Hi everyone,
I have been working on an image processing library that I want to use on android. I know that performance is a bit of a problem on image processing, so I started to study some bit hacking to improve the performance on my integer operations. My problem now is that I don't know why my c-lambdas take so much time to execute. All operations take constant time and there is no branching. Probably is some sort of conversion time between C types and GAMBIT types. I expected min-integer and abs-integer to be a lot faster than min and abs, respectively. Any help is appreciated.
URL REFERENCE: http://graphics.stanford.edu/~seander/bithacks.html#IntegerAbs http://graphics.stanford.edu/~seander/bithacks.html#IntegerMinOrMax
MY ENVIRONMENT: Linux casa 3.6.4-1-ARCH #1 SMP PREEMPT Mon Oct 29 15:13:56 CET 2012 i686 GNU/Linux
gcc -v Using built-in specs. COLLECT_GCC=gcc COLLECT_LTO_WRAPPER=/usr/lib/gcc/i686-pc-linux-gnu/4.7.2/lto-wrapper Target: i686-pc-linux-gnu Configured with: /build/src/gcc-4.7.2/configure --prefix=/usr --libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man --infodir=/usr/share/info --with-bugurl=https://bugs.archlinux.org/--enable-languages=c,c++,ada,fortran,go,lto,objc,... --enable-shared --enable-threads=posix --with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions --enable-clocale=gnu --disable-libstdcxx-pch --enable-libstdcxx-time --enable-gnu-unique-object --enable-linker-build-id --with-ppl --enable-cloog-backend=isl --disable-ppl-version-check --disable-cloog-version-check --enable-lto --enable-gold --enable-ld=default --enable-plugin --with-plugin-ld=ld.gold --with-linker-hash-style=gnu --disable-multilib --disable-libssp --disable-build-with-cxx --disable-build-poststage1-with-cxx --enable-checking=release Thread model: posix gcc version 4.7.2 (GCC)
Gambit v4.6.6 (SINGLE HOST and GCC extensions enabled)
SCHEME CODE: compiled with ---> gambitc -exe bithacks.scm
;; bithacks.scm
(declare (not safe) (fixnum) (inline) (inline-primitives))
;; INTEGER ABSOLUTE (define abs-integer (c-lambda (int) int "// we want to find the absolute value of arg1 int const mask = ___arg1 >> sizeof(int) * CHAR_BIT - 1; ___result = (___arg1 + mask) ^ mask;"))
(define min-integer (c-lambda (int int) int "// find the smaller of arg1 arg2 ___result = ___arg2 ^ ((___arg1 ^ ___arg2) & -(___arg1 < ___arg2));"))
(define (test-abs) (let ((result 0)) (begin (display "abs:\n") (time (let loop-i ((i 0)) (cond ((fx< i 100000) (set! result (abs i)) (loop-i (fx+ 1 i)))))) (display "abs-integer:\n") (time (let loop-j ((j 0)) (cond ((fx< j 100000) (set! result (abs-integer j)) (loop-j (fx+ 1 j)))))))))
(define (test-min) (let ((result 0)) (begin (display "min:\n") (time (let loop-i ((i 0)) (cond ((fx< i 100000) (set! result (min i result)) (loop-i (fx+ 1 i)))))) (display "min-integer:\n") (time (let loop-j ((j 0)) (cond ((fx< j 100000) (set! result (min-integer j result)) (loop-j (fx+ 1 j)))))))))
;; run tests
(test-abs) (test-min)
;;;;;;;;;;;;;;;;;;;;;; RESULTS;;;;;;;;;;;;;;;;;;;;;
abs: (time (let loop-i ((i 0)) (cond ((fx< i 100000) (set! result (abs i)) (loop-i (fx+ 1 i)))))) 2 ms real time 3 ms cpu time (3 user, 0 system) no collections no bytes allocated no minor faults no major faults abs-integer: (time (let loop-j ((j 0)) (cond ((fx< j 100000) (set! result (abs-integer j)) (loop-j (fx+ 1 j)))))) 22 ms real time 23 ms cpu time (23 user, 0 system) no collections no bytes allocated 2 minor faults no major faults min: (time (let loop-i ((i 0)) (cond ((fx< i 100000) (set! result (min i result)) (loop-i (fx+ 1 i)))))) 2 ms real time 3 ms cpu time (0 user, 3 system) no collections no bytes allocated no minor faults no major faults min-integer: (time (let loop-j ((j 0)) (cond ((fx< j 100000) (set! result (min-integer j result)) (loop-j (fx+ 1 j)))))) 28 ms real time 30 ms cpu time (27 user, 3 system) no collections no bytes allocated no minor faults no major faults