[Constraint graph] Make connected components more self-contained.

Have the constraint graph's connected-component implementation be more
self-contained, producing a vector containing each of the actual
components (where each is defined by a list of type variables and a list
of constraints). This simplifies the contract with the client
(SplitterStep) and eliminates a bunch of separate mapping steps to
interpret the results.

It also lets us enrich the Component data structure in the future.

(cherry picked from commit 4c04ced)

Author: DougGregor

lib/Sema/CSStep.cpp

@@ -96,25 +96,31 @@ void SplitterStep::computeFollowupSteps(
   CG.optimize();
 
   // Compute the connected components of the constraint graph.
-  // FIXME: We're seeding typeVars with TypeVariables so that the
-  // connected-components algorithm only considers those type variables within
-  // our component. There are clearly better ways to do this.
-  std::vector<TypeVariableType *> typeVars(CS.TypeVariables);
-  std::vector<unsigned> components;
-  unsigned numComponents = CG.computeConnectedComponents(typeVars, components);
+  auto components = CG.computeConnectedComponents(CS.TypeVariables);
+  unsigned numComponents =
+      components.size() + CG.getOrphanedConstraints().size();
   if (numComponents < 2) {
     componentSteps.push_back(llvm::make_unique<ComponentStep>(
-        CS, 0, /*single=*/true, &CS.InactiveConstraints, Solutions));
+        CS, 0, &CS.InactiveConstraints, Solutions));
     return;
   }
 
   Components.resize(numComponents);
   PartialSolutions = std::unique_ptr<SmallVector<Solution, 4>[]>(
       new SmallVector<Solution, 4>[numComponents]);
 
-  for (unsigned i = 0, n = numComponents; i != n; ++i) {
+  // Add components.
+  for (unsigned i : indices(components)) {
     componentSteps.push_back(llvm::make_unique<ComponentStep>(
-        CS, i, /*single=*/false, &Components[i], PartialSolutions[i]));
+        CS, i, &Components[i], std::move(components[i]), PartialSolutions[i]));
+  }
+
+  // Add components for the orphaned constraints.
+  OrphanedConstraints = CG.takeOrphanedConstraints();
+  for (unsigned i : range(components.size(), numComponents)) {
+    auto orphaned = OrphanedConstraints[i - components.size()];
+    componentSteps.push_back(llvm::make_unique<ComponentStep>(
+        CS, i, &Components[i], orphaned, PartialSolutions[i]));
   }
 
   if (isDebugMode()) {
@@ -129,57 +135,6 @@ void SplitterStep::computeFollowupSteps(
     CG.printConnectedComponents(CS.TypeVariables, log);
   }
 
-  // Map type variables and constraints into appropriate steps.
-  llvm::DenseMap<TypeVariableType *, unsigned> typeVarComponent;
-  llvm::DenseMap<Constraint *, unsigned> constraintComponent;
-  for (unsigned i = 0, n = typeVars.size(); i != n; ++i) {
-    auto *typeVar = typeVars[i];
-    // Record the component of this type variable.
-    typeVarComponent[typeVar] = components[i];
-
-    for (auto *constraint : CG[typeVar].getConstraints())
-      constraintComponent[constraint] = components[i];
-  }
-
-  // Add the orphaned components to the mapping from constraints to components.
-  unsigned firstOrphanedComponent =
-      numComponents - CG.getOrphanedConstraints().size();
-  {
-    unsigned component = firstOrphanedComponent;
-    for (auto *constraint : CG.getOrphanedConstraints()) {
-      // Register this orphan constraint both as associated with
-      // a given component as a regular constrant, as well as an
-      // "orphan" constraint, so it can be proccessed correctly.
-      constraintComponent[constraint] = component;
-      componentSteps[component]->recordOrphan(constraint);
-      ++component;
-    }
-  }
-
-  for (auto *typeVar : CS.TypeVariables) {
-    auto known = typeVarComponent.find(typeVar);
-    // If current type variable is associated with
-    // a certain component step, record it as being so.
-    if (known != typeVarComponent.end()) {
-      componentSteps[known->second]->record(typeVar);
-      continue;
-    }
-  }
-
-  // Transfer all of the constraints from the work list to
-  // the appropriate component.
-  auto &workList = CS.InactiveConstraints;
-  while (!workList.empty()) {
-    auto *constraint = &workList.front();
-    workList.pop_front();
-    assert(constraintComponent.count(constraint) > 0 && "Missed a constraint");
-    componentSteps[constraintComponent[constraint]]->record(constraint);
-  }
-
-  // Remove all of the orphaned constraints; they'll be re-introduced
-  // by each component independently.
-  OrphanedConstraints = CG.takeOrphanedConstraints();
-
   // Create component ordering based on the information associated
   // with constraints in each step - e.g. number of disjunctions,
   // since components are going to be executed in LIFO order, we'd

