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

Alexey Samsonov via llvm-dev llvm-dev at lists.llvm.org
Wed Feb 10 15:55:38 PST 2016

I mostly agree with what Richard and Justin said. Adding a few notes about
the general strategy we use:

(1) lit tests which look "end-to-end" proved to be way more convenient for
testing runtime libraries than unit tests. We do have
the latter, and use them to provide test coverage for utility functions,
but we quite often accompany fix to the runtime library with
"end-to-end" small reproducer extracted from the real-world code that
exposed the issue.
Incidentally, this tests a whole lot of other functionality: Clang driver,
frontend, LLVM passes, etc, but it's not the intent of the test.
These tests are sometimes platform-specific and poorly portable, but they
are more reliable (we make the same steps as the
user of the compiler), and serve the purpose of documentation.

(2) If we change LLVM instrumentation - we add a test to LLVM. If we change
Clang code generation or driver behavior - we add
a test to Clang. No excuses here.

(3) Sometimes we still add a compiler-rt test for the change in LLVM or
Clang: e.g. if we enhance Clang frontend to teach UBSan
to detecting yet another kind of overflow, it makes sense to add a test to
UBSan test-suite that demonstrates it, in addition to
Clang test verifying that we emit a call to UBSan runtime. Also,
compiler-rt test would allow us to verify that the actual error report
we present to the user is sane.

(4) True, we're intimidated by test-suite :) I feel that current use of
compiler-rt test suite to check compiler-rt libs better follows
the doctrine described by David. Also, there's significant complexity in
compiler-rt test suite that narrows the tests executed
to those supported by current host.

On Wed, Feb 10, 2016 at 2:33 PM, Xinliang David Li via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> On Wed, Feb 10, 2016 at 2:11 PM, Justin Bogner via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>> David Blaikie via cfe-dev <cfe-dev at lists.llvm.org> writes:
>> > 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?
>> I think there's a bit of grey area in terms testing the runtime -
>> generally it's pretty hard to use the runtime without a fairly
>> end-to-end test, so tests of the runtime often end up looking pretty
>> close to an end-to-end test.
>> That said, I don't think that should be used as an excuse to sneak
>> arbitrary end-to-end tests into compiler-rt. We should absolutely write
>> tests in clang and llvm that we're inputting what we expect to the
>> runtime and try to keep the tests in compiler-rt as focused on just
>> exercising the runtime code as possible.
> Yes, we should not use compiler-rt tests as an excuse of not adding
> clang/LLVM test. The latter should always be added if possible -- they are
> platform independent and is the first level of defense.  runtime test's
> focus is also more on the runtime lib itself and interaction between
>  runtime, compiler, binutils and other tools.
> David
>> IIUC, the correct place for integration tests in general is somewhere
>> like test-suite, but I think it's a bit intimidating to some people to
>> add new tests there (Are there docs on this?). I suspect some of the
>> profiling related tests in compiler-rt are doing a bit much and should
>> graduate to a spot in the test-suite (but I don't have time to volunteer
>> to do the work, unfortunately).
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Alexey Samsonov
vonosmas at gmail.com
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