The updated version, available at https://github.com/Christewart/bitcoin/tree/64bit-arith-implicit, makes a pivotal shift by removing 64-bit specific opcodes in favor of repurposing the existing ones. This change means that opcodes such as OP_ADD, OP_SUB, OP_MUL, and OP_DIV will now implicitly support 64-bit arithmetic operations without requiring distinct 64-bit versions. Consequently, every opcode within interpreter.cpp that previously accepted a CScriptNum input has been updated to accept an int64_t stack parameter instead. This adjustment allows operations like OP_1ADD to handle int64_t arguments directly, pushing both an int64_t result and a boolean success indicator back onto the stack.

Furthermore, this update eliminates the need for casting opcodes like OP_SCRIPTNUMTOLE64, OP_LE64TOSCRIPTNUM, and OP_LE32TOLE64, streamlining the scripting process. Developers no longer need to decide between using standard or 64-bit specific opcodes for their operations, nor do they have to convert the stack top with casting opcodes, thereby simplifying script development. However, this revision necessitates the modification of existing scripts that use constant numeric arguments. Scripts leveraging operations such as OP_CHECKLOCKTIMEVERIFY and OP_CHECKSEQUENCEVERIFY will require updates to accommodate 8-byte parameters instead of the previous 5-byte parameters.

A key aspect of this overhaul is its reliance on pattern matching on SigVersion to dictate the implementation of opcodes. For example, the handling of OP_DEPTH can be tailored based on the SigVersion, allowing for future redefinitions of opcode semantics through pattern matching. This approach not only facilitates the introduction of new implementations but also ensures that the system robustly handles new SigVersion additions through compiler-enforced exhaustiveness checks. By compelling developers to address any new SigVersion cases, the update lays a solid foundation for future upgrades to the interpreter, promising a more dynamic and flexible development environment.