llvm dev - Jan 2021 - [RFC] Cross-project lit test suite

On Mon, Jan 25, 2021 at 3:36 AM James Henderson via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> Dear all,
>
>
> Recently, I and a number of my colleagues have run into cases where we
would like the ability to write tests that involve components from multiple LLVM
projects, for example using both clang and LLD. Similarly, I have seen a few
instances recently where tests would ideally make use of LLD but only to help
generate input objects for testing an LLVM tool, such as llvm-symbolizer (see
for example https://reviews.llvm.org/D88988). Currently, there is no location
where lit tests that use both clang and LLD can be put, whilst the
llvm-symbolizer cases I’ve hit are testing llvm-symbolizer (and not LLD), so
don’t really fit in the LLD test suite. I therefore have prototyped a lit test
suite that would be part of the monorepo, and which can support tests that use
elements from multiple projects - see https://reviews.llvm.org/D95339. Tests
could be added to this suite as needed. The suite is modelled as an additional
top-level directory, and is enabled by enabling the “cross-project-tests”
project in CMake. I have initially added support for both LLD and clang. At
configuration time, the tests that require LLD or clang will only be enabled
when the respective projects are enabled, so that developers continue to benefit
from the subset of tests that are applicable for the projects they are building.
Note that I am not especially familiar with CMake or lit, so this code may not
be perfect, but it should be sufficient to demonstrate what it can do.
>
>
>
> One could argue that these sorts of tests should belong in the external (to
the monorepo) test-suite, but this is a) quite distant from the existing
testing, and therefore easily forgotten, delaying potential feedback for any
breakages/resulting in potentially duplicate testing etc, and b) is not as easy
to set up and run (owing to the fact that it isn’t part of the monorepo, isn’t
connected to check-all etc), therefore making it harder for developers to
maintain the tests. Back in October 2019, there was an extensive discussion on
end-to-end testing and how to write them (starting from
https://lists.llvm.org/pipermail/cfe-dev/2019-October/063509.html). The
suggestion was that these tests would be lit-based and run as part of check-all,
and would not be inside the clang tree, although there was some opposition. This
concluded with a round table. Unfortunately, I am unaware of what the conclusion
of that round table conversation was, so it’s possible that what I am proposing
is redundant/being worked on by someone else. Additionally, I don’t consider all
classes of tests that the proposed lit suite would be useful for to be
“end-to-end” testing. For example, llvm-symbolizer is usually used on linked
output containing debug information. Usually tests that consume objects that
have debug data in them rely on assembly that has been written by hand or
generated by clang prior to commit, with a limited set able to make use of
yaml2obj to generate the debug data instead. However, the output of these
approaches is typically not a fully linked output (yaml2obj output can be made
to look like one, but getting all the addresses to match up in a maintainable
manner makes this approach not particularly viable). Being able to use LLD to
link the object file produced would make the test significantly more readable,
much as using llvm-mc and assembly to generate test inputs is more preferable to
using prebuilt binaries. Such a test is ultimately not really any more an
end-to-end test than an llvm-symbolizer test that just uses the object produced
by the assembler directly.
>
>
>
> What do people think?
Some concerns (the usual: Things should be tested in isolation, things
should be tested independently - but end to end tests have some value
too), but generally seems good.

Though perhaps debuginfo-tests (this presumably already supports the
multiple-subproject mechanical isssue you're discussing?) could be
generalized/renamed to be all our cross-project lit testing
(Essentially an in-monorepo, lit-only, high-reliability/not-flakey/etc
version of the test-suite).

