Merge pull request swiftlang#41773 from slavapestov/rqm-concrete-protocol-typealias-minimization

RequirementMachine: Overhaul handling of protocol typealiases with concrete underlying type

Author: slavapestov

lib/AST/RequirementMachine/GenericSignatureQueries.cpp

@@ -239,7 +239,8 @@ RequirementMachine::getLongestValidPrefix(const MutableTerm &term) const {
     case Symbol::Kind::Superclass:
     case Symbol::Kind::ConcreteType:
     case Symbol::Kind::ConcreteConformance:
-      llvm_unreachable("Property symbol cannot appear in a type term");
+      llvm::errs() <<"Invalid symbol in a type term: " << term << "\n";
+      abort();
     }
 
     // This symbol is valid, add it to the longest prefix.
@@ -265,6 +266,9 @@ bool RequirementMachine::isCanonicalTypeInContext(Type type) const {
     explicit Walker(const RequirementMachine &self) : Self(self) {}
 
     Action walkToTypePre(Type component) override {
+      if (!component->hasTypeParameter())
+        return Action::SkipChildren;
+
       if (!component->isTypeParameter())
         return Action::Continue;
 
@@ -305,6 +309,9 @@ Type RequirementMachine::getCanonicalTypeInContext(
     TypeArrayView<GenericTypeParamType> genericParams) const {
 
   return type.transformRec([&](Type t) -> Optional<Type> {
+    if (!t->hasTypeParameter())
+      return t;
+
     if (!t->isTypeParameter())
       return None;
 

lib/AST/RequirementMachine/HomotopyReduction.cpp

@@ -65,23 +65,30 @@
 using namespace swift;
 using namespace rewriting;
 
-/// Recompute Useful, RulesInEmptyContext, ProjectionCount and DecomposeCount
-/// if needed.
+/// Recompute various cached values if needed.
 void RewriteLoop::recompute(const RewriteSystem &system) {
   if (!Dirty)
     return;
   Dirty = 0;
 
+  Useful = 0;
   ProjectionCount = 0;
   DecomposeCount = 0;
-  Useful = false;
+  HasConcreteTypeAliasRule = 0;
 
   RewritePathEvaluator evaluator(Basepoint);
   for (auto step : Path) {
     switch (step.Kind) {
-    case RewriteStep::Rule:
+    case RewriteStep::Rule: {
       Useful |= (!step.isInContext() && !evaluator.isInContext());
+
+      const auto &rule = system.getRule(step.getRuleID());
+      if (rule.isProtocolTypeAliasRule() &&
+          rule.getLHS().size() == 3)
+        HasConcreteTypeAliasRule = 1;
+
       break;
+    }
 
     case RewriteStep::LeftConcreteProjection:
       ++ProjectionCount;
@@ -130,6 +137,14 @@ unsigned RewriteLoop::getDecomposeCount(
   return DecomposeCount;
 }
 
+/// Returns true if the loop contains at least one concrete protocol typealias rule,
+/// which have the form ([P].A.[concrete: C] => [P].A).
+bool RewriteLoop::hasConcreteTypeAliasRule(
+    const RewriteSystem &system) const {
+  const_cast<RewriteLoop *>(this)->recompute(system);
+  return HasConcreteTypeAliasRule;
+}
+
 /// The number of Decompose steps, used by the elimination order to prioritize
 /// loops that are not concrete simplifications.
 bool RewriteLoop::isUseful(
@@ -488,7 +503,7 @@ RewritePath::getRulesInEmptyContext(const MutableTerm &term,
 /// \p redundantConformances equal to the set of conformance rules that are
 ///    not minimal conformances.
 Optional<std::pair<unsigned, unsigned>> RewriteSystem::
-findRuleToDelete(llvm::function_ref<bool(unsigned)> isRedundantRuleFn) {
+findRuleToDelete(EliminationPredicate isRedundantRuleFn) {
   SmallVector<std::pair<unsigned, unsigned>, 2> redundancyCandidates;
   for (unsigned loopID : indices(Loops)) {
     auto &loop = Loops[loopID];
@@ -520,7 +535,10 @@ findRuleToDelete(llvm::function_ref<bool(unsigned)> isRedundantRuleFn) {
   }
 
   for (const auto &pair : redundancyCandidates) {
+    unsigned loopID = pair.first;
     unsigned ruleID = pair.second;
+
+    const auto &loop = Loops[loopID];
     const auto &rule = getRule(ruleID);
 
