever rounding mode the
 x87 FPU is set to, which is normally rounding).  The right thing appears
 to be to save/restore the x87 control word around the conversion
 operation, and temporarily set it to truncation.  This is what gcc does.
 Code generated by gcc for truncation:

 {{{
         fnstcw  -2(%ebp)
         movw    -2(%ebp), %ax
         movb    $12, %ah
         movw    %ax, -4(%ebp)
         flds    8(%ebp)
         fldcw   -4(%ebp)
         fistpl  -8(%ebp)
         fldcw   -2(%ebp)
         movl    -8(%ebp), %eax
 }}}

 Test case:

 {{{
 import GHC.Exts
 main = print (f2i (-2.9))

 {-# NOINLINE f2i #-}
 f2i :: Float -> Int
 f2i (F# x) = I# (float2Int# x)
 }}}

 Also `arith005` triggers this, when -O is on and the libraries were
 compiled with `-fasm`.

 Related: #1254

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1819>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler