Hall of shame: Header compilation times

[Aleksey Shipilev]

On 9/6/22 12:31, Magnus Ihse Bursie wrote:
> So, given this list, it seems like it would have a huge potential payoff
> to start looking more closely on assembler.hpp. Is it strictly needed
> everywhere it is included? Can it be split into multiple parts, so most
> users only need to include some smaller subsets? Etc. And then
> continuing doing the same with ciEnv.hpp and the top inline.hpp files...

I agree it would be nice to comb through the includes and see what could be trimmed down.

But it would be weird to do major file splits for minor build time improvements. This unfortunately 
has implications on long-term releases maintainability, where backports would not be cleanly 
applicable, the code archaeology would need to account for these splits, etc.

I think once we combed through the obvious over-includes, we could just rely on precompiled headers 
to handle the rest? I just went and added these to precompiled.hpp:

+#include "asm/assembler.hpp"
+#include "asm/codeBuffer.hpp"
+#include "asm/macroAssembler.hpp"
+#include "code/debugInfo.hpp"
+#include "code/codeCache.hpp"
+#include "oops/access.hpp"

...and got fairly reproducible improvements with PCH with GCC 9.4.0:

Before:

real	1m7.208s
user	43m38.448s
sys	3m15.264s

After:

real	1m6.801s
user	42m6.183s  ; <-- about 3.7% improvement
sys	3m19.828s


-- 
Thanks,
-Aleksey