Wrap external pointers more carefully.

include/v8.h

Index: include/v8.h
diff --git a/include/v8.h b/include/v8.h
index 3a9827b24d3ba2e361c4ba326446fdd87ebee314..7d18107d92d228c8f6de6e6d519d524e0f5de8ce 100644
--- a/include/v8.h
+++ b/include/v8.h
@@ -3353,10 +3353,10 @@ const int kSmiTag = 0;
 const int kSmiTagSize = 1;
 const intptr_t kSmiTagMask = (1 << kSmiTagSize) - 1;
 
-template <size_t ptr_size> struct SmiConstants;
+template <size_t ptr_size> struct SmiTagging;
 
 // Smi constants for 32-bit systems.
-template <> struct SmiConstants<4> {
+template <> struct SmiTagging<4> {
   static const int kSmiShiftSize = 0;
   static const int kSmiValueSize = 31;
   static inline int SmiToInt(internal::Object* value) {
@@ -3364,10 +3364,15 @@ template <> struct SmiConstants<4> {
     // Throw away top 32 bits and shift down (requires >> to be sign extending).
     return static_cast<int>(reinterpret_cast<intptr_t>(value)) >> shift_bits;
   }
+
+  // For 32-bit systems any 2 bytes aligned pointer can be encoded as smi
+  // with a plain reinterpret_cast.
+  static const intptr_t kEncodablePointerMask = 0x1;
+  static const int kPointerToSmiShift = 0;
 };
 
 // Smi constants for 64-bit systems.
-template <> struct SmiConstants<8> {
+template <> struct SmiTagging<8> {
   static const int kSmiShiftSize = 31;
   static const int kSmiValueSize = 32;
   static inline int SmiToInt(internal::Object* value) {
@@ -3375,10 +3380,26 @@ template <> struct SmiConstants<8> {
     // Shift down and throw away top 32 bits.
     return static_cast<int>(reinterpret_cast<intptr_t>(value) >> shift_bits);
   }
+
+  // To maximize the range of pointers that can be encoded
+  // in the available 32 bits, we require them to be 8 bytes aligned.
+  // This gives 2 ^ (32 + 3) = 32G address space covered.
+  // It might be not enough to cover stack allocated objects on some platforms.
+  static const int kPointerAlignment = 3;
+
+  static const intptr_t kEncodablePointerMask =
+      ~(intptr_t(0xffffffff) << kPointerAlignment);
+
+  static const int kPointerToSmiShift =
+      kSmiTagSize + kSmiShiftSize - kPointerAlignment;
 };
 
-const int kSmiShiftSize = SmiConstants<kApiPointerSize>::kSmiShiftSize;
-const int kSmiValueSize = SmiConstants<kApiPointerSize>::kSmiValueSize;
+typedef SmiTagging<kApiPointerSize> PlatformSmiTagging;
+const int kSmiShiftSize = PlatformSmiTagging::kSmiShiftSize;
+const int kSmiValueSize = PlatformSmiTagging::kSmiValueSize;
+const intptr_t kEncodablePointerMask =
+    PlatformSmiTagging::kEncodablePointerMask;
+const int kPointerToSmiShift = PlatformSmiTagging::kPointerToSmiShift;
 
 template <size_t ptr_size> struct InternalConstants;
 
@@ -3426,7 +3447,7 @@ class Internals {
   }
 
   static inline int SmiValue(internal::Object* value) {
-    return SmiConstants<kApiPointerSize>::SmiToInt(value);
+    return PlatformSmiTagging::SmiToInt(value);
   }
 
   static inline int GetInstanceType(internal::Object* obj) {
@@ -3435,9 +3456,14 @@ class Internals {
     return ReadField<uint8_t>(map, kMapInstanceTypeOffset);
   }
 
+  static inline void* GetExternalPointerFromSmi(internal::Object* value) {
+    const intptr_t address = reinterpret_cast<intptr_t>(value);
+    return reinterpret_cast<void*>(address >> kPointerToSmiShift);
+  }
+
   static inline void* GetExternalPointer(internal::Object* obj) {
     if (HasSmiTag(obj)) {
-      return obj;
+      return GetExternalPointerFromSmi(obj);
     } else if (GetInstanceType(obj) == kProxyType) {
       return ReadField<void*>(obj, kProxyProxyOffset);
     } else {