Merge pull request #13056 from xwu/much-of-a-muchness

[stdlib/Sema/AST] Adopt a commonly used implementation for combining hashes

Author: lorentey

include/swift/AST/ASTContext.h

@@ -482,8 +482,8 @@ class ASTContext {
   FuncDecl *getEqualIntDecl() const;
 
   /// Retrieve the declaration of
-  /// Swift._mixForSynthesizedHashValue (Int, Int) -> Int.
-  FuncDecl *getMixForSynthesizedHashValueDecl() const;
+  /// Swift._combineHashValues(Int, Int) -> Int.
+  FuncDecl *getCombineHashValuesDecl() const;
 
   /// Retrieve the declaration of Swift._mixInt(Int) -> Int.
   FuncDecl *getMixIntDecl() const;

lib/AST/ASTContext.cpp

@@ -194,8 +194,8 @@ FOR_KNOWN_FOUNDATION_TYPES(CACHE_FOUNDATION_DECL)
   /// func ==(Int, Int) -> Bool
   FuncDecl *EqualIntDecl = nullptr;
 
-  /// func _mixForSynthesizedHashValue(Int, Int) -> Int
-  FuncDecl *MixForSynthesizedHashValueDecl = nullptr;
+  /// func _combineHashValues(Int, Int) -> Int
+  FuncDecl *CombineHashValuesDecl = nullptr;
 
   /// func _mixInt(Int) -> Int
   FuncDecl *MixIntDecl = nullptr;
@@ -1048,9 +1048,9 @@ FuncDecl *ASTContext::getGetBoolDecl(LazyResolver *resolver) const {
   return decl;
 }
 
-FuncDecl *ASTContext::getMixForSynthesizedHashValueDecl() const {
-  if (Impl.MixForSynthesizedHashValueDecl)
-    return Impl.MixForSynthesizedHashValueDecl;
+FuncDecl *ASTContext::getCombineHashValuesDecl() const {
+  if (Impl.CombineHashValuesDecl)
+    return Impl.CombineHashValuesDecl;
 
   auto resolver = getLazyResolver();
   auto intType = getIntDecl()->getDeclaredType();
@@ -1066,8 +1066,8 @@ FuncDecl *ASTContext::getMixForSynthesizedHashValueDecl() const {
   };
 
   auto decl = lookupLibraryIntrinsicFunc(
-      *this, "_mixForSynthesizedHashValue", resolver, callback);
-  Impl.MixForSynthesizedHashValueDecl = decl;
+      *this, "_combineHashValues", resolver, callback);
+  Impl.CombineHashValuesDecl = decl;
   return decl;
 }
 

lib/Sema/DerivedConformanceEquatableHashable.cpp

@@ -745,34 +745,34 @@ static Expr* integerLiteralExpr(ASTContext &C, int64_t value) {
   return integerExpr;
 }
 
-/// Returns a new assignment expression that mixes the hash value of an
+/// Returns a new assignment expression that combines the hash value of an
 /// expression into a variable.
 /// \p C The AST context.
-/// \p resultVar The variable into which the hash value will be mixed.
-/// \p exprToHash The expression whose hash value should be mixed in.
-/// \return The expression that mixes the hash value into the result variable.
-static Expr* mixInHashExpr_hashValue(ASTContext &C,
-                                     VarDecl* resultVar,
-                                     Expr *exprToHash) {
+/// \p resultVar The variable into which the hash value will be combined.
+/// \p exprToHash The expression whose hash value should be combined.
+/// \return The expression that combines the hash value into the variable.
+static Expr* combineHashValuesAssignmentExpr(ASTContext &C,
+                                             VarDecl* resultVar,
+                                             Expr *exprToHash) {
   // <exprToHash>.hashValue
   auto hashValueExpr = new (C) UnresolvedDotExpr(exprToHash, SourceLoc(),
                                                  C.Id_hashValue, DeclNameLoc(),
                                                  /*implicit*/ true);
 
-  // _mixForSynthesizedHashValue(result, <exprToHash>.hashValue)
-  auto mixinFunc = C.getMixForSynthesizedHashValueDecl();
-  auto mixinFuncExpr = new (C) DeclRefExpr(mixinFunc, DeclNameLoc(),
-                                           /*implicit*/ true);
+  // _combineHashValues(result, <exprToHash>.hashValue)
+  auto combineFunc = C.getCombineHashValuesDecl();
+  auto combineFuncExpr = new (C) DeclRefExpr(combineFunc, DeclNameLoc(),
+                                             /*implicit*/ true);
   auto rhsResultExpr = new (C) DeclRefExpr(resultVar, DeclNameLoc(),
                                            /*implicit*/ true);
-  auto mixinResultExpr = CallExpr::createImplicit(
-      C, mixinFuncExpr, { rhsResultExpr, hashValueExpr }, {});
+  auto combineResultExpr = CallExpr::createImplicit(
+    C, combineFuncExpr, { rhsResultExpr, hashValueExpr }, {});
 
-  // result = _mixForSynthesizedHashValue(result, <exprToHash>.hashValue)
+  // result = _combineHashValues(result, <exprToHash>.hashValue)
   auto lhsResultExpr = new (C) DeclRefExpr(resultVar, DeclNameLoc(),
                                            /*implicit*/ true);
   auto assignExpr = new (C) AssignExpr(lhsResultExpr, SourceLoc(),
-                                       mixinResultExpr, /*implicit*/ true);
+                                       combineResultExpr, /*implicit*/ true);
   return assignExpr;
 }
 
@@ -853,9 +853,9 @@ deriveBodyHashable_enum_hashValue(AbstractFunctionDecl *hashValueDecl) {
 
     // If the enum has no associated values, we use the ordinal alone as the
     // hash value, because that is sufficient for a good distribution. If any
-    // case do have associated values, then the ordinal is used as the first
-    // term mixed into _mixForSynthesizedHashValue, and the final result after
-    // mixing in the payload is passed to _mixInt to improve the distribution.
+    // case does have associated values, then the ordinal is used as the first
+    // term combined into _combineHashValues, and the final result after
+    // combining the payload is passed to _mixInt to improve the distribution.
 
