PGO not only for own code but for D runtime & Phobos too?

[Jon Degenhardt]

On Friday, 4 October 2019 at 08:55:33 UTC, Robert M. Münch wrote:
> On 2019-10-03 16:01:19 +0000, Jon Degenhardt said:
>
>> I did a study of LTO and PGO on the combined application and 
>> libraries (druntime & phobos). It's available here: 
>> https://github.com/eBay/tsv-utils/blob/master/docs/lto-pgo-study.md.
>
> Thanks a lot, very interesting.
>
> How did you include druntime & phobos into your app code build 
> environment? What about extended compile times?
>
> I'm wondering what would the best dev process look like when 
> doing such a overall LTO+PGO optimization.

I'm not quite sure what the best build process would look like. 
For the tsv utilities I put the build steps in make files, but 
its rather clunky and may not work well for larger projects. I 
don't have it working with dub. There appears to be a nice 
opportunity for dub to make the basic cases easier.

For the tsv apps the build steps are relatively easy, as 
individually they are small code bases and have no external 
dependencies. With the LDC 1.9 release the LTO build part became 
very easy. The LTO versions of druntime and phobos are shipped 
with the pre-built binaries, so the LTO part is a couple of 
compiler switches ('ldc2 -flto=thin 
-defaultlib=phobos2-ldc-lto,druntime-ldc-lto' see: 
https://github.com/ldc-developers/ldc/releases/tag/v1.9.0 for 
details).

For PGO I added custom build steps in the makefile, mostly 
following the steps described in Johan Engelen's blog post: 
https://johanengelen.github.io/ldc/2016/07/15/Profile-Guided-Optimization-with-LDC.html

I did create a very simple app built with LTO & PGO using dub, 
including external dependencies here: 
https://github.com/jondegenhardt/dcat-perf. The dub.json file is 
short enough you'll be able to read it and see the basic steps it 
takes. Despite that the dub.json file is short, I found it rather 
tricky to get correct.

--Jon