818750 - IonMonkey: each OutOfLineTruncate is 448 bytes, can we factor this out into per-IonRuntime stub?

Status: NEW

(Core :: JavaScript Engine, enhancement, P3)

Description

[Luke Wagner [:luke]]

The out-of-line double->int conversion path costs .5K for each and every double->int conversion.  From a quick look, it seems like nothing in the out-of-line path is particularly specialized to the individual site except the jump back.  Can we factor this out into a stub (stored in IonRuntime)?  This could quickly add up for large programs.

Comment 1

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

It could be refactored to be a very lightweight call, but we'd lose specialization of the output register. Maybe other things we can try: (1) using bailouts instead of having the js::ToInt32 call, (2) moving only the ToInt32 part to a shared stub, or (3) replacing the logic with something much simpler.

Comment 2

[Luke Wagner [:luke]]

It seems like, in the out-of-line path, there are two paths: one that tries to handle barely-out-of-range doubles and the fallback that calls out to C++.  The former doesn't cost many instructions, so perhaps we can keep that inline in the out-of-line path?

Comment 3

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

Yeah, if I'm reading that right, it would be (2) above.

Comment 4

[Luke Wagner [:luke]]

Agreed

Comment 5

[Luke Wagner [:luke]]

I just measured and WMW generates 144KB of code in visitOutOfLineTruncateSlow (out of 11.8MB total) and Citadel generates 405KB (out of 25MB total), so 1.22% and 1.62%, resp.  Not a huge deal, I suppose.