     // result = <ordinal>
     {
@@ -868,13 +868,14 @@ deriveBodyHashable_enum_hashValue(AbstractFunctionDecl *hashValueDecl) {
     }
 
     if (!hasNoAssociatedValues) {
-      // Generate a sequence of expressions that mix the payload's hash values
-      // into result.
+      // Generate a sequence of expressions that combine the payload's hash
+      // values into result.
       for (auto payloadVar : payloadVars) {
         auto payloadVarRef = new (C) DeclRefExpr(payloadVar, DeclNameLoc(),
                                                  /*implicit*/ true);
-        // result = _mixForSynthesizedHashValue(result, <payloadVar>.hashValue)
-        auto mixExpr = mixInHashExpr_hashValue(C, resultVar, payloadVarRef);
+        // result = _combineHashValues(result, <payloadVar>.hashValue)
+        auto mixExpr = combineHashValuesAssignmentExpr(C, resultVar,
+                                                       payloadVarRef);
         mixExpressions.emplace_back(ASTNode(mixExpr));
       }
 
@@ -953,7 +954,7 @@ deriveBodyHashable_struct_hashValue(AbstractFunctionDecl *hashValueDecl) {
   auto storedProperties =
     structDecl->getStoredProperties(/*skipInaccessible=*/true);
 
-  // For each stored property, generate a statement that mixes its hash value
+  // For each stored property, generate a statement that combines its hash value
   // into the result.
   for (auto propertyDecl : storedProperties) {
     auto propertyRef = new (C) DeclRefExpr(propertyDecl, DeclNameLoc(),
@@ -962,8 +963,9 @@ deriveBodyHashable_struct_hashValue(AbstractFunctionDecl *hashValueDecl) {
                                        /*implicit*/ true);
     auto selfPropertyExpr = new (C) DotSyntaxCallExpr(propertyRef, SourceLoc(),
                                                       selfRef);
-    // result = _mixForSynthesizedHashValue(result, <property>.hashValue)
-    auto mixExpr = mixInHashExpr_hashValue(C, resultVar, selfPropertyExpr);
+    // result = _combineHashValues(result, <property>.hashValue)
+    auto mixExpr = combineHashValuesAssignmentExpr(C, resultVar,
+                                                   selfPropertyExpr);
     statements.emplace_back(ASTNode(mixExpr));
   }
 
@@ -1014,11 +1016,11 @@ deriveHashable_hashValue(TypeChecker &tc, Decl *parentDecl,
   //       result = 0
   //     case B(let a0):
   //       result = 1
-  //       result = _mixForSynthesizedHashValue(result, a0.hashValue)
+  //       result = _combineHashValues(result, a0.hashValue)
   //     case C(let a0, let a1):
   //       result = 2
-  //       result = _mixForSynthesizedHashValue(result, a0.hashValue)
-  //       result = _mixForSynthesizedHashValue(result, a1.hashValue)
+  //       result = _combineHashValues(result, a0.hashValue)
+  //       result = _combineHashValues(result, a1.hashValue)
   //     }
   //     result = _mixInt(result)
   //     return result
@@ -1030,8 +1032,8 @@ deriveHashable_hashValue(TypeChecker &tc, Decl *parentDecl,
   //   var y: String
   //   @derived var hashValue: Int {
   //     var result: Int = 0
-  //     result = _mixForSynthesizedHashValue(result, x.hashValue)
-  //     result = _mixForSynthesizedHashValue(result, y.hashValue)
+  //     result = _combineHashValues(result, x.hashValue)
+  //     result = _combineHashValues(result, y.hashValue)
   //     result = _mixInt(result)
   //     return result
   //   }

stdlib/public/core/Hashing.swift

@@ -188,13 +188,21 @@ func _squeezeHashValue(_ hashValue: Int, _ upperBound: Int) -> Int {
 
 /// Returns a new value that combines the two given hash values.
 ///
+/// Combining is performed using [a hash function][ref] described by T.C. Hoad
+/// and J. Zobel, which is also adopted in the Boost C++ libraries.
+///
 /// This function is used by synthesized implementations of `hashValue` to
 /// combine the hash values of individual `struct` fields and associated values
 /// of `enum`s. It is factored out into a standard library function so that the
 /// specific hashing logic can be refined without requiring major changes to the
 /// code that creates the synthesized AST nodes.
+///
+/// [ref]: http://goanna.cs.rmit.edu.au/~jz/fulltext/jasist-tch.pdf
 @_transparent
 public // @testable
-func _mixForSynthesizedHashValue(_ oldValue: Int, _ nextValue: Int) -> Int {
-  return 31 &* oldValue &+ nextValue
+func _combineHashValues(_ firstValue: Int, _ secondValue: Int) -> Int {
+  let magic = 0x9e3779b9 as UInt // Based on the golden ratio.
+  var x = UInt(bitPattern: firstValue)
+  x ^= UInt(bitPattern: secondValue) &+ magic &+ (x &<< 6) &+ (x &>> 2)
+  return Int(bitPattern: x)
 }