clang-cl gives us the option of changing a function to use the SystemV ABI: https://clang.llvm.org/docs/AttributeReference.html#sysv-abi By using this on functions on the C++ / JIT boundaries, we can use the same codepaths for function calls and the same register mappings between Windows and Linux / Mac / BSD. Enabling the Baseline JS JIT is the next step, and unblocks the possibility of enabling the rest of the JITs: * Wasm LLInt (JIT required for entry thunks) * Wasm BBQ JIT * Wasm OMG JIT * YarrJIT * CSS Selector JIT * DFG JIT * FTL JIT