Fused modulo-equality lane; admit const bindings to comparison-lane reads

Jint.Tests/Runtime/EqualityLaneTests.cs

@@ -119,6 +119,116 @@ public void GlobalLaneDeclinesWhenGlobalRedefinedAsAccessor()
         Assert.Equal("4:2", result);
     }
 
+    [Fact]
+    public void FusedModuloEqualityMatchesGenericPathAcrossSigns()
+    {
+        var engine = new Engine();
+        // negative dividends produce negative (or -0) remainders; the equality consumer
+        // erases the ±0 distinction, so the fused raw-double form must agree everywhere
+        var result = engine.Evaluate("""
+            (function () {
+                var r = [];
+                for (var i = -3; i <= 3; i++) {
+                    r.push(i % 2 == 0, i % 2 === 0, i % 2 != 0, i % 2 !== 0, i % 2 == -1, i % -2 == 1);
+                }
+                return r.join(',');
+            })()
+            """).AsString();
+
+        var expected = string.Join(",",
+            "false,false,true,true,true,false",   // i = -3 → -1
+            "true,true,false,false,false,false",  // i = -2 → -0
+            "false,false,true,true,true,false",   // i = -1 → -1
+            "true,true,false,false,false,false",  // i = 0  → 0
+            "false,false,true,true,false,true",   // i = 1  → 1
+            "true,true,false,false,false,false",  // i = 2  → 0
+            "false,false,true,true,false,true");  // i = 3  → 1
+        Assert.Equal(expected, result);
+    }
+
+    [Fact]
+    public void FusedModuloHandlesSpecialNumbersAndDeclines()
+    {
+        var engine = new Engine();
+        var result = engine.Evaluate("""
+            (function () {
+                var nan = NaN, five = 5, str = '4';
+                return [
+                    nan % 2 == 0, nan % 2 != 0,
+                    five % 0 == 0, five % 0 != 0,
+                    five % Infinity == 5,
+                    str % 2 == 0
+                ].join(',');
+            })()
+            """).AsString();
+
+        Assert.Equal("false,true,false,true,true,true", result);
+    }
+
+    [Fact]
+    public void FusedModuloWorksOnGlobals()
+    {
+        var engine = new Engine();
+        // the stopwatch.js if-chain shape at script top level (globals, not slots)
+        var result = engine.Evaluate("""
+            var hits = 0;
+            for (var x = 0; x < 8; x++) {
+                var z = x ^ 1;
+                if (z % 2 == 0) { hits++; }
+                else if (z % 3 == 0) { hits += 10; }
+            }
+            hits;
+            """).AsNumber();
+
+        Assert.Equal(14, result);
+    }
+
+    [Fact]
+    public void ConstBindingsTakeTheLaneShapes()
+    {
+        var engine = new Engine();
+        // the stopwatch-modern shape: const bindings inside the loop body flow through the
+        // relational, equality and fused-modulo lanes (immutable bindings are readable)
+        var result = engine.Evaluate("""
+            (function () {
+                let hits = 0;
+                for (let x = 0; x < 8; x++) {
+                    const z = x ^ 1;
+                    if (z % 2 == 0) { hits++; }
+                    else if (z % 3 === 0) { hits += 10; }
+                    const lim = 4;
+                    if (z < lim) { hits += 100; }
+                }
+                return hits;
+            })()
+            """).AsNumber();
+
+        Assert.Equal(414, result); // 4 even-z hits + one z=3 (+10) + four z<4 (+400)
+    }
+
+    [Fact]
+    public void TdzReadStillThrowsThroughLaneShapes()
+    {
+        var engine = new Engine();
+        // an uninitialized const slot has neither an unboxed number nor a reference value,
+        // so the lane declines and the generic path raises the ReferenceError
+        var result = engine.Evaluate("""
+            (function () {
+                try {
+                    for (let i = 0; i < 2; i++) {
+                        if (z === 0) { }
+                        const z = i;
+                    }
+                    return 'no-throw';
+                } catch (e) {
+                    return e instanceof ReferenceError ? 'ReferenceError' : 'other';
+                }
+            })()
+            """).AsString();
+
+        Assert.Equal("ReferenceError", result);
+    }
+
     [Fact]
     public void GlobalLaneDeclinesWhenValueTypeChangesInPlace()
     {

Jint/Runtime/Environments/DeclarativeEnvironment.cs

@@ -90,6 +90,38 @@ internal bool TryGetNumberSlot(int slotIndex, out double value)
         return false;
     }
 
+    /// <summary>
+    /// Read-only sibling of <see cref="TryGetNumberSlot"/> for the comparison lanes: admits
+    /// immutable bindings too (a `const` number is the canonical loop-body operand). Uninitialized
+    /// (TDZ) bindings have neither an unboxed number nor a reference value, so they decline here
+    /// and the generic path raises the ReferenceError. Never pair with <see cref="SetNumberSlot"/>.
+    /// </summary>
+    internal bool TryGetNumberSlotForRead(int slotIndex, out double value)
+    {
+        var slots = _slots;
+        if (slots is null || (uint) slotIndex >= (uint) slots.Length)
+        {
+            value = 0;
+            return false;
+        }
+
+        ref var binding = ref slots[slotIndex];
+        if (binding.IsUnboxedNumber)
+        {
+            value = binding.UnboxedNumber;
+            return true;
+        }
+
+        if (binding.HasReferenceValue && binding.Value is JsNumber number)
+        {
+            value = number._value;
+            return true;
+        }
+
+        value = 0;
+        return false;
+    }
+
     /// <summary>
     /// Stores a raw number into a slot previously validated by <see cref="TryGetNumberSlot"/>
     /// without materializing a JsNumber.

