tl;dr

• Simple typer bug, range of BitAnd opcode is assumed to be [1, operand] when in reality it is [0, operand].
• Use range assumptions to create unchecked integer underflow.
• Bypass array bounds checks and obtain OOB write, overwrite size of array to get overlap.
• Use double & object array overlap to create addrOf & fakeObj primitives.
• Create overlapping fake array using StructureID leak to obtain arbitrary R/W.

tl;dr

• Arbitrary type confusion in DFG JIT
• Bug eliminates a single CheckStructure node