Stack ILP issues (Was: Slow recursive functions

[Mladen Adamovic]

Andrew Haley wrote on Fri, 15 Apr 2005 11:43:39 +0100 :

(the original thread was about bad performance in recursive functions -
Akerman test ran slow)

Andrew> We use the x86 system calling
Andrew> convention throughout gcj, and this is slower than passing args in
Andrew> registers.  There's also the possibility that some JITs might be
Andrew> optimized for this kind of benchmark.

I guess this means that stack functions push, pop etc. don't exploit ILP.
I found that it might be true
http://csdl2.computer.org/persagen/DLAbsToc.jsp?resourcePath=/dl/proceedings/&toc=comp/proceedings/dsd/2004/2203/00/2203toc.xml&DOI=10.1109/DSD.2004.1333267

But real performance issue is that STACK instructions have AFA I remember 8%
in SPEC2000 tests. I don't have book "Modern computer design" at the moment to
check the real percentage.

I think that for compilers the easiest way to compile expresssions like
(expr1 * expr2) / expr3
is extensivly using stack.

So, way to awoid using stack might be important performance issue.
MOV is better idea because it can exploit ILP better.
Somebody might check the stack performance in x86-64.

Anyway, can somebody of developers say which techniques did they use to
exploit ILP in GCJ?

Also, maybe good idea will be to ask in gcc mailing list about stack ilp issues?

Speed of gcj might be important because JVM have awfull performance in matrix
multiplication and nested loops. Probably they don't do compiler techniques to
exploit ILP for loops. JVM language was done in 1995 so maybe they will have a
lot problems with ILP and TLP in the future because in 1995 just few people
think about that.

I'm new here, I'm graduate student a bit involved in ILP and compiler issues.
If you think that I can help somehow in gcj development in these issues you
can let me know.


Mladen Adamovic
home page: http://home.blic.net/adamm