Jint/Runtime/Interpreter/Expressions/JintBinaryExpression.cs

@@ -88,7 +88,7 @@ public bool TryGetOperands(EvaluationContext context, out double left, out doubl
 
             var env = engine.ExecutionContext.LexicalEnvironment;
             if (!_slotCache.TryResolve(engine, env, identifier.Identifier, out var environment, out var slotIndex)
-                || !environment.TryGetNumberSlot(slotIndex, out left))
+                || !environment.TryGetNumberSlotForRead(slotIndex, out left))
             {
                 // top-level vars are global-object properties, never slots: fall back to the
                 // validated global-descriptor cache (the stopwatch-shape loop test). The null
@@ -106,31 +106,91 @@ public bool TryGetOperands(EvaluationContext context, out double left, out doubl
             }
 
             if (_rightSlotCache.TryResolve(engine, env, rightIdentifier.Identifier, out var rightEnvironment, out var rightSlotIndex)
-                && rightEnvironment.TryGetNumberSlot(rightSlotIndex, out right))
+                && rightEnvironment.TryGetNumberSlotForRead(rightSlotIndex, out right))
             {
                 return true;
             }
 
             return rightIdentifier._cachedGlobalEnv is not null && TryReadGlobalNumber(rightIdentifier, engine, env, out right);
         }
+    }
 
-        /// <summary>
-        /// Global-binding arm of the lane operand read: validated-cache probe plus a field read,
-        /// both pure — so a decline (or a left read followed by a right-side decline) has no
-        /// observable effect. The value type is re-checked on every read because in-place value
-        /// mutations bump no version. Out of line so lane misses don't grow the inlined probe.
-        /// </summary>
-        [MethodImpl(MethodImplOptions.NoInlining)]
-        private static bool TryReadGlobalNumber(JintIdentifierExpression identifier, Engine engine, Environments.Environment env, out double value)
+    /// <summary>
+    /// Global-binding arm of the lane operand reads: validated-cache probe plus a field read,
+    /// both pure — so a decline (or a left read followed by a right-side decline) has no
+    /// observable effect. The value type is re-checked on every read because in-place value
+    /// mutations bump no version. Out of line so lane misses don't grow the inlined probes.
+    /// Callers pre-check <see cref="JintIdentifierExpression._cachedGlobalEnv"/> for null.
+    /// </summary>
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private protected static bool TryReadGlobalNumber(JintIdentifierExpression identifier, Engine engine, Environments.Environment env, out double value)
+    {
+        if (identifier.TryGetValidatedGlobalDescriptor(engine, env) is { _value: JsNumber number })
         {
-            if (identifier.TryGetValidatedGlobalDescriptor(engine, env) is { _value: JsNumber number })
+            value = number._value;
+            return true;
+        }
+
+        value = 0;
+        return false;
+    }
+
+    /// <summary>
+    /// Fused lane for the `identifier % numericConstant == numericConstant` test — the
+    /// stopwatch.js if-chain shape. Reads the identifier through the same slot/global arms as
+    /// <see cref="NumericConstantComparisonLane"/> and computes the whole test on raw doubles.
+    /// C# double % implements Number::remainder exactly, and the ways integer and double
+    /// remainders can disagree (−0 vs +0 results) are erased by the equality consumer
+    /// (IEEE == treats ±0 as equal), so the fused result is spec-exact for any proven-Number
+    /// operand: NaN dividends/divisors (and % 0) compare false, v % ±Infinity == v.
+    /// </summary>
+    private protected struct ModuloEqualityLane
+    {
+        private JintIdentifierExpression? _identifier;
+        private double _divisor;
+        private double _expected;
+        private SlotLocationCache _slotCache;
+
+        public void Initialize(JintExpression left, JintExpression right)
+        {
+            if (left is ModuloBinaryExpression { _left: JintIdentifierExpression identifier, _right: JintConstantExpression { Value: JsNumber divisor } }
+                && right is JintConstantExpression { Value: JsNumber expected })
             {
-                value = number._value;
-                return true;
+                _identifier = identifier;
+                _divisor = divisor._value;
+                _expected = expected._value;
             }
+        }
 
