Re: [gambit-list] Slow Bit Hacking with c-lambda - Gambit-list

4 Nov 2012


      Ah, also throw in the following:
 * (declare (not interrupts-enabled))
This might be a big deal - when it's off, Gambit doesn't produce any
hooks for stack overflow or thread system interrupts but executes your code
according to a typical C model
* (declare (standard-bindings) (extended-bindings)) too?
What is the performance you get from Gambit now, and what kind of
performance is it you want to get?
2012/11/4 tcl super dude tcleval@gmail.com
...
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@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@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,... --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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