Re: GNAT's stack checking in Ubuntu 9.04 (and Shootout regex-dna)

[jonathan <johnscpg@googlemail.com>]

On Aug 9, 8:13 pm, Georg Bauhaus <rm.tsoh.plus-
bug.bauh...@maps.futureapps.de> wrote:


I was going to give up on regexdna.adb, but I had to
try one last thing.  (That's the trouble with these
danged benchmarks).  I wanted to recompile the
GNAT.spitbol packages .. the quickest thing I could
think of was copy the source into my working directory
and rename them. GNAT.spitbol to GNAT_spitbol, and
GNAT.spitbol.patterns to GNAT_spitbol.patterns.
The advantage of this is you can modify them.  The
first thing I tried worked rather well: I replaced

   Stack_Size : constant Positive := 2000;

with

   Stack_Size : constant Positive := 129;

in GNAT_spitbol.patterns.

The program running time (single-core) dropped from
47 seconds to 32 seconds.  (You can set Stack_Size to
17, with all validity checks enabled, and still get
the right answer.) (Used the latest GNAT GPL, which
seems to have fewer optimizations disabled than
earlier GNAT's. Also, I ran it on a 3.2 GHz 4-core
INTEL .. faster than the one used at that
benchmarking site.)

I used gnatmake with

-O3 -gnatnp -funroll-all-loops -march=native regexdna.adb

Unfortunately, you need to do more work to
get earlier GNAT's to do this ... missing
"System.String_Hash" or something.

Last remark: someone with experience using the
spitbol routines might be able to optimize
further.  There are tricks using "anchors"
mentioned in spec of GNAT.spitbol.patterns.


Jonathan