[cmucl-ticket] Re: #20: Modular arith bug?

[cmucl]

#20: Modular arith bug?
---------------------+------------------------------------------------------
  Reporter:  rtoy    |       Owner:  somebody
      Type:  defect  |      Status:  new     
  Priority:  major   |   Milestone:          
 Component:  Core    |     Version:  19e     
Resolution:          |    Keywords:          
---------------------+------------------------------------------------------
Old description:

> {{{
> (defun mat3neg (tt v)
>   (ldb (byte 32 0) (ash v (- tt))))
>
> (defun mat3neg-a (tt v)
>   (logand #xffffffff (ash v (- tt))))
>
> (defun zot (z2)
>   (declare (type (unsigned-byte 32) z2)
>            (optimize speed (safety 0)))
>   (mat3neg -28 z2))
>
> (defun zot-a (z2)
>   (declare (type (unsigned-byte 32) z2)
>            (optimize speed (safety 0)))
>   (mat3neg-a -28 z2))
> }}}
>
> Compile {{{zot}}} and there are lots of compiler notes, from
> {{{mat3neg}}}.  But compile {{{zot-a}}} and there's just one for boxing
> up the result.
>
> However, {{{zot}}} and {{{zot-a}}} are functionally identical, so
> {{{zot}}} and {{{zot-a}}} should produce the same code.  But {{{zot}}}
> has to do a full call to {{{ash}}} and {{{two-arg-and}}}.
>
> The {{{ldb}}} must be confusing the compiler somehow.

New description:

 {{{
 (declaim (inline mat3neg mat3neg-a))
 (defun mat3neg (tt v)
   (ldb (byte 32 0) (ash v (- tt))))

 (defun mat3neg-a (tt v)
   (logand #xffffffff (ash v (- tt))))

 (defun zot (z2)
   (declare (type (unsigned-byte 32) z2)
            (optimize speed (safety 0)))
   (mat3neg -28 z2))

 (defun zot-a (z2)
   (declare (type (unsigned-byte 32) z2)
            (optimize speed (safety 0)))
   (mat3neg-a -28 z2))
 }}}

 Compile {{{zot}}} and there are lots of compiler notes, from
 {{{mat3neg}}}.  But compile {{{zot-a}}} and there's just one for boxing up
 the result.

 However, {{{zot}}} and {{{zot-a}}} are functionally identical, so
 {{{zot}}} and {{{zot-a}}} should produce the same code.  But {{{zot}}} has
 to do a full call to {{{ash}}} and {{{two-arg-and}}}.

 The {{{ldb}}} must be confusing the compiler somehow.

-- 
Ticket URL: <http://trac.common-lisp.net/cmucl/ticket/20>
cmucl <http://common-lisp.net/project/cmucl>
cmucl