-            value = 0;
-            return false;
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public bool TryEvaluate(EvaluationContext context, out bool equal)
+        {
+            equal = false;
+
+            var identifier = _identifier;
+            if (identifier is null || context.OperatorOverloadingAllowed)
+            {
+                return false;
+            }
+
+            var engine = context.Engine;
+            if (engine.ExecutionContext.Suspendable is not null)
+            {
+                return false;
+            }
+
+            var env = engine.ExecutionContext.LexicalEnvironment;
+            if (!_slotCache.TryResolve(engine, env, identifier.Identifier, out var environment, out var slotIndex)
+                || !environment.TryGetNumberSlotForRead(slotIndex, out var value))
+            {
+                if (identifier._cachedGlobalEnv is null || !TryReadGlobalNumber(identifier, engine, env, out value))
+                {
+                    return false;
+                }
+            }
+
+            equal = value % _divisor == _expected;
+            return true;
         }
     }
 
@@ -398,10 +458,12 @@ internal static void ValidateBigIntPowSize(Realm realm, BigInteger baseValue, in
     private sealed class StrictlyEqualBinaryExpression : JintBinaryExpression
     {
         private NumericConstantComparisonLane _numericLane;
+        private ModuloEqualityLane _moduloLane;
 
         public StrictlyEqualBinaryExpression(NonLogicalBinaryExpression expression) : base(expression)
         {
             _numericLane.Initialize(_left, _right);
+            _moduloLane.Initialize(_left, _right);
         }
 
         protected override object EvaluateInternal(EvaluationContext context)
@@ -411,6 +473,11 @@ protected override object EvaluateInternal(EvaluationContext context)
                 return unboxedLeft == unboxedRight ? JsBoolean.True : JsBoolean.False;
             }
 
+            if (_moduloLane.TryEvaluate(context, out var moduloEqual))
+            {
+                return moduloEqual ? JsBoolean.True : JsBoolean.False;
+            }
+
             if (!TryEvaluateOperands(context, out var left, out var right))
             {
                 return JsValue.Undefined;
@@ -427,6 +494,11 @@ public override bool GetBooleanValue(EvaluationContext context)
                 return unboxedLeft == unboxedRight;
             }
 
+            if (_moduloLane.TryEvaluate(context, out var moduloEqual))
+            {
+                return moduloEqual;
+            }
+
             if (!TryEvaluateOperands(context, out var left, out var right))
             {
                 return false;
@@ -439,10 +511,12 @@ public override bool GetBooleanValue(EvaluationContext context)
     private sealed class StrictlyNotEqualBinaryExpression : JintBinaryExpression
     {
         private NumericConstantComparisonLane _numericLane;
+        private ModuloEqualityLane _moduloLane;
 
         public StrictlyNotEqualBinaryExpression(NonLogicalBinaryExpression expression) : base(expression)
         {
             _numericLane.Initialize(_left, _right);
+            _moduloLane.Initialize(_left, _right);
         }
 
         protected override object EvaluateInternal(EvaluationContext context)
@@ -452,6 +526,11 @@ protected override object EvaluateInternal(EvaluationContext context)
                 return unboxedLeft == unboxedRight ? JsBoolean.False : JsBoolean.True;
             }
 
+            if (_moduloLane.TryEvaluate(context, out var moduloEqual))
+            {
+                return moduloEqual ? JsBoolean.False : JsBoolean.True;
+            }
+
             if (!TryEvaluateOperands(context, out var left, out var right))
             {
                 return JsValue.Undefined;
@@ -467,6 +546,11 @@ public override bool GetBooleanValue(EvaluationContext context)
                 return unboxedLeft != unboxedRight;
             }
 
+            if (_moduloLane.TryEvaluate(context, out var moduloEqual))
+            {
+                return !moduloEqual;
+            }
+
             if (!TryEvaluateOperands(context, out var left, out var right))
             {
                 return false;
@@ -848,11 +932,13 @@ private sealed class EqualBinaryExpression : JintBinaryExpression
     {
         private readonly bool _invert;
         private NumericConstantComparisonLane _numericLane;
+        private ModuloEqualityLane _moduloLane;
 
         public EqualBinaryExpression(NonLogicalBinaryExpression expression, bool invert = false) : base(expression)
         {
             _invert = invert;
             _numericLane.Initialize(_left, _right);
+            _moduloLane.Initialize(_left, _right);
         }
 
         protected override object EvaluateInternal(EvaluationContext context)
@@ -862,6 +948,11 @@ protected override object EvaluateInternal(EvaluationContext context)
                 return (unboxedLeft == unboxedRight) == !_invert ? JsBoolean.True : JsBoolean.False;
             }
 
+            if (_moduloLane.TryEvaluate(context, out var moduloEqual))
+            {
+                return moduloEqual == !_invert ? JsBoolean.True : JsBoolean.False;
+            }
+
             if (!TryEvaluateOperands(context, out var left, out var right))
             {
                 return JsValue.Undefined;
@@ -889,6 +980,11 @@ public override bool GetBooleanValue(EvaluationContext context)
                 return _invert ? !equal : equal;
             }
 
+            if (_moduloLane.TryEvaluate(context, out var moduloEqual))
+            {
+                return _invert ? !moduloEqual : moduloEqual;
+            }
+
             if (!TryEvaluateOperands(context, out var left, out var right))
             {
                 return false;