[Constraint graph] Restrict connected components to requested type variables.

The API of the connected-components algorithm asks clients to
provide the set of type variables of interest. However, the connected
components algorithm itself was operating across the entire set of
type variables, then narrowing the result down to the type variables
of interest. Instead, only perform connected components on those type
variables of interest, so that we are only doing work proportional to
the subgraph we're working in.

Author: DougGregor

lib/Sema/ConstraintGraph.cpp

@@ -526,7 +526,6 @@ unsigned ConstraintGraph::computeConnectedComponents(
   // Track those type variables that the caller cares about.
   llvm::SmallPtrSet<TypeVariableType *, 4> typeVarSubset(typeVars.begin(),
                                                          typeVars.end());
-  typeVars.clear();
 
   // Initialize the components with component == # of type variables,
   // a sentinel value indicating that we have yet to assign a component to
@@ -538,9 +537,7 @@ unsigned ConstraintGraph::computeConnectedComponents(
   // what component it is in.
   llvm::DenseSet<Constraint *> visitedConstraints;
   unsigned numComponents = 0;
-  for (unsigned i = 0; i != numTypeVariables; ++i) {
-    auto typeVar = TypeVariables[i];
-
+  for (auto typeVar : typeVars) {
     // Look up the node for this type variable.
     auto nodeAndIndex = lookupNode(typeVar);
 
@@ -576,6 +573,10 @@ unsigned ConstraintGraph::computeConnectedComponents(
   // are the only components and type variables we want to report.
   SmallVector<bool, 4> componentHasUnboundTypeVar(numComponents, false);
   for (unsigned i = 0; i != numTypeVariables; ++i) {
+    // If we didn't look at this type variable, there's nothing to do.
+    if (components[i] == numTypeVariables)
+      continue;
+
     // If this type variable has a fixed type, skip it.
     if (CS.getFixedType(TypeVariables[i]))
       continue;
@@ -592,6 +593,10 @@ unsigned ConstraintGraph::computeConnectedComponents(
   SmallVector<unsigned, 4> componentRenumbering(numComponents, 0);
   numComponents = 0;
   for (unsigned i = 0, n = componentHasUnboundTypeVar.size(); i != n; ++i) {
+    // If we didn't look at this type variable, there's nothing to do.
+    if (components[i] == numTypeVariables)
+      continue;
+
     // Skip components that have no unbound type variables.
     if (!componentHasUnboundTypeVar[i])
       continue;
@@ -601,8 +606,13 @@ unsigned ConstraintGraph::computeConnectedComponents(
 
   // Copy over the type variables in the live components and remap
   // component numbers.
+  typeVars.clear();
   unsigned outIndex = 0;
   for (unsigned i = 0, n = TypeVariables.size(); i != n; ++i) {
+    // If we didn't look at this type variable, there's nothing to do.
+    if (components[i] == numTypeVariables)
+      continue;
+
     // Skip type variables in dead components.
     if (!componentHasUnboundTypeVar[components[i]])
       continue;