[llvm-dev] CMake dependencies and building LLVM

[Nemanja Ivanovic via llvm-dev]

Hi everyone,
I have felt for quite some time that during my regular LLVM workflow, CMake
recompiles a lot of files after I update to the latest ToT (which I do more
than once per week). Of course, this wasn't really based on any real data,
just felt like every time I update (even if it's a day or two later), we
end up recompiling everything. For a while I assumed that patches touch a
few headers that are used all over the place or something like that, but
this was happening with what seems like unexpected frequency.

So what I've done is do a few update/rebuild cycles for revisions that are
200 apart, then 100, then 50, then 25. And it seems that quite commonly, we
recompile more than 50% of the source with just a few landed revisions.
I've included the numbers below (and I do realize that this is a fairly
small sample).

If this seems reasonable to most of the community, so be it - I work on a
large server that chews through this stuff pretty quickly so I can
certainly live with it. On the other hand, if this seems way higher than
what you expect, it could very well be that something is wrong in my setup
and I'd appreciate some pointers about how to go about investigating this.
Finally, if others are seeing this and we as a community do feel that
something can/should be done about this (although I don't know what that
might be), perhaps we can start that discussion.

Setup: CMake with -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_ASSERTIONS=ON -G
Ninja

The numbers here are the total numbers of targets ninja reports as having
rebuilt. Ninja is invoked only `-j <N>` for parameters (i.e. not check-all
or any specific targets).

RevFrm, RevTo,  NumNinjaObjsRebuilt
300000, 300200, 2957
300200, 300400, 2692
300400, 300600, 3001
300600, 300800, 2877
300800, 301000, 3003
301000, 301200, 2750
301200, 301400, 2579
301400, 301600, 2709
301600, 301700, 2325
301700, 301800, 2357
301800, 301900, 2667
301900, 302000, 2466
302000, 302100, 2499
302100, 302200, 2063
302200, 302250, 2884
302250, 302300, 2007
302300, 302350, 2209
302350, 302400, 737
302400, 302450, 2398
302450, 302500, 2856
302500, 302550, 2090
302550, 302600, 2866
302600, 302625, 1786
302625, 302650, 1966
302650, 302675, 1583
302675, 302700, 307
302700, 302725, 2246
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