Proof of concept for NaN boxing (#54)

* Proof of concept implementation of NaN boxing

* Added note about how to contribute

* Added homeworks post

* Fixed windows build

* Fixed windows build

1 files changed, 116 insertions, 0 deletions

diff --git a/JSImpl/test/empty.cpp b/JSImpl/test/empty.cpp
index 2787b00..2725467 100644
--- a/JSImpl/test/empty.cpp
+++ b/JSImpl/test/empty.cpp
@@ -1,4 +1,6 @@
 #include <gtest/gtest.h>
+#include <memory>
+#include <cmath>
 
 TEST(Empty, Empty)
 {
@@ -58,5 +60,119 @@ TEST(Pointer, int64)
 	ASSERT_EQ(data + 9, PC);
 }
 
+/*
+ * Value will be either a double, undefined, null, string, array, object,
+ * function, boolean.
+ *
+ * Pointer values (like string, array, object, function) can be shared pointers
+ * via intrusive pointer implementation. This way we can get a reference
+ * counting garbage collection.
+ * 
+ */
+struct Value
+{
+    Value(double v)
+    {
+        m_value.as_double = v;
+    }
+
+    Value(int tag, void* pointer)
+    {
+        assert(tag && "Zero tag is plain double");
+        assert(!((size_t)pointer >> 48) && "Use only pointers to the heap.");
+        m_value.as_pointer.pointer = (size_t)pointer;
+        m_value.as_pointer.tag = tag;
+        m_value.as_pointer.nan = 0x1fff;
+    }
+    struct NanPointer
+    {
+        size_t pointer:48;
+        size_t tag:3;
+        size_t nan:13;
+    };
+
+    struct CheckType
+    {
+        size_t payload: 48;
+        size_t check:16;
+    };
+    union {
+        double as_double;
+        NanPointer as_pointer;
+        CheckType to_check;
+
+    } m_value;
+
+    static_assert(sizeof(size_t) == 8, "unsupported arch");
+    static_assert(sizeof(double) == 8, "unsupported arch");
+
+    bool is_double() const
+    {
+        return m_value.to_check.check <= 0xFFF8;
+    }
+
+    double get_double() const
+    {
+        assert(is_double());
+        return m_value.as_double;
+    }
+
+    int get_tag() {
+        assert(!is_double());
+        return m_value.as_pointer.tag;
+    }
+
+    void* get_pointer() const {
+        return (void*)(size_t) m_value.as_pointer.pointer;
+    }
+};
+
+
+TEST(NaNBox, PlainDoubles)
+{
+	double a = floor(rand() / RAND_MAX);
+    double values[] = {
+        0.0, 2.89, 3.14, 0.0 / 1.0, 1.0 / a
+    };
+    for (auto value: values)
+    {
+        Value box(value);
+        ASSERT_TRUE(box.is_double());
+        ASSERT_EQ(value, box.get_double());
+
+        Value box2(-value);
+        ASSERT_TRUE(box2.is_double());
+        ASSERT_EQ(-value, box2.get_double());
+    }
+}
 
+TEST(NaNBox, Border)
+{
+    Value v(2, nullptr);
+    ASSERT_TRUE(std::isnan(v.m_value.as_double));
+    ASSERT_FALSE(std::isinf(v.m_value.as_double));
+}
 
+TEST(NaNBox, IsNaN)
+{
+    Value v2(std::sqrt(-1.0));
+    ASSERT_TRUE(v2.is_double());
+}
+
+TEST(NaNBox, Pointers)
+{
+    std::vector<std::unique_ptr<int>> pointers;
+    int n = 1024;
+    pointers.reserve(n);
+    for (int i = 0; i < n; ++i)
+    {
+        pointers.emplace_back(std::make_unique<int>(i));
+    }
+
+    for (auto& value: pointers)
+    {
+        Value box(1, value.get());
+        ASSERT_FALSE(box.is_double());
+        ASSERT_EQ(value.get(), box.get_pointer());
+    }
+}