555425 - JM: Remove sp updates

Status: RESOLVED FIXED

(Core :: JavaScript Engine, defect)

Description

[David Anderson [:dvander] - inactive, e-mail if emergency]

Attachment: WIP

This patch removes sp updates in JIT'd code. For example, JSOP_ZERO might go from:
     mov [rbx], 1
     add rbx, 8

To:
     mov [rbx + 0xOFFSET], 1

Unfortunately, this approach seems to have slowed down SunSpider by just a hair (consistent 5ms) and v8 more. Slow paths need to dump out state to |cx| and now the previously cheap MOV of sp into cx->regs->sp is now a more expensive LEA.

Fortunately, a microbenchmark where we don't hit slow paths suggests that this patch is in fact a win (not a big one, but measurable). This probably means that we're spending a lot of time - too much time - in stub calls.

In the meantime, I'm going to try and reduce the stubcall overhead.

Comment 1

[David Anderson [:dvander] - inactive, e-mail if emergency]

I pushed this early because it's got refactoring that I want now rather than later. Also re-measuring it looks like a slight win JM-only. The 5ms JM+TM perf loss will go away as stub calls get smarter.

http://hg.mozilla.org/users/danderson_mozilla.com/jaegermonkey/rev/9116c7f46321