     // We should not find a rule that has already been marked redundant
@@ -538,18 +556,18 @@ findRuleToDelete(llvm::function_ref<bool(unsigned)> isRedundantRuleFn) {
     // Homotopy reduction runs multiple passes with different filters to
     // prioritize the deletion of certain rules ahead of others. Apply
     // the filter now.
-    if (!isRedundantRuleFn(ruleID)) {
+    if (!isRedundantRuleFn(loopID, ruleID)) {
       if (Debug.contains(DebugFlags::HomotopyReductionDetail)) {
         llvm::dbgs() << "** Skipping rule " << rule << " from loop #"
-                     << pair.first << "\n";
+                     << loopID << "\n";
       }
 
       continue;
     }
 
     if (Debug.contains(DebugFlags::HomotopyReductionDetail)) {
       llvm::dbgs() << "** Candidate rule " << rule << " from loop #"
-                   << pair.first << "\n";
+                   << loopID << "\n";
     }
 
     if (!found) {
@@ -561,7 +579,6 @@ findRuleToDelete(llvm::function_ref<bool(unsigned)> isRedundantRuleFn) {
     // we've found so far.
     const auto &otherRule = getRule(found->second);
 
-    const auto &loop = Loops[pair.first];
     const auto &otherLoop = Loops[found->first];
 
     {
@@ -712,7 +729,7 @@ void RewriteSystem::deleteRule(unsigned ruleID,
 }
 
 void RewriteSystem::performHomotopyReduction(
-    llvm::function_ref<bool(unsigned)> isRedundantRuleFn) {
+    EliminationPredicate isRedundantRuleFn) {
   while (true) {
     auto optPair = findRuleToDelete(isRedundantRuleFn);
 
@@ -803,14 +820,21 @@ void RewriteSystem::minimizeRewriteSystem() {
   // First pass:
   // - Eliminate all LHS-simplified non-conformance rules.
   // - Eliminate all RHS-simplified and substitution-simplified rules.
-  // - Eliminate all rules with unresolved symbols.
+  //
+  // An example of a conformance rule that is LHS-simplified but not
+  // RHS-simplified is (T.[P] => T) where T is irreducible, but there
+  // is a rule (V.[P] => V) for some V with T == U.V.
+  //
+  // Such conformance rules can still be minimal, as part of a hack to
+  // maintain compatibility with the GenericSignatureBuilder's minimization
+  // algorithm.
   if (Debug.contains(DebugFlags::HomotopyReduction)) {
-    llvm::dbgs() << "---------------------------------------------\n";
-    llvm::dbgs() << "First pass: simplified and unresolved rules -\n";
-    llvm::dbgs() << "---------------------------------------------\n";
+    llvm::dbgs() << "------------------------------\n";
+    llvm::dbgs() << "First pass: simplified rules -\n";
+    llvm::dbgs() << "------------------------------\n";
   }
 
-  performHomotopyReduction([&](unsigned ruleID) -> bool {
+  performHomotopyReduction([&](unsigned loopID, unsigned ruleID) -> bool {
     const auto &rule = getRule(ruleID);
 
     if (rule.isLHSSimplified() &&
@@ -821,8 +845,31 @@ void RewriteSystem::minimizeRewriteSystem() {
         rule.isSubstitutionSimplified())
       return true;
 
-    if (rule.containsUnresolvedSymbols() &&
-        !rule.isProtocolTypeAliasRule())
+    return false;
+  });
+
+  // Second pass:
+  // - Eliminate all rules with unresolved symbols which were *not*
+  //   simplified.
+  //
+  // Two examples of such rules:
+  //
+  //  - (T.X => T.[P:X]) obtained from resolving the overlap between
+  //    (T.[P] => T) and ([P].X => [P:X]).
+  //
+  // - (T.X.[concrete: C] => T.X) obtained from resolving the overlap
+  //   between (T.[P] => T) and a protocol typealias rule
+  //   ([P].X.[concrete: C] => [P].X).
+  if (Debug.contains(DebugFlags::HomotopyReduction)) {
+    llvm::dbgs() << "-------------------------------\n";
+    llvm::dbgs() << "Second pass: unresolved rules -\n";
+    llvm::dbgs() << "-------------------------------\n";
+  }
+
+  performHomotopyReduction([&](unsigned loopID, unsigned ruleID) -> bool {
+    const auto &rule = getRule(ruleID);
+
+    if (rule.containsUnresolvedSymbols())
       return true;
 
     return false;
@@ -838,14 +885,14 @@ void RewriteSystem::minimizeRewriteSystem() {
   llvm::DenseSet<unsigned> redundantConformances;
   computeMinimalConformances(redundantConformances);
 
