RequirementMachine: Completion depth limit

Author: slavapestov

include/swift/AST/RewriteSystem.h

@@ -192,6 +192,10 @@ class Rule final {
     deleted = true;
   }
 
+  unsigned getDepth() const {
+    return LHS.size();
+  }
+
   void dump(llvm::raw_ostream &out) const;
 };
 
@@ -212,7 +216,15 @@ class RewriteSystem final {
 
   bool simplify(Term &term) const;
 
-  void computeConfluentCompletion(unsigned maxIterations);
+  enum class CompletionResult {
+    Success,
+    MaxIterations,
+    MaxDepth
+  };
+
+  CompletionResult computeConfluentCompletion(
+      unsigned maxIterations,
+      unsigned maxDepth);
 
   void dump(llvm::raw_ostream &out) const;
 };

lib/AST/RequirementMachine.cpp

@@ -338,6 +338,10 @@ RequirementMachine::~RequirementMachine() {
 void RequirementMachine::addGenericSignature(CanGenericSignature sig) {
   PrettyStackTraceGenericSignature debugStack("building rewrite system for", sig);
 
+  auto *Stats = Context.Stats;
+
+  FrontendStatsTracer(Stats, "build-rewrite-system");
+
   if (Context.LangOpts.DebugRequirementMachine) {
     llvm::dbgs() << "Adding generic signature " << sig << " {\n";
   }
@@ -351,7 +355,24 @@ void RequirementMachine::addGenericSignature(CanGenericSignature sig) {
     Impl->System.addRule(rule.first, rule.second);
 
   // FIXME: Add command line flag
-  Impl->System.computeConfluentCompletion(10000);
+  auto result = Impl->System.computeConfluentCompletion(5000, 10);
+
+  switch (result) {
+  case RewriteSystem::CompletionResult::Success:
+    break;
+
+  case RewriteSystem::CompletionResult::MaxIterations:
+    llvm::errs() << "Generic signature " << sig
+                 << " exceeds maximum completion step count\n";
+    break;
+    // abort();
+
+  case RewriteSystem::CompletionResult::MaxDepth:
+    llvm::errs() << "Generic signature " << sig
+                 << " exceeds maximum completion depth\n";
+    break;
+    // abort();
+  }
 
   markComplete();
 

lib/AST/RewriteSystem.cpp

@@ -246,8 +246,10 @@ bool RewriteSystem::simplify(Term &term) const {
   return changed;
 }
 
-void RewriteSystem::computeConfluentCompletion(
-    unsigned maxIterations) {
+RewriteSystem::CompletionResult
+RewriteSystem::computeConfluentCompletion(
+    unsigned maxIterations,
+    unsigned maxDepth) {
   SmallVector<std::pair<unsigned, unsigned>, 16> worklist;
 
   for (unsigned i : indices(Rules)) {
@@ -286,15 +288,14 @@ void RewriteSystem::computeConfluentCompletion(
     if (!addRule(first, second))
       continue;
 
-    if (maxIterations == 0) {
-      dump(llvm::errs());
-      llvm::errs() << "Completion procedure exceeded max iteration count\n";
-      abort();
-    }
+    if (maxIterations == 0)
+      return CompletionResult::MaxIterations;
 
     maxIterations--;
 
     const auto &newRule = Rules[i];
+    if (newRule.getDepth() > maxDepth)
+      return CompletionResult::MaxDepth;
 
     for (unsigned j : indices(Rules)) {
       if (i == j)
@@ -317,6 +318,8 @@ void RewriteSystem::computeConfluentCompletion(
         rule.markDeleted();
     }
   }
+
+  return CompletionResult::Success;
 }
 
 void RewriteSystem::dump(llvm::raw_ostream &out) const {