How Link-Time Optimization (LTO) breaks the translation unit compilation boundary:

Normally, the compiler cannot inline functions defined in util.cpp when compiling main.cpp because translation units are isolated.

Full LTO vs ThinLTO:

• Full LTO (-flto): Emits LLVM bitcode instead of machine code, merges entire project into one giant module at link time. Slower compilation, maximum inlining.
• ThinLTO (-flto=thin): Generates lightweight function summaries and imports only required functions across modules concurrently. Scales to million-line projects with 95% of full LTO speedups!

ThinLTO enabled cross-module inlining across our 4,000 source files in only 45 seconds of link time. Performance increased by 14%.

ThinLTO is one of the best compiler innovations from the LLVM team.