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

[Zachary Turner via llvm-dev]

But it will still re-write the output files, updating their timestamps and
trigger rebuild of all dependencies (at worst) or a diff-and-noop (at
best), no?  Since the original proposal was about reducing the number of
situations in which tablegen runs at all, I would think that the best
comparison would be one in which tablegen doesn't run at all.

On Tue, May 30, 2017 at 2:29 PM Mehdi AMINI <joker.eph at gmail.com> wrote:

> 2017-05-30 14:28 GMT-07:00 Zachary Turner <zturner at google.com>:
>
>> Doesn't that just tell it what tablegen to use?  I was looking for an
>> option to make it not run *anything*, and just assume that all tablegen
>> definitions were up to date
>>
>
> If you tell it to run an external llvm-tblgen, this external tablegen
> becomes the producers of all the generated file. And since this external
> tablegen binary is not produced by your CMake, it does not depends on any
> source file.
>
> --
> Mehdi
>
>
>
>
>>
>> On Tue, May 30, 2017 at 2:27 PM Mehdi AMINI <joker.eph at gmail.com> wrote:
>>
>>> 2017-05-30 13:50 GMT-07:00 Zachary Turner via llvm-dev <
>>> llvm-dev at lists.llvm.org>:
>>>
>>>>
>>>>
>>>> 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.
>>>>
>>>
>>> I mentioned earlier in the thread that there is already a CMake option
>>> for this: -DLLVM_TABLEGEN=path/to/llvm-tblgen
>>>
>>>
>>> --
>>> Mehdi
>>>
>>
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