[llvm-dev] Incrementally compiling LLVM

[Mehdi AMINI via llvm-dev]

On Wed, Apr 14, 2021 at 6:36 PM Stephen Neuendorffer via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> This flow is being used successfully by both npcomp and circt, which are
> llvm incubator projects.  You should be able to leverage/build off of the
> github actions flows that they define.  see https://github.com/llvm/circt
> and https://github.com/llvm/mlir-npcomp
>

Is it the same flow?
As far as I can tell you're not getting a build directory in order to do an
incremental rebuild with modified sources, but instead getting the build
artifacts in order to build a downstream project entirely.

The only way I can see incremental compilation to work would be to get the
entire source tree from the cache alongside with the build tree, and then
run `git fetch && git checkout <rev>`. This would mark only the source file
changed by git during the checkout.
Otherwise a fresh `git clone` will have the source files modified data as
the time of the checkout, so more recent than the cached build dir.

-- 
Mehdi



>
> Steve
>
> On Wed, Apr 14, 2021 at 12:59 PM Francesco Bertolaccini via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> On 14/04/2021 21:18, David Blaikie wrote:
>> > Sounds like something's going wrong for sure. How are you measuring
>> > the time? (how much time is it taking) and what are you testing by
>> > "partial recompilation"? Touching an ADT .h file is likely to
>> > recompile nearly everything so might not be much better than a clean
>> > build - but touching a .cpp file (especially a .cpp file for a single
>> > tool, rather than a library) might be quite quick. So try timing a
>> > full clean build (ninja -t clean, ninja clang) and then incrementally
>> > touching just a .cpp file (eg: clang/tools/driver/driver.cpp  ) and
>> > see how they compare
>>
>> I currently do not have sufficient resources to compile LLVM on my
>> personal machine, so I am (ab?)using GitHub Actions. My workflow is
>> setup such that the build artifacts are cached and restored on every
>> worflow run. The time is measured automatically by GitHub itself, and is
>> around ~2h for each run, whether from scratch or starting from the cache.
>>
>> I don't generally edit .h files directly, unless they are generated by
>> modifying TableGen files.
>>
>> The fact that someone else has successfully been able to do incremental
>> builds _does_ make it seem like it's a configuration issue on my part.
>>
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