ZJIT: Reduce cost of PushInlineFrame

[tekknolagi]

Currently, we:

• Check for stack overflow (lea, cmp, jbe)
• Save PC
• Save SP
• Save all locals
• Save all stack
• Push a frame
  • Write CME
  • Write specval
  • Write ENV_FLAGS
  • Write block code
  • Maybe write SP, JIT_RETURN
  • Write self
  • Write EP
• Write JITFrame
• Write JIT_RETURN
• Write SP
• Write CFP

We need to cut most of this.

[k0kubun]

Save PC

Maybe the function name confused you but I don't think we do that right now. PC should be updated through "Write JITFrame".

Write block code

This is also optimized away when iseq_may_write_block_code(iseq).

[k0kubun]

In the issue description, it'd be nice to distinguish operations for the callee from ones for the caller. On PushInlineFrame, we update the context for the caller and also set up a frame for the callee. It's not clear whether "Write JITFrame" is about the caller or the callee, for example.

[tekknolagi]

"Save PC" is maybe a misnomer but it does write a pointer to JIT frame to the frame

[k0kubun]

Yeah. The suggestion to separate caller vs callee in the issue description #1008 (comment) was to clarify that. It's for the caller, not the callee frame it pushes.