- Dave

On Mon, Jan 25, 2021 at 12:16 PM David Blaikie via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> On Mon, Jan 25, 2021 at 3:36 AM James Henderson via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> >
> > Dear all,
> >
> >
> > Recently, I and a number of my colleagues have run into cases where we
> would like the ability to write tests that involve components from multiple
> LLVM projects, for example using both clang and LLD. Similarly, I have seen
> a few instances recently where tests would ideally make use of LLD but only
> to help generate input objects for testing an LLVM tool, such as
> llvm-symbolizer (see for example https://reviews.llvm.org/D88988).
> Currently, there is no location where lit tests that use both clang and LLD
> can be put, whilst the llvm-symbolizer cases I’ve hit are testing
> llvm-symbolizer (and not LLD), so don’t really fit in the LLD test suite. I
> therefore have prototyped a lit test suite that would be part of the
> monorepo, and which can support tests that use elements from multiple
> projects - see https://reviews.llvm.org/D95339. Tests could be added to
> this suite as needed. The suite is modelled as an additional top-level
> directory, and is enabled by enabling the “cross-project-tests” project in
> CMake. I have initially added support for both LLD and clang. At
> configuration time, the tests that require LLD or clang will only be
> enabled when the respective projects are enabled, so that developers
> continue to benefit from the subset of tests that are applicable for the
> projects they are building. Note that I am not especially familiar with
> CMake or lit, so this code may not be perfect, but it should be sufficient
> to demonstrate what it can do.
> >
> >
> >
> > One could argue that these sorts of tests should belong in the
external
> (to the monorepo) test-suite, but this is a) quite distant from the
> existing testing, and therefore easily forgotten, delaying potential
> feedback for any breakages/resulting in potentially duplicate testing etc,
> and b) is not as easy to set up and run (owing to the fact that it isn’t
> part of the monorepo, isn’t connected to check-all etc), therefore making
> it harder for developers to maintain the tests. Back in October 2019, there
> was an extensive discussion on end-to-end testing and how to write them
> (starting from
> https://lists.llvm.org/pipermail/cfe-dev/2019-October/063509.html). The
> suggestion was that these tests would be lit-based and run as part of
> check-all, and would not be inside the clang tree, although there was some
> opposition. This concluded with a round table. Unfortunately, I am unaware
> of what the conclusion of that round table conversation was, so it’s
> possible that what I am proposing is redundant/being worked on by someone
> else. Additionally, I don’t consider all classes of tests that the proposed
> lit suite would be useful for to be “end-to-end” testing. For example,
> llvm-symbolizer is usually used on linked output containing debug
> information. Usually tests that consume objects that have debug data in
> them rely on assembly that has been written by hand or generated by clang
> prior to commit, with a limited set able to make use of yaml2obj to
> generate the debug data instead. However, the output of these approaches is
> typically not a fully linked output (yaml2obj output can be made to look
> like one, but getting all the addresses to match up in a maintainable
> manner makes this approach not particularly viable). Being able to use LLD
> to link the object file produced would make the test significantly more
> readable, much as using llvm-mc and assembly to generate test inputs is
> more preferable to using prebuilt binaries. Such a test is ultimately not
> really any more an end-to-end test than an llvm-symbolizer test that just
> uses the object produced by the assembler directly.
> >
> >
> >
> > What do people think?
>
> Some concerns (the usual: Things should be tested in isolation, things
> should be tested independently - but end to end tests have some value
> too), but generally seems good.
>
Indeed this is a usual concern: such tests shouldn't be seen as replacing
isolated lit tests ("unit tests").
But I have another question about the cost of maintenance here: are we
gonna revert patches to either project when one of the integration tests
fails?
What about integration tests that require to be updated manually when
changing another component?
I find the cost of maintenance of end-to-end tests is often hard to carry
over, especially since they are only supplementing and not replacing lit
"unit-tests".

Best,

-- 
Mehdi


>
> Though perhaps debuginfo-tests (this presumably already supports the
> multiple-subproject mechanical isssue you're discussing?) could be
> generalized/renamed to be all our cross-project lit testing
> (Essentially an in-monorepo, lit-only, high-reliability/not-flakey/etc
> version of the test-suite).
>
> - Dave
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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