[Bug c++/54000] New: Performance breakdown for gcc-4.{6,7} vs. gcc-4.5 using std::vector in matrix vector multiplication

["benedict.geihe at ins dot uni-bonn.de" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54000

             Bug #: 54000
           Summary: Performance breakdown for gcc-4.{6,7} vs. gcc-4.5
                    using std::vector in matrix vector multiplication
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: benedict.geihe@ins.uni-bonn.de


Dear experts,

I got here from the gcc-help mailing list but have not submitted any bug
reports before. So I hope for your patience.

In a self-written library used for numerical computations we have some typical
programs serving as benchmarks for new compiler versions or optimization flags.
When gcc-4.6 was released we noticed a performance breakdown. The problem
persisted with gcc-4.7. I tried to produce a minimal stand-alone example and
followed the instructions at http://gcc.gnu.org/bugs/minimize.html. As
std::vector is included I was however not able to arrive at a really small
file.

What you see at the end of the file is actually just 1000 times matrix-vector
multiplication. However the matrix has a highly specific structure which is
encountered when performing numerical computations using the Finite Element
Method (FEM), i.e.:

std::vector<MinimalVec3> rows[9];

Thus it consists of 9 bands of triples of doubles. The length of each band
corresponds to the length of the vector it is applied to.


Compiling with gcc-4.5.0 (our current standard) 'time' command gives:
  real    1m32.606s
Using gcc-4.7.0 we get:
  real    2m6.923s
When removing member variable "double stuff" in "class MinimalVector" and using
gcc-4.7.0 we get:
  real    1m27.354s


Using a C array instead of std::vector above resolves this issue.


The specifications of the two compilers used are:

Using built-in specs.
COLLECT_GCC=/home/prog/gcc-4.5.0-64/bin/g++
COLLECT_LTO_WRAPPER=/home/prog/gcc-4.5.0-64/libexec/gcc/x86_64-unknown-linux-gnu/4.5.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ./configure --prefix=/home/prog/gcc-4.5.0-64/
--enable-languages=c,c++,fortran --disable-multilib --enable-lto
--with-libelf=/home/prog/libelf-64/ --with-ppl=/home/prog/ppl-64/
--with-cloog=/home/prog/cloog-ppl-64/
Thread model: posix
gcc version 4.5.0 (GCC)

and

Using built-in specs.
COLLECT_GCC=/home/prog/gcc-4.7.0-64/bin/g++
COLLECT_LTO_WRAPPER=/home/prog/gcc-4.7.0-64/libexec/gcc/x86_64-unknown-linux-gnu/4.7.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ../gcc-4.7.0/configure --prefix=/home/prog/gcc-4.7.0-64/
--enable-languages=c,c++,fortran --with-gmp=/home/prog/gmp-5.0.4-64/
--with-ppl=/home/prog/ppl-0.12-64/ --enable-cloog-backend=isl
--with-cloog=/home/prog/cloog-0.16.3-64/ --disable-multilib
--enable-libstdcxx-debug
Thread model: posix
gcc version 4.7.0 (GCC)

They have been compiled manually on a machine running openSuse 11.3.


The command line was: g++ -O2 -o minmvmult minmvmult.ii


There were no warnings or error messages.



We'd be grateful for any suggestions.
Best regards
Benedict