[PR] Now That's What I Call Fashion - February 2010
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GCC freezing for a Multiply Chain
Hello Everyone, I am creating a benchmark, where I have a following code: x1 = a * b x2 = x1 * a; x3 = x1* x2; x4 = x2 * x3; x5 = x3 * x4; x6 = x5 * x6; . . . . . . x1000 = x999 * x998; When I do this, and compile using -O3/-O2/-O1, the compiler freezes. Am I doing anything wrong? GCC Version: 4.2.1 Platform: x86_64 (in macOS Snow Leopard). I also tested this on a i386 (Red Hat 4.3.2-7) machine with gcc 4.3.2 and I had the same response. Please CC me in the response since I m not a subscribed member of this mailing list. Any help is highly appreciated! Thanks, Balaji V. Iyer.
Re: GCC freezing for a Multiply Chain
balaji.i...@gtri.gatech.edu wrote:
Hello Everyone,
I am creating a benchmark, where I have a following code:
x1 = a * b
x2 = x1 * a;
x3 = x1* x2;
x4 = x2 * x3;
x5 = x3 * x4;
x6 = x5 * x6;
. . .
. . .
x1000 = x999 * x998;
When I do this, and compile using -O3/-O2/-O1, the compiler freezes. Am I doing anything wrong?
GCC Version: 4.2.1
Platform: x86_64 (in macOS Snow Leopard).
I also tested this on a i386 (Red Hat 4.3.2-7) machine with gcc 4.3.2 and I had
the same response.
Please CC me in the response since I m not a subscribed member of this mailing list.
I have no real idea on this particular issue, but I did empirically observe that on some programs (for me, it was the
MELT generated C code on my MELT [1] branch eg before rev 154917) the register allocator (or some optimizing passes)
might have in practice an O(n^2) behavior, and might need a lot of memory
I would suggest:
* trying a very recent gcc (that you probably should build from its released source code), at least gcc 4.4.3
http://gcc.gnu.org/gcc-4.4/ or perhaps a gcc 4.5 snapshot.
* reducing the number of variables (to e.g. 300 or 500 instead of 1000).
* compiling first with -O1
* compiling with -ftime-report
* observe the cpu time and memory consumption of the compilation, eg by running
the top utility in a separate terminal.
So try first to reduce the number of variables (perhaps even to 200) and to
compile with
gcc-4.4 -O1 -ftime-report -v -c benchbalaji.c
I believe that recent GCC (ie 4.4 at least) had serious progress both on register allocation and on some optimization
passes, so they probably should work better on your [pathological] case.
Regards.
Note [1]: MELT is a lisp like dialect -translated into C- for coding GCC passes in plugins, and it used to generate huge
initialization routines. This is no more the case since a few weeks (because there is no more huge routines, only
several big ones), MELT can be reasonably compiled on a desktop even with optimization like -O2.
--
Basile STARYNKEVITCH http://starynkevitch.net/Basile/
email: basilestarynkevitchnet mobile: +33 6 8501 2359
8, rue de la Faiencerie, 92340 Bourg La Reine, France
*** opinions {are only mines, sont seulement les miennes} ***
Exception handling mechanism
Howdy all, I am in middle of designing program analysis for C++ code, but with concerns about exception handling. I used to disable exception handling but now have to take into account exception handling somehow. May someone give me a brief introduction on how GCC processes C++ exceptions or recommend some docs about exception handling? (Thus I can quickly evaluate the overhead arising from taking into account exceptions during program analysis.) Thank you very much! Xiaolong
Re: Exception handling mechanism
... I would suggest first reading this document, in detail: http://www.codesourcery.com/public/cxx-abi/ and then asking very specific questions. Paolo.
Re: gcc -O1 performs better than gcc -O2
Joakim Tjernlund wrote: Noticed while optimizing crc16 that gcc -O performed much better than gcc -O2 while doing crc16: I have also seen -O1 beat -O2 when with a large FORTRAN program. http://www.hep.manchester.ac.uk/u/sam/zgoubi-optimise/ Sam
RE: GCC freezing for a Multiply Chain
Hi Balaji. If you have access to GCC source and are willing to change it, there is a way to turn off IRA (the Integrated Register Allocator) and just use a "fast allocation" algorithm that otherwise only gets used for -O0. Should IRA be the issue, this may be a workaround for you in the short-term: In GCC version 4.4.1, the last function of ira-color.c is called ira-color(). Simply make it always call fast_allocation instead of only if ira_conflict_p is false. Best regards, Ian > -Original Message- > From: gcc-ow...@gcc.gnu.org [mailto:gcc-ow...@gcc.gnu.org] On Behalf Of > balaji.i...@gtri.gatech.edu > Sent: 15 February 2010 17:04 > To: gcc@gcc.gnu.org > Subject: GCC freezing for a Multiply Chain > > Hello Everyone, > I am creating a benchmark, where I have a following code: > > x1 = a * b > x2 = x1 * a; > x3 = x1* x2; > x4 = x2 * x3; > x5 = x3 * x4; > x6 = x5 * x6; > . . . > . . . >x1000 = x999 * x998; > > > > When I do this, and compile using -O3/-O2/-O1, the compiler freezes. > Am I doing anything wrong? > > GCC Version: 4.2.1 > Platform: x86_64 (in macOS Snow Leopard). > > I also tested this on a i386 (Red Hat 4.3.2-7) machine with gcc 4.3.2 > and I had the same response. > > Please CC me in the response since I m not a subscribed member of this > mailing list. > > Any help is highly appreciated! > > Thanks, > > > Balaji V. Iyer.
Re: Exception handling mechanism
On 15/02/2010 18:07, Xiaolong Tang wrote: > May someone give > me a brief introduction on how GCC processes C++ exceptions or recommend > some docs about exception handling? http://gcc.gnu.org/wiki/Dwarf2EHNewbiesHowto cheers, DaveK
