DMD Performance Regression Publisher [GSoC 2026]

[Abul Hossain Khan]

Hi everyone,

Last time we had switched the host compiler from DMD to 
ldc2-1.42.0, which gave a nice drop in instruction counts and 
compiler size. Since then I finished my semester exams and got 
back to the work.


#### PGO
  DMD already has a `dmd-pgo` build target that does the full LDC 
PGO cycle, So the first approach was just use that from the 
workflow for base and head separately. Got that working, but the 
training was actually failing silently cause Phobos was building 
after `dmd-pgo`, so the instrumented compiler couldn't import 
stdlib during training, so it barely trained on anything. Then I 
fixed the ordering and it worked!

But then another problem was that it was showing weird result 
after that like ~5.5% delta on sizes and also the Instruction 
Count was showing weird deltas. That's because base and head were 
each training their own PGO profile independently, in parallel, 
the merge data comes out slightly different every single run. So 
the two sides end up with two different profiles even on 
identical source, which shows up as noise in the diff. Fixed it 
by training PGO once and reusing that same profile for both 
builds. After that It was stable across the runs.

Here's how the numbers moved across every stage so far:

| Metric | DMD host | LDC2 host | LDC2 + PGO |
|--------|----------|-----------|------------|
| compile hello.d (instr) | 422.8 M | 257.6 M | 224.1 M |
| compile hello.d -O (instr) | 445.9 M | 277.9 M | 242.7 M |
| dmd binary size (stripped) | 11.93 MB | 6.96 MB | 6.89 MB |
| hello binary size | 0.72 MB | 0.72 MB | 0.72 MB |
| peak RSS | 56 MB | 50 MB | 50 MB |


#### Fork PRs

The system works fine on my forked repo, but a PR that comes from 
a fork repo gets a read-only token, so the comment step would 
just fail there.
Did some research and the usual fix for this is to split it into 
two workflows -
`perf.yml` - does the build + measure and just uploads the 
results as an artifact,
`perf-comment.yml` - runs on `workflow_run` after that finishes, 
in the base repo context, so it gets a writable token, downloads 
the artifact and posts the comment. It should works fine.

Now the thing is, `workflow_run` only triggers off the copy of 
the workflow that's on the default branch. So until this is 
merged to `master`, `perf-comment.yml` just never fires, even 
though `perf.yml` runs fine and uploads the artifact.

PR link - https://github.com/dlang/dmd/pull/23380

#### What's next:
- After the PR got Reviewed and merged We can add more workloads 
into it, and start working on the dashboard

Thanks again to Dennis for the amazing mentorship.
As always, feel free to leave any feedback or suggestions!