In a C++ template library (a matrix library with expression templates)
upgrading from g++-4.2 to g++-4.3 results in 3x slower programs.
Compiler versions:
g++-4.2 (GCC) 4.2.1 (Debian 4.2.1-5)
g++-4.3 (Debian 4.3-20070902-1) 4.3.0 20070902 (experimental) [trunk revision
128028]
$ g++-4.2 -O3 -DNDEBUG benchmark.cpp -o benchmark && time ./benchmark
1.0001 0 0
0 1.0001 0
0 0 1.0001
real 0m4.495s
user 0m4.416s
sys 0m0.003s
$ g++-4.3 -O3 -DNDEBUG benchmark.cpp -o benchmark && time ./benchmark
1.0001 0 0
0 1.0001 0
0 0 1.0001
real 0m15.891s
user 0m15.595s
sys 0m0.018s
I looked at the assembler code but I didn't see anything obvious (I don't
know much about assembler so I may have missed something.
I did notice that adding -fforce-addr changes the result for 4.3 but not
for 4.2.
$ g++-4.3 -O3 -DNDEBUG benchmark.cpp -fforce-addr -o benchmark && time
./benchmark
1.0001 0 0
0 1.0001 0
0 0 1.0001
real 0m8.779s
user 0m8.662s
sys 0m0.007s
--
Summary: significantly slower results with 4.3 compared to 4.2
Product: gcc
Version: 4.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: michael dot olbrich at gmx dot net
GCC build triplet: i486-linux-gnu
GCC host triplet: i486-linux-gnu
GCC target triplet: i486-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33604
