In the early days of Chrome when we used WebKit there was no support for
ASCII strings on the C++ side, so we put a hint onto these two-byte
strings that said "string only contains one byte data", such that
internally in V8 when these were involved in string operations, we could
instead create the *cheaper* one byte strings.
Nowadays Blink properly supports one-byte string representations and
this additional hint only comes with overhead, since we check it in
quite a few places (i.e. on the hot path for string concatenation), plus
we end up consuming more memory due to the additional string maps.
Removing the hint also frees one bit in the InstanceType zoo for
strings.
This alone improves performance on the `bench-dom-serialize.js` test case
by around **3%**.
Tbr: mstarzinger@chromium.org
Bug: v8:6622, v8:8834, v8:8939
Cq-Include-Trybots: luci.chromium.try:linux-blink-rel
Change-Id: I0753f2859cee7b5a37b6f0da64d8ec39fcb044ff
Doc: https://bit.ly/fast-string-concatenation-in-javascript
Reviewed-on: https://chromium-review.googlesource.com/c/1498478
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60006}
With 32-bit kTaggedSize small strings may be not externalizable.
Bug: v8:7703
Change-Id: I34002568214742dadb2358fca97dfb4b92a5342a
Reviewed-on: https://chromium-review.googlesource.com/c/1480373
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59770}
This reverts commit e42e7fc800.
Reason for revert: Speculative revert for:
https://crbug.com/v8/7149
Original change's description:
> [objects] No longer create short external strings.
>
> This fixes String::MakeExternal() to bail out if the subject string
> doesn't fit a regular ExternalString, instead of creating a short
> external string. The observation here is that for short external strings
> the overhead of having to have the StringResource plus going to the
> runtime/C++ for each and every character access from JavaScript land
> is probably bigger than the anticipated benefits.
>
> If this turns out to be wrong and there's a real benefit, we should make
> use of ThinStrings instead of having a separate way to represent
> external strings.
>
> Bug: v8:6621, v8:7109, v8:7145
> Change-Id: I4b75da08b82a72027c782a69de9c8eaf3cca1d4d
> Reviewed-on: https://chromium-review.googlesource.com/799750
> Reviewed-by: Yang Guo <yangguo@chromium.org>
> Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#49735}
TBR=yangguo@chromium.org,bmeurer@chromium.org
Change-Id: I3f5cfa9ab5c99ddce1d61ede9ed9515cb3936cdd
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:6621, v8:7109, v8:7145, v8:7149
Reviewed-on: https://chromium-review.googlesource.com/801675
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49770}
This fixes String::MakeExternal() to bail out if the subject string
doesn't fit a regular ExternalString, instead of creating a short
external string. The observation here is that for short external strings
the overhead of having to have the StringResource plus going to the
runtime/C++ for each and every character access from JavaScript land
is probably bigger than the anticipated benefits.
If this turns out to be wrong and there's a real benefit, we should make
use of ThinStrings instead of having a separate way to represent
external strings.
Bug: v8:6621, v8:7109, v8:7145
Change-Id: I4b75da08b82a72027c782a69de9c8eaf3cca1d4d
Reviewed-on: https://chromium-review.googlesource.com/799750
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49735}
If a two-byte string only contains ascii characters, then we can save
memory when flattening a cons string containing it. Similarly we can
use this in Array.prototype.join implementation. To track this a new
bit is added to instance type. This bit is used as a hint in generated
code and in runtime functions.
To enable testing a new V8 extension is added controlled by
--expose-externalize-string flag.
Review URL: http://codereview.chromium.org/2762008
git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4894 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
