How many memory the bit-vector using in sbcl?
Does per bit spend 1 bit memory? Does per bit spend 1 byte memory? Does per bit spend 1 word memory?
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2 Answers
From Common Lisp one can ask if there is a special array type for bit vectors:
* (UPGRADED-ARRAY-ELEMENT-TYPE 'bit) BIT That means that when you ask for a bit vector, then CL provides you with a bit vector and not a, say, vector with 8 bit elements.
Size of an object in SBCL
Alastair Bridgewater provided this function as an attempt to get the 'size' of an object in SBCL:
(defun get-object-size/octets (object) (sb-sys:without-gcing (nth-value 2 (sb-vm::reconstitute-object (ash (logandc1 sb-vm:lowtag-mask (sb-kernel:get-lisp-obj-address object)) (- sb-vm:n-fixnum-tag-bits)))))) * (get-object-size/octets (make-array 40 :element-type 'bit :initial-element 1)) 32 * (get-object-size/octets (make-array 400 :element-type 'bit :initial-element 1)) 80 * (get-object-size/octets (make-array 4000 :element-type 'bit :initial-element 1)) 528 1 Comment
Nicolas56
The "bug" report involving Alastair Bridgewater (bugs.launchpad.net/sbcl/+bug/1636910) seems to have led to an upgrade of SBCL:
(sb-ext:primitive-object-size (make-array 4000 :element-type 'bit :initial-element 1)) --> 528Bit vectors in SBCL are stored efficiently with one bit per bit, plus some small housekeeping overhead per vector.
They are also very efficient at bitwise operations, working a full word at a time. For example, BIT-XOR on a 64-bit platform will work on 64 bits of a bit-vector at once.