lib/Sema/CSStep.h

@@ -19,6 +19,7 @@
 #define SWIFT_SEMA_CSSTEP_H
 
 #include "Constraint.h"
+#include "ConstraintGraph.h"
 #include "ConstraintSystem.h"
 #include "swift/AST/Types.h"
 #include "llvm/ADT/ArrayRef.h"
@@ -336,7 +337,7 @@ class ComponentStep final : public SolverStep {
   std::unique_ptr<Scope> ComponentScope = nullptr;
 
   /// Type variables and constraints "in scope" of this step.
-  std::vector<TypeVariableType *> TypeVars;
+  TinyPtrVector<TypeVariableType *> TypeVars;
   /// Constraints "in scope" of this step.
   ConstraintList *Constraints;
 
@@ -349,29 +350,51 @@ class ComponentStep final : public SolverStep {
   Constraint *OrphanedConstraint = nullptr;
 
 public:
-  ComponentStep(ConstraintSystem &cs, unsigned index, bool single,
+  /// Create a single component step.
+  ComponentStep(ConstraintSystem &cs, unsigned index,
                 ConstraintList *constraints,
                 SmallVectorImpl<Solution> &solutions)
-      : SolverStep(cs, solutions), Index(index), IsSingle(single),
+      : SolverStep(cs, solutions), Index(index), IsSingle(true),
         OriginalScore(getCurrentScore()), OriginalBestScore(getBestScore()),
         Constraints(constraints) {}
 
-  /// Record a type variable as associated with this step.
-  void record(TypeVariableType *typeVar) { TypeVars.push_back(typeVar); }
+  /// Create a component step from a constraint graph component.
+  ComponentStep(ConstraintSystem &cs, unsigned index,
+                ConstraintList *constraints,
+                ConstraintGraph::Component &&component,
+                SmallVectorImpl<Solution> &solutions)
+      : SolverStep(cs, solutions), Index(index), IsSingle(false),
+        OriginalScore(getCurrentScore()), OriginalBestScore(getBestScore()),
+        Constraints(constraints) {
+    TypeVars = std::move(component.typeVars);
+
+    for (auto constraint : component.constraints) {
+      constraints->erase(constraint);
+      record(constraint);
+    }
+  }
+
+  /// Create a component step for an orphaned constraint.
+  ComponentStep(ConstraintSystem &cs, unsigned index,
+                ConstraintList *constraints,
+                Constraint *orphaned,
+                SmallVectorImpl<Solution> &solutions)
+      : SolverStep(cs, solutions), Index(index), IsSingle(false),
+        OriginalScore(getCurrentScore()), OriginalBestScore(getBestScore()),
+        Constraints(constraints), OrphanedConstraint(orphaned) {
+    constraints->erase(orphaned);
+    record(orphaned);
+  }
 
+private:
   /// Record a constraint as associated with this step.
   void record(Constraint *constraint) {
     Constraints->push_back(constraint);
     if (constraint->getKind() == ConstraintKind::Disjunction)
       ++NumDisjunctions;
   }
 
-  /// Record a constraint as associated with this step but which doesn't
-  /// have any free type variables associated with it.
-  void recordOrphan(Constraint *constraint) {
-    assert(!OrphanedConstraint);
-    OrphanedConstraint = constraint;
-  }
+public:
 
   StepResult take(bool prevFailed) override;
   StepResult resume(bool prevFailed) override;