[cmucl-ticket] #15: x86 double-float issue

[cmucl]

#15: x86 double-float issue
--------------------+-------------------------------------------------------
 Reporter:  rtoy    |       Owner:  somebody
     Type:  defect  |      Status:  new     
 Priority:  minor   |   Milestone:          
Component:  Core    |     Version:  19d     
 Keywords:          |  
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 Consider this sample code
 {{{
 (defun rbug (z tt betain beta)
  (declare (double-float z tt betain beta)
       (optimize (speed 3) (safety 0)))
  (= (* (* (* z tt) betain) beta) z))
 }}}

 Compile it and run:
 {{{
 (rbug 5.562684646268004d-309 (1+ double-float-epsilon) .5d0 2d0)
 }}}

 This is t on sparc, nil on x86.

 This is caused by x86 issues with double-float (53-bit) precision and the
 extra range of the exponent in long-double (64-bit) format.  Denormals in
 this format are not handled the same as denormals on sparc or ppc.

 There is a fix for this.  When computing x*y, we scale x by an appropriate
 value, multiply by y, and scale back.  This will produce the correctly
 rounded denormal.  The only issue would be the exponent range.  That is
 fixed by storing the number to memory and reloaded.  (This solution taken
 from a proposed solution for Java numerics).

 This could be easily implemented, but potentially slows down double-float
 arithmetic by a factor of 2-4 times.  I don't think we want to do that.

-- 
Ticket URL: <http://trac.common-lisp.net/cmucl/ticket/15>
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