[AIE] Protect against empty sets

Put in some runtime control, a real one based on timeouts, and a deterministic
one pre-deciding based on estimated milliseconds

llvm/lib/Target/AIE/AIEPostPipeliner.cpp

@@ -676,7 +676,7 @@ bool PostPipeliner::tryHeuristics() {
 
   DEBUG_SUMMARY(dbgs() << "-- MinLength=" << MinLength << "\n");
 
-  if (II <= 10 && solve()) {
+  if (solve(MinLength / II)) {
     return true;
   }
 
@@ -752,13 +752,12 @@ class FixedStrategy : public PostPipelinerStrategy {
   std::string name() override { return "FixedStrategy"; }
 };
 
-bool PostPipeliner::solve() {
+bool PostPipeliner::solve(int NS) {
   if (!UseSolver) {
     return false;
   }
 
   Z3BinarySolver Solver;
-  Solver.setScheduleSize(II, 3);
   for (int N = 0; N < NInstr; N++) {
     SUnit &SU = DAG->SUnits[N];
     auto *MI = SU.getInstr();
@@ -786,7 +785,7 @@ bool PostPipeliner::solve() {
     }
   }
 
-  // Add loop-carried true dependences to future iterations. The iteration
+  // Add loop-carried dependences to future iterations. The iteration
   // distance is taken into account
   for (int N = 0; N < NInstr; N++) {
     SUnit &SU = DAG->SUnits[N];
@@ -801,7 +800,11 @@ bool PostPipeliner::solve() {
     }
   }
 
-  if (!Solver.genModel()) {
+  Solver.setScheduleSize(II, NS);
+  Solver.genModel();
+  if (!Solver.solveModel()) {
+    // Note: If we can't solve it, it doesn't mean the II isn't feasible,
+    // so we don't need to avoid running the heuristics.
     return false;
   }
   auto Schedule = Solver.getCycles();

llvm/lib/Target/AIE/AIEPostPipeliner.h

@@ -190,7 +190,7 @@ class PostPipeliner {
   int computeMinScheduleLength() const;
 
   // try to find a solution using a solver
-  bool solve();
+  bool solve(int NS);
 
   /// Try all heuristics, stop at the first that fits the II
   /// If it returns true, a valid schedule is laid down in Info.