Module value_word 

ValueWord: 8-byte NaN-boxed value representation for the VM stack.

Uses IEEE 754 quiet NaN space to pack type tags and payloads into 8 bytes. Simple types (f64, i48, bool, None, Unit, Function) are stored inline. Complex types are heap-allocated as Arc<HeapValue> with the raw pointer in the payload.

NaN-boxing scheme

All tagged values use sign bit = 1 with a quiet NaN exponent, giving us 51 bits for tag + payload. Normal f64 values (including NaN, which is canonicalized to a positive quiet NaN) are stored directly and never collide with our tagged range.

Tagged: 0xFFF[C-F]_XXXX_XXXX_XXXX
  Bit 63    = 1 (sign, marks as tagged)
  Bits 62-52 = 0x7FF (NaN exponent)
  Bit 51    = 1 (quiet NaN bit)
  Bits 50-48 = tag (3 bits)
  Bits 47-0  = payload (48 bits)

Tag	Meaning
0b000	Heap pointer to Arc<HeapValue> (48-bit ptr)
0b001	i48 (48-bit signed integer, sign-extended)
0b010	Bool (payload bit 0)
0b011	None
0b100	Unit
0b101	Function(u16) (payload = function_id)
0b110	ModuleFunction(u32) (payload = index)
0b111	Ref (absolute stack slot index, 48 bits)

Structs

ValueWord

An 8-byte value word for the VM stack (NaN-boxed encoding).

Enums

ArrayView

Unified read-only view over all array variants (generic, int, float, bool, width-specific).

ArrayViewMut

Mutable view over all array variants. Uses Arc::make_mut for COW semantics.

NanTag

Tag discriminator for ValueWord values.

RefTarget