commit 5d8b494557e17992b64d8df548f49ef1f9ef4e05
author Mike Vitus <vitus@google.com> Mon Apr 09 10:10:03 2018 -0700
committer Mike Vitus <vitus@google.com> Mon Apr 09 10:10:03 2018 -0700
tree 4bd48ad6864f19b75ca91a2f3a30fb31fccfa696
parent 8d7b2528bc64e6f4d7f281917004ec6511f073b4

Adds a ParallelFor wrapper for no threads and OpenMP.

With the addition of C++11 support we can simplify the parallel for code by
removing the ifdef branching.  Converts coordinate_descent_minimizer.cc to use
the thread_id ParallelFor API.

Tested by building with OpenMP, C++11 threads, TBB, and no threads.  All tests
pass.

Also compared timing via the bundle adjuster.

./bin/bundle_adjuster --input=../problem-744-543562-pre.txt

With OpenMP num_threads=8

Head:
Time (in seconds):
  Residual only evaluation           0.807753 (5)
  Jacobian & residual evaluation     4.489404 (6)
  Linear solver                     41.826481 (5)
Minimizer                           50.745857
Total                               73.294424

CL:
Time (in seconds):
  Residual only evaluation           0.970483 (5)
  Jacobian & residual evaluation     4.647438 (6)
  Linear solver                     41.781892 (5)
Minimizer                           50.848904
Total                               73.089983

With OpenMP num_threads=1

HEAD:
Time (in seconds):
  Residual only evaluation           2.990246 (5)
  Jacobian & residual evaluation    14.132090 (6)
  Linear solver                     79.631951 (5)
Minimizer                          100.281847
Total                              122.946267

CL:
Time (in seconds):
  Residual only evaluation           3.075178 (5)
  Jacobian & residual evaluation    13.966451 (6)
  Linear solver                     77.005441 (5)
Minimizer                           97.568712
Total                              120.410454

Change-Id: I1857d7943073be7465b6c6476bf46ab11c5475a3