-  // Second pass: Eliminate all non-minimal conformance rules.
+  // Third pass: Eliminate all non-minimal conformance rules.
   if (Debug.contains(DebugFlags::HomotopyReduction)) {
-    llvm::dbgs() << "--------------------------------------------\n";
-    llvm::dbgs() << "Second pass: non-minimal conformance rules -\n";
-    llvm::dbgs() << "--------------------------------------------\n";
+    llvm::dbgs() << "-------------------------------------------\n";
+    llvm::dbgs() << "Third pass: non-minimal conformance rules -\n";
+    llvm::dbgs() << "-------------------------------------------\n";
   }
 
-  performHomotopyReduction([&](unsigned ruleID) -> bool {
+  performHomotopyReduction([&](unsigned loopID, unsigned ruleID) -> bool {
     const auto &rule = getRule(ruleID);
 
     if (rule.isAnyConformanceRule() &&
@@ -855,17 +902,22 @@ void RewriteSystem::minimizeRewriteSystem() {
     return false;
   });
 
-  // Third pass: Eliminate all other redundant non-conformance rules.
+  // Fourth pass: Eliminate all remaining redundant non-conformance rules.
   if (Debug.contains(DebugFlags::HomotopyReduction)) {
-    llvm::dbgs() << "---------------------------------------\n";
-    llvm::dbgs() << "Third pass: all other redundant rules -\n";
-    llvm::dbgs() << "---------------------------------------\n";
+    llvm::dbgs() << "----------------------------------------\n";
+    llvm::dbgs() << "Fourth pass: all other redundant rules -\n";
+    llvm::dbgs() << "----------------------------------------\n";
   }
 
-  performHomotopyReduction([&](unsigned ruleID) -> bool {
+  performHomotopyReduction([&](unsigned loopID, unsigned ruleID) -> bool {
+    const auto &loop = Loops[loopID];
     const auto &rule = getRule(ruleID);
 
-    if (!rule.isAnyConformanceRule())
+    if (rule.isProtocolTypeAliasRule())
+      return true;
+
+    if (!loop.hasConcreteTypeAliasRule(*this) &&
+        !rule.isAnyConformanceRule())
       return true;
 
     return false;

lib/AST/RequirementMachine/InterfaceType.cpp

@@ -311,7 +311,8 @@ getTypeForSymbolRange(const Symbol *begin, const Symbol *end, Type root,
       case Symbol::Kind::Superclass:
       case Symbol::Kind::ConcreteType:
       case Symbol::Kind::ConcreteConformance:
-        llvm_unreachable("Term has invalid root symbol");
+        llvm::errs() << "Invalid root symbol: " << MutableTerm(begin, end) << "\n";
+        abort();
       }
     }
 

lib/AST/RequirementMachine/MinimalConformances.cpp

@@ -365,7 +365,8 @@ static const ProtocolDecl *getParentConformanceForTerm(Term lhs) {
     break;
   }
 
-  llvm_unreachable("Bad symbol kind");
+  llvm::errs() << "Bad symbol in " << lhs << "\n";
+  abort();
 }
 
 /// Collect conformance rules and parent paths, and record an initial

lib/AST/RequirementMachine/RewriteContext.h

@@ -55,9 +55,6 @@ class RewriteContext final {
   /// Cache for associated type declarations.
   llvm::DenseMap<Symbol, AssociatedTypeDecl *> AssocTypes;
 
-  /// Cache for merged associated type symbols.
-  llvm::DenseMap<std::pair<Symbol, Symbol>, Symbol> MergedAssocTypes;
-
   /// Requirement machines built from generic signatures.
   llvm::DenseMap<GenericSignature, RequirementMachine *> Machines;
 

lib/AST/RequirementMachine/RewriteLoop.h

@@ -457,6 +457,9 @@ class RewriteLoop {
   /// Cached value for getDecomposeCount().
   unsigned DecomposeCount : 15;
 
+  /// Cached value for hasConcreteTypeAliasRule().
+  unsigned HasConcreteTypeAliasRule : 1;
+
   /// A useful loop contains at least one rule in empty context, even if that
   /// rule appears multiple times or also in non-empty context. The only loops
   /// that are elimination candidates contain a rule in empty context *exactly
@@ -478,6 +481,7 @@ class RewriteLoop {
     : Basepoint(basepoint), Path(path) {
     ProjectionCount = 0;
     DecomposeCount = 0;
+    HasConcreteTypeAliasRule = 0;
     Useful = 0;
     Deleted = 0;
 
@@ -509,6 +513,8 @@ class RewriteLoop {
 
   unsigned getDecomposeCount(const RewriteSystem &system) const;
 
