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:
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,obj-c++ --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
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