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

[Zachary Turner via llvm-dev]

ninja, MSVC compiler, MSVC linker, Windows 10, 48 logical cores, 64GB ram.

On Tue, May 30, 2017 at 2:00 PM David Blaikie <dblaikie at gmail.com> wrote:

> On Tue, May 30, 2017 at 1:50 PM Zachary Turner via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> 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.
>>
>
> Changing the header seems liable to rebuild a bunch of things that include
> the header - not sure any layering changes would change that, would they?
> (I don't think ninja at least waits for the library to build before
> building things that include the library's headers)
>
> Out of curiosity - what build system/linker/host compiler/OS are you
> using? (& cores/ram/etc too)
>
>
>>
>> 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.
>>
>
> That's why I made an equivalent change above tablegen to see what the
> relative cost was - I think that's probably pretty representative. Without
> tablegen touching any cpp file should (& with ninja, seems to) rebuild that
> object, the library, and the resulting binary & not much/anything else.
>
> - Dave
>
>
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>
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