+  bool hasConcreteTypeAliasRule(const RewriteSystem &system) const;
+
   void findProtocolConformanceRules(
       llvm::SmallDenseMap<const ProtocolDecl *,
                           ProtocolConformanceRules, 2> &result,

lib/AST/RequirementMachine/RewriteSystem.cpp

@@ -174,8 +174,13 @@ Optional<Identifier> Rule::isProtocolTypeAliasRule() const {
     //
     // We shouldn't have unresolved symbols on the right hand side;
     // they should have been simplified away.
-    if (RHS.containsUnresolvedSymbols())
-      return None;
+    if (RHS.containsUnresolvedSymbols()) {
+      if (RHS.size() != 2 ||
+          RHS[0] != LHS[0] ||
+          RHS[1].getKind() != Symbol::Kind::Name) {
+        return None;
+      }
+    }
   } else {
     // This is the case where the underlying type is concrete.
     assert(LHS.size() == 3);
@@ -660,6 +665,28 @@ void RewriteSystem::verifyRewriteRules(ValidityPolicy policy) const {
     for (unsigned index : indices(lhs)) {
       auto symbol = lhs[index];
 
+      // The left hand side can contain a single name symbol if it has the form
+      // T.N or T.N.[p], where T is some prefix that does not contain name
+      // symbols, N is a name symbol, and [p] is an optional property symbol.
+      //
+      // In the latter case, we have a protocol typealias, or a rule derived
+      // via resolving a critical pair involving a protocol typealias.
+      //
+      // Any other valid occurrence of a name symbol should have been reduced by
+      // an associated type introduction rule [P].N, marking the rule as
+      // LHS-simplified.
+      if (!rule.isLHSSimplified() &&
+          (rule.isPropertyRule()
+           ? index != lhs.size() - 2
+           : index != lhs.size() - 1)) {
+        // This is only true if the input requirements were valid.
+        if (policy == DisallowInvalidRequirements) {
+          ASSERT_RULE(symbol.getKind() != Symbol::Kind::Name);
+        } else {
+          // FIXME: Assert that we diagnosed an error
+        }
+      }
+
       if (index != lhs.size() - 1) {
         ASSERT_RULE(symbol.getKind() != Symbol::Kind::Layout);
         ASSERT_RULE(!symbol.hasSubstitutions());
@@ -677,14 +704,18 @@ void RewriteSystem::verifyRewriteRules(ValidityPolicy policy) const {
     for (unsigned index : indices(rhs)) {
       auto symbol = rhs[index];
 
-      // RHS-simplified rules might have unresolved name symbols on the
-      // right hand side. Also, completion can introduce rules of the
-      // form T.X.[concrete: C] => T.X, where T is some resolved term,
-      // and X is a name symbol for a protocol typealias.
-      if (!rule.isLHSSimplified() &&
-          !rule.isRHSSimplified() &&
-          !(rule.isPropertyRule() &&
-            index == rhs.size() - 1)) {
+      // The right hand side can contain a single name symbol if it has the form
+      // T.N, where T is some prefix that does not contain name symbols, and
+      // N is a name symbol.
+      //
+      // In this case, we have a protocol typealias, or a rule derived via
+      // resolving a critical pair involving a protocol typealias.
+      //
+      // Any other valid occurrence of a name symbol should have been reduced by
+      // an associated type introduction rule [P].N, marking the rule as
+      // RHS-simplified.
+      if (!rule.isRHSSimplified() &&
+          index != rhs.size() - 1) {
         // This is only true if the input requirements were valid.
         if (policy == DisallowInvalidRequirements) {
           ASSERT_RULE(symbol.getKind() != Symbol::Kind::Name);

lib/AST/RequirementMachine/RewriteSystem.h

@@ -506,13 +506,15 @@ class RewriteSystem final {
 
   void processConflicts();
 
+  using EliminationPredicate = llvm::function_ref<bool(unsigned loopID,
+                                                       unsigned ruleID)>;
+
   Optional<std::pair<unsigned, unsigned>>
-  findRuleToDelete(llvm::function_ref<bool(unsigned)> isRedundantRuleFn);
+  findRuleToDelete(EliminationPredicate isRedundantRuleFn);
 
   void deleteRule(unsigned ruleID, const RewritePath &replacementPath);
 
-  void performHomotopyReduction(
-      llvm::function_ref<bool(unsigned)> isRedundantRuleFn);
+  void performHomotopyReduction(EliminationPredicate isRedundantRuleFn);
 
   void computeMinimalConformances(
       llvm::DenseSet<unsigned> &redundantConformances);