Re: Please try out FFI speed improvement

Hoehle, Joerg-Cyril writes:
 > Aurelio Bignoli wrote:
 > >with-c-var/cast
 > >   Real time: 1.273894 sec. (it was 1.494392)
 > >with-foreign-object/%cast
 > >   Real time: 0.521461 sec. (it was 0.638666)
 > 
 > This is strange, since the compiler macro using LOAD-TIME-VALUE is
 > supposed to do exactly what I did by hand using %CAST
 > directly. Therefore, one should not see any difference in speed (or
 > slowness).

You are right. For my tests I used the following two functions:

(defun get-foreign-data-2 (data size type)
  (with-c-var (p 'c-pointer data)
    (cast p (cond
             ((eq type 'integer) '(c-ptr int))
             ((eq type 'string)
              `(c-ptr (c-array character ,size)))
             (t (multiple-value-bind (ft byte-size)
                    (foreign-element-type type)
                  `(c-ptr (c-array ,ft ,(/ size byte-size)))))))))

(defun get-foreign-data-3 (data size type)
  (declare (ignore size))
  (cond
   ((eq type 'integer)
    (with-foreign-object (p 'c-pointer data)
       (ffi::foreign-value
        (ffi::%cast p (load-time-value (ffi::parse-c-type '(c-ptr int)))))))
   ((eq type 'string) "")
   (t 0)))

In both cases the COND macro expands into 

(IF (EQ TYPE 'INTEGER)
  ...)

so, in my opinion, the execution time of the two functions should be
equivalent to the execution time of WITH-C-VAR or WITH-FOREIGN-OBJECT
plus the execution time of IF. I was wrong: the following, simpler functions:

(defun get-foreign-int-1 (data)
  (with-c-var (p 'c-pointer data)
    (cast p '(c-ptr int))))

(defun get-foreign-int-2 (data)
  (with-foreign-object (p 'c-pointer data)
       (ffi::foreign-value
        (ffi::%cast p (load-time-value (ffi::parse-c-type '(c-ptr int)))))))

have exactly the same execution time.


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