V8’s version numbering scheme

V8 version numbers are of the form x.y.z.w, where:

x.y is the Chromium milestone divided by 10 (e.g. M60 → 6.0)
z is automatically bumped whenever there’s a new LKGR (typically a few times per day)
w is bumped for manually backmerged patches after a branch point

If w is 0, it’s omitted from the version number. E.g. v5.9.211 (instead of “v5.9.211.0”) gets bumped up to v5.9.211.1 after backmerging a patch.

Which V8 version should I use? #

Embedders of V8 should generally use the head of the branch corresponding to the minor version of V8 that ships in Chrome.

Finding the minor version of V8 corresponding to the latest stable Chrome #

To find out what version this is,

Go to https://chromiumdash.appspot.com/releases
Find the latest stable Chrome version in the table
Click on the (i) and check the V8 column

Finding the head of the corresponding branch #

V8’s version-related branches do not appear in the online repository at https://chromium.googlesource.com/v8/v8.git; instead only tags appear. To find the head of that branch, go to the URL in this form:

https://chromium.googlesource.com/v8/v8.git/+/branch-heads/<minor-version>

Example: for the V8 minor version 12.1 found above, we go to https://chromium.googlesource.com/v8/v8.git/+/branch-heads/12.1, finding a commit titled “Version 12.1.285.2.

Caution: You should not simply find the numerically-greatest tag corresponding to the above minor V8 version, as sometimes those are not supported, e.g. they are tagged before deciding where to cut minor releases. Such versions do not receive backports or similar.

Example: the V8 tags 5.9.212, 5.9.213, 5.9.214, 5.9.214.1, …, and 5.9.223 are abandoned, despite being numerically greater than the branch head of 5.9.211.33.

Checking out the head of the corresponding branch #

If you have the source already, you can check out the head somewhat directly. If you’ve retrieved the source using depot_tools then you should be able to do

git branch --remotes | grep branch-heads/

to list the relevant branches. You'll want to check out the one corresponding to the minor V8 version you found above, and use that. The tag that you end up on is the appropriate V8 version for you as the embedder.

If you did not use depot_tools, edit .git/config and add the line below to the [remote "origin"] section:

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