[cfe-dev] RFC: End-to-end testing

[David Greene via cfe-dev]

[ I am initially copying only a few lists since they seem like
  the most impacted projects and I didn't want to spam all the mailing
  lists.  Please let me know if other lists should be included. ]

I submitted D68230 for review but this is not about that patch per se.
The patch allows update_cc_test_checks.py to process tests that should
check target asm rather than LLVM IR.  We use this facility downstream
for our end-to-end tests.  It strikes me that it might be useful for
upstream to do similar end-to-end testing.

Now that the monorepo is about to become the canonical source of truth,
we have an opportunity for convenient end-to-end testing that we didn't
easily have before with svn (yes, it could be done but in an ugly way).
AFAIK the only upstream end-to-end testing we have is in test-suite and
many of those codes are very large and/or unfocused tests.

With the monorepo we have a place to put lit-style tests that exercise
multiple subprojects, for example tests that ensure the entire clang
compilation pipeline executes correctly.  We could, for example, create
a top-level "test" directory and put end-to-end tests there.  Some of
the things that could be tested include:

- Pipeline execution (debug-pass=Executions)
- Optimization warnings/messages
- Specific asm code sequences out of clang (e.g. ensure certain loops
  are vectorized)
- Pragma effects (e.g. ensure loop optimizations are honored)
- Complete end-to-end PGO (generate a profile and re-compile)
- GPU/accelerator offloading
- Debuggability of clang-generated code

Each of these things is tested to some degree within their own
subprojects, but AFAIK there are currently no dedicated tests ensuring
such things work through the entire clang pipeline flow and with other
tools that make use of the results (debuggers, etc.).  It is relatively
easy to break the pipeline while the individual subproject tests
continue to pass.

I realize that some folks prefer to work on only a portion of the
monorepo (for example, they just hack on LLVM).  I am not sure how to
address those developers WRT end-to-end testing.  On the one hand,
requiring them to run end-to-end testing means they will have to at
least check out and build the monorepo.  On the other hand, it seems
less than ideal to have people developing core infrastructure and not
running tests.

I don't yet have a formal proposal but wanted to put this out to spur
discussion and gather feedback and ideas.  Thank you for your interest
and participation!

                        -David