648698 - perhaps add fast call ic paths that handle arity mismatch

Status: RESOLVED WONTFIX

(Core :: JavaScript Engine, defect)

Description

[Luke Wagner [:luke]]

During a single run, v8-raytrace makes 1.2M mjit calls of which 320K (25%) have an arity mismatch.  This means we call stubs::FixupArity 320K times for an estimated total of 86ms (out of 500ms total) with the average FixupArity call taking 698 cycles (via rdtsc).  The potential speedup is even greater since an arity mismatch takes the slowest ic path.

I haven't studied whether v8-raytrace contains polymorphic call sites or not but, in the best case it seems like we could generate a fast path that isn't much slower than our fastest call path.  It also seems like this optimization would benefit the real webz too since I think what v8-raytrace is doing:
  function foo() { bar.apply(y, arguments) }
seems to be a real pattern that shows up.

Comment 1

[Luke Wagner [:luke]]

Oops, that was a debug build.  The % arity-mismatch calls is still high, but an opt build only shows ~4.7ms out of ~110ms (or less, depending on how much of that is rdtsc).  Not trivial, but not such a big deal.

Comment 2

[Luke Wagner [:luke]]

IonMonkey will handle differently.