Comment by ahartmetz
2 days ago
I think your rant isn't long enough to include everything relevant ;) The Blink web engine (which I sometimes compile for qtwebengine) takes a really long time to compile, several times longer than Gecko according to some info I found online. Google has a policy of not using forward declarations, including everything instead. That's a pretty big WTF for anyone who has ever optimized build time. Google probably just throws hardware and (distributed) caching at the problem, not giving a shit about anyone else building it. Oh, it also needs about 2 GB of RAM per build thread - basically nothing else does.
Even with Firefox using Rust and requiring a build of many crates, qtwebengine takes more time. It was so bad that I had to remove packaged from my system (Gentoo) that were pulling qtwebengine.
And I build all Rust crates (including rustc) with -O3, same as C/C++.
Chromium deviates from Google-wide policy and allows forward-declarations: https://chromium.googlesource.com/chromium/src/+/main/styleg..., "Forward declarations vs. #includes".
That is really nice to hear, but AFAICS it only means that it may change in the future. Because in current code, it was ~all includes last time I checked.
Well, I remember one - very biased - example where I had a look at a class that was especially expensive to compile, like 40 seconds (on a Ryzen 7950X) and maybe 2 GB of RAM. It had under 200 LOC and didn't seem to do anything that's typically expensive to compile... except for the stuff it included. Which also didn't seem to do anything fancy. But transitive includes can snowball if you don't add any "compile firewalls".
> Because in current code, it was ~all includes last time I checked.
That's another matter - just because forward-declares are allowed, doesn't mean they are mandated, but in my experience the reviewers were paying attention to that pretty well.
Counter-exeamples to "~all includes": https://source.chromium.org/chromium/chromium/src/+/main:thi..., https://source.chromium.org/chromium/chromium/src/+/main:thi..., https://source.chromium.org/chromium/chromium/src/+/main:thi....
I picked couple random headers from the directory where I've contributed the most to blink, and from what I'm seeing, most of the classes that could be forward-declared, were. I have not looked at .cc files given that those tend to need to see the declaration (except when it's unused, but then why have a forward-decl at all?) or the compiler would complain about access into incomplete type.
> Well, I remember one - very biased - example where I had a look at a class that was especially expensive to compile, like 40 seconds (on a Ryzen 7950X) and maybe 2 GB of RAM. It had under 200 LOC and didn't seem to do anything that's typically expensive to compile... except for the stuff it included.
Maybe the stuff was actually being compiled because of some member in a class (so it was actually expensive to compile). Or maybe you stumbled upon a place where folks weren't paying attention. Hard to say without a concrete example. The "compile firewall" was added pretty recently I think, but I don't know if it's going to block anything from landing.
Edit: formatting (switched bulleted list into comma-separated because clearly I don't know how to format it).
This is actually tracked at a publicly visible URL: https://commondatastorage.googleapis.com/chromium-browser-cl...
And the include graph analysis: https://commondatastorage.googleapis.com/chromium-browser-cl...
The annotated red dots correspond to the last time Chrome developers did a big push to prune the include graph to optimize build time. It was effective, but there was push back. C++ developers just want magic, they don't want to think about dependency management, and it's hard to blame them. But, at the end of the day, builds scale with sources times dependencies, and if you aren't disciplined, you can expect superlinear build times.
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