more fun

[Miles Bader <miles@cogsci.edinburgh.ac.uk>]

(defconstant max-byte (ash 1 vm:byte-bits))

(deftype byte ()
  `(unsigned-byte ,vm:byte-bits))

(defconstant bit-reversed-byte-table
  (make-array max-byte
	      :element-type 'byte
	      :initial-contents (loop for n from 0 to (1- max-byte) collecting
				      (%reverse-fixnum-bits n vm:byte-bits))))

(defun reverse-byte-bits (byte)
  (declare (type byte byte) (optimize (speed 3)))
  (aref bit-reversed-byte-table byte))

* (TYPE-OF BIT-REVERSED-BYTE-TABLE)

(SIMPLE-ARRAY (UNSIGNED-BYTE 8) (256))
* (DISASSEMBLE 'REVERSE-BYTE-BITS)

0B307778:       .ENTRY "Top-Level Form"()    ; FUNCTION
	...
     7B0:       .ENTRY REVERSE-BYTE-BITS(byte) ; (FUNCTION ((UNSIGNED-BYTE ..)))
	...
     7DC:       TADDCCTV %ZERO, %A0
     7E0:       MOVE   %OCFP, %NL1
     7E4:       LD     [%CODE+17], %A1       ; #(0 128 64 192 32 ...)
						[Why did it load this?]
     7E8:       ADD    %ZERO, 1024, %NL0
     7EC:       CMP    %A0, %NL0
     7F0:       BGEU   L1
     7F4:       NOP
     7F8:       LD     [%CODE+17], %A1       ; #(0 128 64 192 32 ...)
     7FC:       SRL    %A0, 2, %CFUNC
     800:       ADD    1, %CFUNC
     804:       LDUB   [%A1+%CFUNC], %NL0
     808:       MOVE   %NL0, %CFUNC
     80C:       SRA    %CFUNC, 29, %NL0	 	[So if BIT... is a constant,
						 with elements of type
						 (unsigned byte 8), why does
						 it bother?]
     810:       CMP    %NL0, 0
     814:       BEQ    L2
	... etc (non-fixnum integer allocation)
     870: L0:   UNIMP  10                    ; Error trap
	...
     87C: L1:   UNIMP  10                    ; Error trap
	...
NIL
* 

I hope I'm not being *too* annoying...  I suppose a lot of these things
you're already aware of...

-Miles

--
Miles Bader  --  HCRC, University of Edinburgh  --  Miles.Bader@ed.ac.uk
99% of everything is grunge