https://github.com/lightpanda-io/browser/pull/3271 introduce an orderfile so
that hot sections of code were grouped together in the binary, resulting in more
efficient loading. But, it excluded v8 functions because v8 is compiled with
`-fno-unique-section-names` so each section gets the same name. Because of this
the orderfile can't target specific "hot" or "cold" functions.
As a follow up to 3271, I thought we'd be able to re-compile v8 without that
flag, but:
1 - The flag isn't directly exposed by v8, so we'd need to change v8's own build
BUILD.gn in our build process
2 - It would make the .a file ~ 40MB larger (though the final lightpanda binary
would stay the same size
3 - If we wanted to created two .a files (one with -fno-unique-section-names and
one without), it would require a full rebuild (for both x86-64 and arm).
I didn't love those compromises, so I asked Claude if the sections names could
be generated in a separate file and then that could be used when generating out
lightpanda.ld. What claude came up with was a small Zig script
(mark_hot_sections.zig) which is now run as part of the build. It takes the
v8.a file (which still has `-fno-unique-section-names`), it takes a text list
of hot functions, and it re-generates v8.a with a special .text.hot and
.rodata.hot sections. lightpanda.ld can include these.
This PR depends on https://github.com/lightpanda-io/zig-v8-fork/pull/202 but
202 doesn't require a v8 rebuild. It merely allows prebuilt_v8_path to be
a lazypath, which we need because the prebuilt_v8_path that we pass is now
generated from this build script.
CI build, for e2e-test and nightly now build with:
-Dorderfile/lightpanda.ld
This file informs the build on how to organize the code in the binary, grouping
hot code together so that we have to load less of the binary into memory.
lightpanda.ld will drift: we'll refactor our code, add new features, update
dependencies, update Zig, ... So it has to be re-generated. But we can do that
automatically in the CI (say, before the nightly build). That's for a follow up
PR.
This does not currently cover V8. V8 is being build with
`-no-unique-section-names`, so we don't get names that we can correctly
organize. The real win comes from doing this in V8, since a lot of V8 is cold.
This PR can land as-is, a zig-v8-fork PR will remove that flag, and then we can
have a follow up PR with an lightpanda.ld that includes the v8 symbols.
This is opt-in (via the -Dorderfile flag) because it adds ~20 seconds of
linking time.
Karl Seguin