PSA: V8 Build Changes

[Dan Elphick]

If you don't modify v8/BUILD.gn then you can stop reading now.

I'm planning to land https://chromium-review.googlesource.com/c/v8/v8/+/2712569, which should get us a lot closer to gn-check-compliance. What this means is that we can run "gn check" to ensure that all the build dependencies are correct, which amongst other benefits ensures that generated headers are correctly rebuilt before being used.

On the negative side, the change is 1000 lines long AND changes the way you add files to the most common V8 build targets. From now on headers that would have gone into v8_base_without_compiler or v8_compiler should go into v8_internal_headers. The reason for this is that v8_base_without_compiler and v8_compiler really comprise a single component that is artificially split so the parts needed for mksnapshot can be compiled with optimisations turned on to make builds faster when v8_enable_fast_mksnapshot is enabled. This split arbitrarily places the header files on either side of the divide, which means that for gn check to be clean we'd need v8_base_without_compiler to depend on v8_compiler as well as the other way around.

  (We should be able to move some back into their true components at a later time, but this will be much safer to do when we can turn gn check on in presubmits). I plan to land this on 2021-03-01 when everything is back to normal after the branch cut.

Thanks,

Dan