This patch makes ArraySpeciesCreate fast in V8 by avoiding two property reads
when the following conditions are met:
- No Array instance has had its __proto__ reset
- No Array instance has had a constructor property defined
- Array.prototype has not had its constructor changed
- Array[Symbol.species] has not been reset
For subclasses of Array, or for conditions where one of these assumptions is
violated, the full lookup of species is done according to the ArraySpeciesCreate
algorithm. Although this is a "performance cliff", it does not come up in the
expected typical use case of @@species (Array subclassing), so it is hoped that
this can form a good start. Array subclasses will incur the slowness of looking
up @@species, but their use won't slow down invocations of, for example,
Array.prototype.slice on Array base class instances.
Possible future optimizations:
- For the fallback case where the assumptions don't hold, optimize the two
property lookups.
- For Array.prototype.slice and Array.prototype.splice, even if the full lookup
of @@species needs to take place, we still could take the rest of the C++
fastpath. However, to do this correctly requires changing the calling convention
from C++ to JS to pass the @@species out, so it is not attempted in this patch.
With this patch, microbenchmarks of Array.prototype.slice do not suffer a
noticeable performance regression, unlike their previous 2.5x penalty.
TBR=hpayer@chromium.org
Review URL: https://codereview.chromium.org/1689733002
Cr-Commit-Position: refs/heads/master@{#34199}
This patch makes Array.prototype.concat support subclassing Arrays
and constructing instances properly with Symbol.species. It is
guarded by the --harmony-species flag.
R=cbruni
LOG=Y
BUG=v8:4093
Review URL: https://codereview.chromium.org/1577043002
Cr-Commit-Position: refs/heads/master@{#33503}
This patch implements @@species, guarded behind the --harmony-species
flag, on Arrays. Methods which return an Array will instead return
the appropriate instance based on the ArraySpeciesCreate algorithm.
The algorithm is implemented in C++ to get access to realm information
and to implement some Array methods in C++, but it is also accessed
from JavaScript through a new runtime function. A couple interactive
Octane runs show no performance regression with the flag turned off,
but turning --harmony-species on will surely have a significant
regression, as Array methods now heavily use ObjectDefineProperty.
BUG=v8:4093
LOG=Y
R=adamk,cbruni
Review URL: https://codereview.chromium.org/1560763002
Cr-Commit-Position: refs/heads/master@{#33144}
