wanted: efficient endian swap

[Nikodemus Siivola]

Endian-swapping binary data of all sorts would be useful. On x86 there is even a specialized instruction
(not sure how CISC-slow it is, though) BSWAP.

Either in SB-EXT, or as an SB-ROTATE-BYTE-style contrib.

[Nathan Froyd (froydnj)]

I think to make this really useful/more applicable, you need a version that operates not only on bare integer values, but also loads endian-reversed data from arrays/saps. Then you can at least let PPC get into the act and you even have the option of specializing code for Atom.

[Nikodemus Siivola (nikodemus)]

;;; Here's a quick version for x86-64 as a discussion point re. API.
;;;
;;; Specifying the word-width in the API is an issue.
;;;
;;; It should also be able to byte-swap vectors of unsigned 8 using
;;; any byte-width upto, dunno, 128 bits?

(in-package :sb-c)

(defknown byte-swap (word) word
    (unsafe))

(in-package :sb-vm)

(define-vop (fast-byte-swap fast-safe-arith-op)
  (:translate sb-c::byte-swap)
  (:args (x :scs (unsigned-reg) :target res))
  (:arg-types unsigned-num)
  (:results (res :scs (unsigned-reg)))
  (:result-types unsigned-num)
  (:generator 1
   (move res x)
   (inst bswap res)))

(defun byte-swap-vector (vector)
  (declare (type (simple-array (unsigned-byte 64) (*)) vector))
  (dotimes (i (length vector))
    (setf (aref vector i) (sb-c::byte-swap (aref vector i))))
  vector)

[Nikodemus Siivola (nikodemus)]

"It should also be able to byte-swap vectors of unsigned 8 using any byte-width upto, dunno, 128 bits?"

...what didn't come out right.

What I meant was that a vector of unsigned 8 should be byte-swappable as if it was a vector of unsigned 16, 32, 64, etc.

[Stas Boukarev (stassats)]

FWIW, here is a portable swap-bytes with VOPS for x86 and x86-64 SBCL: http://github.com/stassats/swap-bytes

[Douglas Katzman (dougk)]

Maybe we could bring the code from 'nibbles' in as a contrib? Then subsequent versions of 'nibbles' can just do (REQUIRE :sb-nibbles) and call it done.