Sema: Performance optimization for associated type solver

Don't explore parts of the solution space that would lead to a
worse solution than what we already have.

Author: slavapestov

lib/Sema/AssociatedTypeInference.cpp

@@ -2956,6 +2956,14 @@ void AssociatedTypeInference::findSolutionsRec(
           unsigned numTypeWitnesses,
           unsigned numValueWitnessesInProtocolExtensions,
           unsigned reqDepth) {
+  // If this solution is going to be worse than what we've already recorded,
+  // give up now.
+  if (!solutions.empty() &&
+      solutions.front().NumValueWitnessesInProtocolExtensions
+          < numValueWitnessesInProtocolExtensions) {
+    return;
+  }
+
   using TypeWitnessesScope = decltype(typeWitnesses)::ScopeTy;
 
   // If we hit the last requirement, record and check this solution.
@@ -3089,6 +3097,13 @@ void AssociatedTypeInference::findSolutionsRec(
     solution.NumValueWitnessesInProtocolExtensions
       = numValueWitnessesInProtocolExtensions;
 
+    // If this solution was clearly better than the previous best solution,
+    // swap them.
+    if (solutionList.back().NumValueWitnessesInProtocolExtensions
+          < solutionList.front().NumValueWitnessesInProtocolExtensions) {
+      std::swap(solutionList.front(), solutionList.back());
+    }
+
     // We're done recording the solution.
     return;
   }
@@ -3441,15 +3456,10 @@ bool AssociatedTypeInference::findBestSolution(
   if (solutions.empty()) return true;
   if (solutions.size() == 1) return false;
 
-  // Find the smallest number of value witnesses found in protocol extensions.
-  // FIXME: This is a silly heuristic that should go away.
+  // The solution at the front has the smallest number of value witnesses found
+  // in protocol extensions, by construction.
   unsigned bestNumValueWitnessesInProtocolExtensions
     = solutions.front().NumValueWitnessesInProtocolExtensions;
-  for (unsigned i = 1, n = solutions.size(); i != n; ++i) {
-    bestNumValueWitnessesInProtocolExtensions
-      = std::min(bestNumValueWitnessesInProtocolExtensions,
-                 solutions[i].NumValueWitnessesInProtocolExtensions);
-  }
 
   // Erase any solutions with more value witnesses in protocol
   // extensions than the best.

test/decl/protocol/req/associated_type_inference_exponential.swift

@@ -0,0 +1,145 @@
+// RUN: %target-typecheck-verify-swift
+
+// Inspired by GRDB.swift.
+
+protocol P {
+  associatedtype A
+  associatedtype B
+  func f1(_: A)
+  func f2(_: A)
+  func f3(_: A)
+  func f4(_: A)
+  func f5(_: A)
+  func f6(_: A)
+  func f7(_: A)
+  func f8(_: A)
+  func f9(_: A)
+  func f10(_: A)
+  func f11(_: A)
+  func f12(_: A)
+  func f13(_: A)
+  func f14(_: A)
+  func f15(_: A)
+  func f16(_: B)
+  func f17(_: B)
+  func f18(_: B)
+  func f19(_: B)
+  func f20(_: B)
+  func f21(_: B)
+  func f22(_: B)
+  func f23(_: B)
+  func f24(_: B)
+  func f25(_: B)
+  func f26(_: B)
+  func f27(_: B)
+  func f28(_: B)
+  func f29(_: B)
+  func f30(_: B)
+  func f31(_: B)
+  func f32(_: B)
+}
+
+extension P {
+  func f1(_: A) {}
+  func f2(_: A) {}
+  func f3(_: A) {}
+  func f4(_: A) {}
+  func f5(_: A) {}
+  func f6(_: A) {}
+  func f7(_: A) {}
+  func f8(_: A) {}
+  func f9(_: A) {}
+  func f10(_: A) {}
+  func f11(_: A) {}
+  func f12(_: A) {}
+  func f13(_: A) {}
+  func f14(_: A) {}
+  func f15(_: A) {}
+  func f16(_: A) {}
+  func f17(_: A) {}
+  func f18(_: A) {}
+  func f19(_: A) {}
+  func f20(_: A) {}
+  func f21(_: A) {}
+  func f22(_: A) {}
+  func f23(_: A) {}
+  func f24(_: A) {}
+  func f25(_: A) {}
+  func f26(_: A) {}
+  func f27(_: A) {}
+  func f28(_: A) {}
+  func f29(_: A) {}
+  func f30(_: A) {}
+  func f31(_: A) {}
+  func f32(_: A) {}
+}
+
+extension P {
+  func f1(_: B) {}
+  func f2(_: B) {}
+  func f3(_: B) {}
+  func f4(_: B) {}
+  func f5(_: B) {}
+  func f6(_: B) {}
+  func f7(_: B) {}
+  func f8(_: B) {}
+  func f9(_: B) {}
+  func f10(_: B) {}
+  func f11(_: B) {}
+  func f12(_: B) {}
+  func f13(_: B) {}
+  func f14(_: B) {}
+  func f15(_: B) {}
+  func f16(_: B) {}
+  func f17(_: B) {}
+  func f18(_: B) {}
+  func f19(_: B) {}
+  func f20(_: B) {}
+  func f21(_: B) {}
+  func f22(_: B) {}
+  func f23(_: B) {}
+  func f24(_: B) {}
+  func f25(_: B) {}
+  func f26(_: B) {}
+  func f27(_: B) {}
+  func f28(_: B) {}
+  func f29(_: B) {}
+  func f30(_: B) {}
+  func f31(_: B) {}
+  func f32(_: B) {}
+}
+
+struct S: P {
+  func f1(_: Int) {}
+  func f2(_: Int) {}
+  func f3(_: Int) {}
+  func f4(_: Int) {}
+  func f5(_: Int) {}
+  func f6(_: Int) {}
+  func f7(_: Int) {}
+  func f8(_: Int) {}
+  func f9(_: Int) {}
+  func f10(_: Int) {}
+  func f11(_: Int) {}
+  func f12(_: Int) {}
+  func f13(_: Int) {}
+  func f14(_: Int) {}
+  func f15(_: Int) {}
+  func f16(_: Int) {}
+  func f17(_: Int) {}
+  func f18(_: Int) {}
+  func f19(_: Int) {}
+  func f20(_: Int) {}
+  func f21(_: Int) {}
+  func f22(_: Int) {}
+  func f23(_: Int) {}
+  func f24(_: Int) {}
+  func f25(_: Int) {}
+  func f26(_: Int) {}
+  func f27(_: Int) {}
+  func f28(_: Int) {}
+  func f29(_: Int) {}
+  func f30(_: Int) {}
+  func f31(_: Int) {}
+  func f32(_: Int) {}
+}