[llvm-dev] Should we split llvm Support and ADT?

[Zachary Turner via llvm-dev]

On Tue, May 30, 2017 at 12:52 PM Bob Haarman <llvm at inglorion.net> wrote:

> I would like to better understand how you came to conclude that the
> tablegen re-runs based on changes in Support are what's causing your build
> to be slow and what part specifically is taking all that time. I can do a
> clean release + assertions build of LLVM, Clang, compiler-rt and lld in
> about 5 minutes, plus 40 seconds to run cmake, on what I think is similar
> hardware to what Zach is using. If I swap Ptr += ret and Size -= Ret in
> raw_fd_ostream::write_impl so that Support changes, I can do an incremental
> build in about 10 seconds, including the regeneration of various .inc
> files. How do we get from there to 10 minutes for an incremental build?
>

For the sake of comparison, I made the same change and it took  1:58.39.  A
slightly different but more intrusive change to Format.h in
format_object_base::print() took 5:6.54.  A clean build on the same machine
takes about 12 minutes.

I tried a few hacks to the CMake to say "don't run tablegen, no matter
what, and don't make anything depend on tablegen's output", but I couldn't
come up with the right magic.  If anyone knows, I can test a side-by-side
comparison without tablegen.
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