ZJIT: Support JIT-to-JIT calls with more than C_ARG_OPNDS arguments

[k0kubun]

Every basic block, including the first block after a JIT entry, takes method parameters as basic block arguments. Therefore, the callee expects arguments in ALLOC_REGS registers. The first 6-8 of which agree with C_ARG_OPNDS.

The caller uses a CCall to jump to the callee block. At the moment, we pass method parameters using the arguments of the CCall. So the first 6-8 of them are stored in C_ARG_OPNDS. However, because the arguments of CCall are assigned to stack slots beyond C_ARG_OPNDS, they don't get stored in the rest of ALLOC_REGS registers.

So parameters read by JIT entry blocks and ones written by SendWithoutBlockDirect need to agree with each other. At the moment, the former reads from ALLOC_REGS because compile_jit_entry_state pushes hir::Insn::Param, and the latter writes to C_ARG_OPNDS because it uses the arguments of CCall.

We can either:

• Let the JIT entry blocks read parameters from C arguments, or:
• Implement a custom calling convention used only for JIT-to-JIT calls
  • This option allows you to use more registers for passing method parameters, especially for arm64