[GHC] #3033: numerical error introduced by -O1

[GHC]

#3033: numerical error introduced by -O1
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 Reporter:  roland            |          Owner:                  
     Type:  bug               |         Status:  new             
 Priority:  normal            |      Milestone:                  
Component:  Compiler          |        Version:  6.10.1          
 Severity:  normal            |     Resolution:                  
 Keywords:                    |       Testcase:                  
       Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
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Comment (by guest):

 So, interestingly gcc prints non-zero with no optimization and 0 with
 optimization.

 The problem is due to the fact the floating point on the 387 compatible
 FPU stores a double in 64 bits, but internally computes with 80 bits.  So
 the first (f x) is computed and stored, then the second one is computed,
 and the first one is subtracted.  But one of the (f x) calls has 80 bits
 all the way and the other has been truncated to 64 bits and lost some
 bits.

 This is life with the x86, unless you really want to cripple performance
 and truncate after each FP operation.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3033#comment:1>
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