In the following program, removing the line
(declare (type (simple-array bit) arr))
makes running time increase by more than a factor of 3, using SBCL. The type information given in the defclass macro via :type on the other hand appears to have no impact on performance.
(defclass class-1 () ((arr :type (simple-array bit))))
(defun sample (inst)
(declare (type class-1 inst))
(let ((arr (slot-value inst 'arr)))
(declare (type (simple-array bit) arr)) ;; 3x running time without
(map-into arr #'(lambda (dummy) (if (< (random 1.0) 0.5) 0 1)) arr)))
(let ((inst (make-instance 'class-1)))
(setf (slot-value inst 'arr) (make-array 10000 :element-type 'bit))
(loop for i from 1 to 10000 do (sample inst)))
How can I have the same performance benefit without having to declare the arr slot a simple-array bit each time I use it? The latter is particularly annoying since (as far as I have found out) is requires to introduce a binding via let or similar each time; I cannot just write (slot-value inst 'arr) in the place where I need it.
First of all, this is an SBCL-specific question, you might get a better answer on the SBCL user list. Different compilers do different optimizations, and most ignore at least some declarations.
Second, you should let-bind arr because you are using it twice.
Third, you can use the if you want to avoid let-bind:
(the (simple-array bit) (slot-value inst 'arr))
Fourth, if you want the compiler to infer the type, use a specific reader instead of slot-value:
(defclass c () ((arr :type (simple-array bit) :reader c-arr)))
(defun sample (inst)
(declare (type class-1 inst))
(let ((arr (c-arr inst)))
(map-into arr #'(lambda (dummy) (random 2)) arr)))
c-arr should allow the compiler to infer the value type easier, but (as you have discovered yourself!) you might need to declare its return type:
(declaim (ftype (function (c) (simple-array bit)) c-arr))
The reason is, apparently, that SBCL ignores slot type declarations.