[cfe-dev] Testing Best Practices/Goals (in the context of compiler-rt)

[Richard Smith via cfe-dev]

On Wed, Feb 10, 2016 at 11:06 AM, David Blaikie via cfe-dev
<cfe-dev at lists.llvm.org> wrote:
> Recently had a bit of a digression in a review thread related to some tests
> going in to compiler-rt (
> http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20160208/330759.html
> ) and there seems to be some disconnect at least between my expectations and
> reality. So I figured I'd have a bit of a discussion out here on the dev
> lists where there's a bit more visibility.
>
> My basic expectation is that the lit tests in any LLVM project except the
> test-suite are targeted tests intended to test only the functionality in the
> project. This seems like a pretty well accepted doctrine across most LLVM
> projects - most visibly in Clang, where we make a concerted effort not to
> have tests that execute LLVM optimizations, etc.
>
> There are exceptions/middle ground to this - DIBuilder is in LLVM, but
> essentially tested in Clang rather than writing LLVM unit tests. It's
> somewhat unavoidable that any of the IR building code (IRBuilder, DIBuilder,
> IR asm printing, etc) is 'tested' incidentally in Clang in process of
> testing Clang's IR generation. But these are seen as incidental, not
> intentionally trying to cover LLVM with Clang tests (we don't add a Clang
> test if we add a new feature to IRBuilder just to test the IRBuilder).
>
> Another case with some middle ground are things like linker tests and
> objdump, dwarfdump, etc - in theory to isolate the test we would checkin
> binaries (or the textual object representation lld had for a while, etc) to
> test those tools. Some tests instead checkin assembly and assemble it with
> llvm-mc. Again, not to cover llvm-mc, but on the assumption that llvm-mc is
> tested, and just using it as a tool to make tests easier to maintain.
>
>
> So I was surprised to find that the compiler-rt lit tests seem to diverge
> from this philosophy & contain more intentional end-to-end tests (eg: adding
> a test there when making a fix to Clang to add a counter to a function that
> was otherwise missing a counter - I'd expect that to be tested in Clang and
> that there would already be coverage in compiler-rt for "if a function has a
> counter, does compiler-rt do the right thing with that counter" (testing
> whatever code in compiler-rt needs to be tested)).
>
> Am I off base here? Are compiler-rt's tests fundamentally different to the
> rest of the LLVM project? Why? Should they continue to be?

If a change is made to Clang to make it emit different IR, there
should be a test for that in Clang's test suite. Adding a test to
compiler-rt should generally not be considered sufficient, especially
as plenty of people hack on Clang without compiler-rt checked out /
without running the compiler-rt tests.

On the compiler-rt side, taking the sanitizer tests as an example: I
would view testing that the compiler produces the right machine code
as being an incidental part of the unit test coverage in the same way
that an objdump test covering llvm-mc is incidental coverage -- it
should not be the point of the test, but it may be the most direct way
to demonstrate that the runtime library behaves in the right way in
the presence of the expected set of calls to it.

If the compiler-rt maintainers want to go beyond that and include
end-to-end tests alongside their unit tests, and doing so doesn't make
their tests fragile, I think that's their call, but it might make
organizational sense to separate those tests out into a different
directory at least.