Ifpack2: Add Schur complements to the BlockTriDiag solver. (#12344)

* Add Schur complements to the BlockTriDiag solver.

* Build warning free

* Update the PR with Luc's comments

* Reorder the loops to avoid branching

packages/ifpack2/example/BlockTriDi.cpp

@@ -18,7 +18,7 @@ namespace { // (anonymous)
 
 // Values of command-line arguments.
 struct CmdLineArgs {
-  CmdLineArgs ():blockSize(-1),numIters(10),numRepeats(1),tol(1e-12),nx(172),ny(-1),nz(-1),mx(1),my(1),mz(1),sublinesPerLine(1),useStackedTimer(false),overlapCommAndComp(false){}
+  CmdLineArgs ():blockSize(-1),numIters(10),numRepeats(1),tol(1e-12),nx(172),ny(-1),nz(-1),mx(1),my(1),mz(1),sublinesPerLine(1),sublinesPerLineSchur(1),useStackedTimer(false),overlapCommAndComp(false){}
 
   std::string mapFilename;
   std::string matrixFilename;
@@ -35,6 +35,7 @@ struct CmdLineArgs {
   int my;
   int mz;
   int sublinesPerLine;
+  int sublinesPerLineSchur;
   bool useStackedTimer;
   bool overlapCommAndComp;
   std::string problemName;
@@ -71,6 +72,7 @@ getCmdLineArgs (CmdLineArgs& args, int argc, char* argv[])
 		  "Whether to run with overlapCommAndComp)");
   cmdp.setOption("problemName", &args.problemName, "Human-readable problem name for Watchr plot");
   cmdp.setOption("matrixType", &args.matrixType, "matrixType");
+  cmdp.setOption("sublinesPerLineSchur", &args.sublinesPerLineSchur, "sublinesPerLineSchur");
   auto result = cmdp.parse (argc, argv);
   return result == Teuchos::CommandLineProcessor::PARSE_SUCCESSFUL;
 }
@@ -573,7 +575,7 @@ main (int argc, char* argv[])
 
   {
     Teuchos::TimeMonitor precSetupTimeMon (*precSetupTime);
-    precond = rcp(new BTDC(Ablock,parts,args.overlapCommAndComp));
+    precond = rcp(new BTDC(Ablock,parts,args.sublinesPerLineSchur,args.overlapCommAndComp));
 
     if(rank0) std::cout<<"Initializing preconditioner..."<<std::endl;
     precond->initialize ();