[gambit-list] Slow Bit Hacking with c-lambda

tcl super dude tcleval at gmail.com
Sun Nov 4 16:40:27 EST 2012


I assumed I could get a bit of performance improvement over min procedure
because I don't do any branch. I' thought  min-integer to be at least close
to gambit-c time. The problem is that those operations and a few more are
performed thousands of times. Gambit loops are usually 2x slower than C, so
I hoped I could improve some basic operations as I only perform them on
integers. I don't Understand how a simple shift to extract the lower bits
could make so much difference. Maybe if someone point me to the
implementation of min and abs on gambit, I can understand the difference. I
ve looked for these procedures on gambit's source but could not point it
out.

I'll try to implement them using vectors and see what happens.

now compiled with (block):


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))))))
    19 ms real time

    20 ms cpu time (17 user, 3 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))))))
    27 ms real time
    27 ms cpu time (27 user, 0 system)
    no collections
    no bytes allocated
    no minor faults
    no major faults


On Sun, Nov 4, 2012 at 5:15 PM, Mikael <mikael.rcv at gmail.com> wrote:

> First, please add (declare (block)) and re-compile, this way Gambit won't
> need to re-look up the procedures in the global namespace that you call, on
> each invocation. Please report what kind of improvement you got with this.
>
>
> Now just to make this round of emails complete:
>
> What would be good/very good performance in your view?
>
> Could you send jobs in some kind of batch format to C?
>
> Note that Gambit-internal fxnum variables have a special encoding (the
> lowermost two(?) bits are object type identifier, which are shifted out
> when converting them to C int), this takes a little bit of time.
>
> If for instance you make u32vectors/s32vectors/etc. [and send batch ops
> made on those to C to be performed etc.] then you can just read out the
> content of those vectors 'raw' in C, there's no interconversion there.
>
> Brgds
>
> 2012/11/4 tcl super dude <tcleval at gmail.com>
>
>> 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,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
>>
>>
>> _______________________________________________
>> Gambit-list mailing list
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>>
>>
>
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