[Bug tree-optimization/46032] New: openmp inhibits loop vectorization

[vincenzo.innocente at cern dot ch]

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: unassigned@gcc.gnu.org
        ReportedBy: vincenzo.innocente@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.