[Sema] Add counter to track number of constraints considered by each edge contraction attempt

Author: xedin

include/swift/Basic/Statistics.def

@@ -149,6 +149,11 @@ FRONTEND_STATISTIC(Sema, NumConformancesDeserialized)
 /// expression typechecker did".
 FRONTEND_STATISTIC(Sema, NumConstraintScopes)
 
+/// Number of constraints considered per attempt to
+/// contract constraint graph edges.
+/// This is a measure of complexity of the contraction algorithm.
+FRONTEND_STATISTIC(Sema, NumConstraintsConsideredForEdgeContraction)
+
 /// Number of declarations that were deserialized. A rough proxy for the amount
 /// of material loaded from other modules.
 FRONTEND_STATISTIC(Sema, NumDeclsDeserialized)

lib/Sema/ConstraintGraph.cpp

@@ -18,6 +18,7 @@
 #include "ConstraintGraph.h"
 #include "ConstraintGraphScope.h"
 #include "ConstraintSystem.h"
+#include "swift/Basic/Statistic.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/SaveAndRestore.h"
 #include <algorithm>
@@ -27,6 +28,8 @@
 using namespace swift;
 using namespace constraints;
 
+#define DEBUG_TYPE "ConstraintGraph"
+
 #pragma mark Graph construction/destruction
 
 ConstraintGraph::ConstraintGraph(ConstraintSystem &cs) : CS(cs) { }
@@ -650,7 +653,9 @@ static bool shouldContractEdge(ConstraintKind kind) {
 
 bool ConstraintGraph::contractEdges() {
   SmallVector<Constraint *, 16> constraints;
-  CS.findConstraints(constraints, [](const Constraint &constraint) {
+  CS.findConstraints(constraints, [&](const Constraint &constraint) {
+    // Track how many constraints did contraction algorithm iterated over.
+    incrementConstraintsPerContractionCounter();
     return shouldContractEdge(constraint.getKind());
   });
 
@@ -767,6 +772,14 @@ void ConstraintGraph::optimize() {
   while (contractEdges()) {}
 }
 
+void ConstraintGraph::incrementConstraintsPerContractionCounter() {
+  SWIFT_FUNC_STAT;
+  auto &context = CS.getASTContext();
+  if (context.Stats)
+    context.Stats->getFrontendCounters()
+        .NumConstraintsConsideredForEdgeContraction++;
+}
+
 #pragma mark Debugging output
 
 void ConstraintGraphNode::print(llvm::raw_ostream &out, unsigned indent) {

lib/Sema/ConstraintGraph.h

@@ -332,6 +332,10 @@ class ConstraintGraph {
   /// Constraints that are "orphaned" because they contain no type variables.
   SmallVector<Constraint *, 4> OrphanedConstraints;
 
+  /// Increment the number of constraints considered per attempt
+  /// to contract constrant graph edges.
+  void incrementConstraintsPerContractionCounter();
+
   /// The kind of change made to the graph.
   enum class ChangeKind {
     /// Added a type variable.

validation-test/Sema/type_checker_perf/fast/rdar29358447.swift.gyb

@@ -0,0 +1,10 @@
+// RUN: %scale-test --begin 0 --end 25000 --step 1000 --typecheck --select incrementConstraintsPerContractionCounter %s
+// REQUIRES: OS=macosx
+// REQUIRES: asserts
+
+let _: [Int] = [
+%for i in range(0, N):
+  1,
+%end
+  1
+]