When the compiler inlines foo(), it turns into

Not bad - saves us the function call and all the setup that goes with having a function. But the compiler can in fact even do better - because now all the information is in one place. The compiler can apply that knowledge and deduce that fiver_inline() adds the number three and does so 5 times - and so the entire code is


There's just one downside - that approach needs to generate all of the machine code for Chrome, all the time. Change one line in one file, compile all of Chrome. And there's a lot of Chrome. It also effectively disables caching build results and so makes remote compilation much less useful. (And we rely a lot on remote compilation & caching so we can quickly build new versions of Chrome.)

So, back to the drawing board. The core insight is that each source file only needs to include a few functions - it doesn't need to see every single other file. All that's needed is "cleverly" mixing the right inline functions into the right source files.


. And to know which functions calls which other ones a lot… yep. You guessed it. Our profiles from the PGO work pay off again.

One more thing.

It turns out that the compiler can make even more use of that profile data for PGO. (Not a surprise - once you know where the slow spots are, exactly, you can do a lot to improve!). To make use of that, and enable further improvements, LLVM has something called the "new pass manager". In a nutshell, it's a new way to run optimizations within LLVM, and it helps a lot with PGO. For much more detail, I'd suggest reading the


And since this work requires changes to compilers and linkers, that would mean changing the build - and testing it - across 5 compilers and 4 linkers. But, thankfully, we've simplified our toolchain (Simplicity - another one of the 4S's!). To be able to do this, we worked with the LLVM community to make clang a great


And suddenly, it's only a single toolchain to fix. (Almost. LTO for lld on MacOS is .