For reference, AFIO a very simple library sees this with precompiled headers enabled:
GCC: 12m => 10m MSVC: 7m => 2m clang: No change
AFIO also has the ability to both use precompiled headers and not be header only:
GCC: 4m MSVC: 1m
So especially big wins for GCC.
I suspect that C++ Modules cannot deliver any improvements over the non-header only with precompiled headers configuration. Good build config always beats compiler magic.
Actually it can - if implemented well - way back when, Borland's C++ compiler had better PCH support than anything else out there, it wasn't quite C++ modules, but it was quite a decent part of the way there. They had a little server app that managed and loaded all required PCH's as AST's into memory, so when a file was being compiled it would just map the memory from the other process and start compiling from there. It was lightning fast with near enough instant compile times. And this was 10+ years ago on hardware that folks would laugh at today. It was more or less automatic too - if there were no changes to a files #includes it would just go ahead and cache everything till after the last #include without the user having to excplicitly mess with PCH's at all. Of course both templates and C++ in general are a lot more complex now: this was all pre-STL days! Still it would be nice, if we could have a compiler/IDE release that wasn't actually 3 times slower than the one before ;) With rose tinted spectacles yours, John.