http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46032
Summary: openmp inhibits loop vectorization
Product: gcc
Version: 4.5.1
Status: UNCONFIRMED
Severity: major
Priority: P3
Component: tree-optimization
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: vincenzo.innoce...@cern.ch
The use of openmp to parallelize loop inhibits auto-vectorization.
This defeats all benefits of parallelization making the parallel code slower
than the "sequential one".
Is it foreseen a version of openmp that preserve auto-vectorization?
Example
on
Linux 2.6.18-194.11.3.el5.cve20103081 #1 SMP Thu Sep 16 15:17:10 CEST 2010
x86_64 x86_64 x86_64 GNU/Linux
using
GNU C++ (GCC) version 4.6.0 20100408 (experimental) (x86_64-unknown-linux-gnu)
compiled by GNU C version 4.6.0 20100408 (experimental), GMP version 4.3.2,
MPFR version 2.4.2, MPC version 0.8.1
GGC heuristics: --param ggc-min-expand=30 --param ggc-min-heapsize=4096
compiling this simple example
cat openmpvector.cpp
int main()
{
const unsigned int nEvents = 1000;
double results[nEvents] = {0};
double pData[nEvents] = {0};
double coeff = 12.2;
#pragma omp parallel for
for (int idx = 0; idx<(int)nEvents; idx++) {
results[idx] = coeff*pData[idx];
}
return resultsCPU[0]; // avoid optimization of "dead" code
}
gives
g++ -O2 -fopenmp -ftree-vectorize -ftree-vectorizer-verbose=7 openmpvector.cpp
openmpvector.cpp:11: note: not vectorized: loop contains function calls or data
references that cannot be analyzed
openmpvector.cpp:9: note: vectorized 0 loops in function.
