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Wenyu Zhao 5e0b94c4dc Allowing map word to be used for other state in GC header. 
This CL adds features to pack/unpack map words.

Currently V8 cannot store extra metadata in object headers -- because V8
objects do not have a proper header, but only a map pointer at the start
of the object. To store per-object metadata like marking data, a side
table is required as the per-object metadata storage.

This CL enables V8 to use higher unused bits in a 64-bit map word as
per-object metadata storage. Map pointer stores come with an extra step
to encode the metadata into the pointer (we call it "map packing").
Map pointer loads will also remove the metadata bits as well (we call it
"map packing").

Since the map word is no longer a valid pointer after packing, we also
change the tag of the packed map word to make it looks like a Smi. This
helps various GC and barrier code to correctly skip them instead of
blindly dereferencing this invalid pointer.

A ninja flag `v8_enable_map_packing` is provided to turn this
map-packing feature on and off. It is disabled by default.

* Only works on x64 platform, with `v8_enable_pointer_compression`
  set to `false`

Bug: v8:11624
Change-Id: Ia2bdf79553945e5fc0b0874c87803d2cc733e073
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2247561
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73915}
2021-04-12 17:34:13 +00:00
.github [github] Close PRs created on the V8 GitHub mirror automatically 2021-01-12 08:39:59 +00:00
build_overrides [build] Remove obsolete gn flag 2021-01-11 15:50:59 +00:00
custom_deps Use relative paths to OWNERS files 2019-08-12 13:52:52 +00:00
docs Add OWNERS for docs/ 2019-05-15 12:06:15 +00:00
gni [logging] Add runtime-call-stats compile-time flag 2021-04-12 15:53:03 +00:00
include Allowing map word to be used for other state in GC header. 2021-04-12 17:34:13 +00:00
infra Reland "Reland "[test] Extend testing on Mac on arm64"" 2021-04-08 15:54:53 +00:00
samples cppgc: Avoid initializing cppgc platform through V8 2021-02-09 08:26:11 +00:00
src Allowing map word to be used for other state in GC header. 2021-04-12 17:34:13 +00:00
test Allowing map word to be used for other state in GC header. 2021-04-12 17:34:13 +00:00
testing build: Remove no-op calls to set_sources_assignment_filter 2020-10-08 14:44:01 +00:00
third_party [language] Fix Array.prototype.sort 2021-03-24 11:20:58 +00:00
tools Allowing map word to be used for other state in GC header. 2021-04-12 17:34:13 +00:00
.clang-format
.clang-tidy
.editorconfig
.flake8 Add .flake8 to allow for python style checking. 2019-07-19 21:44:03 +00:00
.git-blame-ignore-revs [infra] Update .git-blame-ignore-revs 2020-03-03 12:59:13 +00:00
.gitattributes
.gitignore [build] Remove legacy binutils 2021-01-08 12:33:31 +00:00
.gn [build] Dynamically decide if to use system xcode 2019-07-29 08:20:24 +00:00
.vpython [tools] Implement confidence-based number of runs 2019-05-09 09:42:28 +00:00
.ycm_extra_conf.py Fix ycm config for headers without source 2019-12-09 11:59:21 +00:00
AUTHORS Allowing map word to be used for other state in GC header. 2021-04-12 17:34:13 +00:00
BUILD.gn Allowing map word to be used for other state in GC header. 2021-04-12 17:34:13 +00:00
CODE_OF_CONDUCT.md
codereview.settings
COMMON_OWNERS Remove petermarshall from OWNERS 2021-03-31 11:00:22 +00:00
DEPS Update V8 DEPS. 2021-04-12 03:28:10 +00:00
DIR_METADATA Add DIR_METADATA files to v8. 2020-10-20 22:12:28 +00:00
ENG_REVIEW_OWNERS Update ENG_REVIEW_OWNERS 2021-02-25 14:28:32 +00:00
INFRA_OWNERS Add team members as owners 2020-08-18 08:29:04 +00:00
INTL_OWNERS add ftang as owner 2020-04-24 19:19:36 +00:00
LICENSE
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LICENSE.strongtalk
LICENSE.v8
MIPS_OWNERS [mips] Request to be an owner of MIPS files 2020-11-23 09:43:49 +00:00
OWNERS Add RISC-V backend 2021-02-09 17:06:36 +00:00
PPC_OWNERS Adding myself to relevant OWNERS files. 2020-11-02 16:37:28 +00:00
PRESUBMIT.py cppgc: Add testing API structure 2021-02-03 16:00:37 +00:00
README.md
RISCV_OWNERS Add RISC-V backend 2021-02-09 17:06:36 +00:00
S390_OWNERS Adding myself to relevant OWNERS files. 2020-11-02 16:37:28 +00:00
WATCHLISTS Add a watchlist for ieee754.(cc|h) changes. 2021-01-11 18:10:19 +00:00

README.md

V8 JavaScript Engine

V8 is Google's open source JavaScript engine.

V8 implements ECMAScript as specified in ECMA-262.

V8 is written in C++ and is used in Google Chrome, the open source browser from Google.

V8 can run standalone, or can be embedded into any C++ application.

V8 Project page: https://v8.dev/docs

Getting the Code

Checkout depot tools, and run

    fetch v8

This will checkout V8 into the directory v8 and fetch all of its dependencies. To stay up to date, run

    git pull origin
    gclient sync

For fetching all branches, add the following into your remote configuration in .git/config:

    fetch = +refs/branch-heads/*:refs/remotes/branch-heads/*
    fetch = +refs/tags/*:refs/tags/*

Contributing

Please follow the instructions mentioned at v8.dev/docs/contribute.