[GHC] #13212: Support abs as a primitive operation on floating point numbers.

31 Jan 2017

      #13212: Support abs as a primitive operation on floating point numbers.
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           Reporter:  dominic        |             Owner:
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Haskell differs from C and FORTRAN on the manner in which it computes the
 absolute value of floating point numbers.     Both FORTRAN and C support a
 `fabs` primitive function that is compiled directly to the underlying
 `fabs` machine instruction on either AMD64 or Intel x86  processors (with
 a small amount of stack manipulation).

 Haskell, however, does not support `abs` as a primitive operation on
 floating point numbers.   Instead, Haskell desugars `abs` to the
 following:

    abs x    | x == 0    = 0 -- handles (-0.0)
              | x >  0    = x
              | otherwise = negateFloat x

 Rather than calling the utilizing the  `fabs` mnemonic or twiddling the
 sign bit, both of which can be executed in a single instruction, this
 implementation results in ~15 machine instructions and requires ~4-5 times
 the number of clock cycles to execute.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13212
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