http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51041
Bug #: 51041
Summary: g++ strange optimisation behaviour
Classification: Unclassified
Product: gcc
Version: 4.6.2
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: other
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: fb.programm...@gmail.com
The attached code repeatedly executes a vector * vector product to test the
performance of the system. Compiled with
g++ -Wall -O2 file.cpp
it results in a performance of about 1.7 Gflops on an Intel i5-750, ie
the output is
adding: 0.059 s, 1.695 GFlops, sum=0.000000
However, when adding another printf (remove the comment in front of the
last printf) the performance deteriorates strongly (same compiler
options):
adding: 0.195 s, 0.512 GFlops, sum=0.000000
sum=0.000000
It seems the last printf confuses the compiler optimisation completely,
although it shouldn't make a difference at all, as the same variable
is already printed a few lines above.
This is worrying as it seems the compiler fails to fully optimise the
code under odd circumstances. I've used compiler version 4.6.2 as well as
4.4.1 which is the default compiler on the system.
$ gcc-4.6.2 --version
gcc-4.6.2 (GCC) 4.6.2
$ gcc --version
gcc (SUSE Linux) 4.4.1 [gcc-4_4-branch revision 150839]
$ uname -a
Linux localhost 2.6.31.14-0.8-desktop #1 SMP PREEMPT 2011-04-06 18:09:24 +0200
x86_64 x86_64 x86_64 GNU/Linux
