Faster compilation through parallel module build

I saw in the “What’s New in Xcode” WWDC session that y’all sped up compilation by building a library’s module in parallel with its object files, instead of doing a separate module-merging step. That’s basically what happens in whole-module builds today, and SourceKit has to do something similar, so I always knew it was possible, but I didn’t expect it to be faster, especially in an incremental build. I’m a little ashamed of having not tested it while at Apple. (Of course, maybe it was slower and not worth it in the past.) Kudos to y’all inside or outside Apple who made this happen!

(Just for my own edification, I assume this is not done in [edit: non-WMO] optimized builds? Cause how would the inlinable SIL get in there?)


The second part of Demystify parallelization in Xcode builds has more details on eager emission of modules, and eager linking.

I don't know if there's an answer to your question in the video.

Thanks. There is not, but that is the better video to cite! It sounds like the existence of non-WMO-but-optimized is downplayed, which matches what Xcode's default project template has been for years.

I guess the other option besides giving up on eager module emission is to have the eager module emission generate inlinable code. (And the third option is that inlinable code is broken in this scenario, but that seems unlikely—there are tests for it in the compiler repo!)