On x86-64: (sb-bignum:%bignum-ref
#x1234567890123456789012345678901234567890 0) => #x5678901234567890. For
any other platform, you'd have to mask it down from there to
positive-fixnum or similar. All backends should support %BIGNUM-REF. Good
enough?


On Thu, Dec 3, 2015 at 9:40 AM, Christophe Rhodes <
<email address hidden>> wrote:

> Christophe Rhodes <email address hidden> writes:
>
> > Lutz Euler <email address hidden> writes:
> >
> >> In case anyone wants to investigate this: There was some discussion on
> >> #sbcl about this on 2014-02-07 and 2014-02-08.
> >
> > For this bug, we have to do generic arithmetic at some point (because
> > the argument is generic).  Does the problem boil down to the fact that
> > the out-of-line implementation of mask-signed-field is itself very
> > generic?  If it were redefined along the lines of two-arg-and, to handle
> > small-argument cases (and maybe word-sized cases) more effectively,
> > maybe some of these performance problems would go away...
>
> Proof-of-concept patch attached.  It "fixes" the logtest issue, at least
> with fixnum arguments, with a 90% speedup in the test case in this bug.
>
> It's not a complete fix.  If backends had a primitive to get the low
> N-FIXNUM-BITS of a bignum, then we could use that to improve the
> (MASK-SIGNED-FIELD [1,63] <BIGNUM>) case, which would make LOGTEST not
> cons even on BIGNUM arguments -- as it is, it will do all the
> out-of-line DPB consing.
>
> Thoughts?
>
>
>
> Christophe
>
>
> ** Patch added: "0001-better-out-of-line-MASK-SIGNED-FIELD.patch"
>
> https://bugs.launchpad.net/bugs/1277690/+attachment/4529063/+files/0001-better-out-of-line-MASK-SIGNED-FIELD.patch
>
> --
> You received this bug notification because you are subscribed to SBCL.
> https://bugs.launchpad.net/bugs/1277690
>
> Title:
>   Generic LOGTEST conses even when given fixnum arguments.
>
> Status in SBCL:
>   Triaged
>
> Bug description:
>   See this transcript (SBCL 1.1.15.33-142dc0d on Linux/x86-64):
>
>   * (defun f (g)
>       (let ((x 0))
>         (dotimes (i 100000)
>           (when (funcall g i)
>             (incf x)))
>         x))
>
>   F
>   * (let ((before (sb-ext:get-bytes-consed)))
>       (f #'(lambda (i) (logtest i 1)))
>       (- (sb-ext:get-bytes-consed) before))
>
>   12779520
>
>   This is bad because (LOGTEST X 1) is used in ODDP which is used in
>   INTEXP, so the generic ODDP and EXPT (and EVENP and surely more) on
>   fixnums all cons unnecessarily.
>
>   ROOM shows that the consing consists of bignums:
>
>   * (room)
>   […]
>     12,551,712 bytes for   647,394 bignum objects.
>   […]
>
>   This is a regression. (Before the regression the second expression
>   above returned 0 and ROOM shows no bignums). I have bisected it to
>   sbcl-1.1.8-32-ga8419eb:
>
>     a8419eb994f3b59b70cfa12e1004711a830a43fa is the first bad commit
>     commit a8419eb994f3b59b70cfa12e1004711a830a43fa
>     Author: Paul Khuong <email address hidden>
>     Date:   Fri Jun 7 18:46:25 2013 -0400
>
>     Complete cut-to-width for modular arithmetic
>
>     For each modular argument, go through the nodes that provide its
>     value and try to narrow down their bitwidth. If we fail on any and
>     the result might be too wide, splice in an explicit call to LOGAND
>     or MASK-SIGNED-FIELD. Skip that last step if the value is an argument
>     to an equivalent LOGAND or MASK-SIGNED-FIELD.
>
>     Test case by Eric Marsden.
>
>   Disassembling ODDP shows a call to TWO-ARG-AND in the SBCL versions
> before
>   this commit and a call to SB-C::MASK-SIGNED-FIELD from this commit on.
>
>   Greetings,
>
>   Lutz
>
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