[llvm-dev] [FileCheck] Error for unused check prefixes (was: [RFC] FileCheck: (dis)allowing unused prefixes)

[Nikita Popov via llvm-dev]

On Wed, Feb 3, 2021 at 12:50 AM Fāng-ruì Sòng <maskray at google.com> wrote:

> Last October Mircea sent an RFC about (dis)allowing unused check prefixes
> in FileCheck:
> https://lists.llvm.org/pipermail/llvm-dev/2020-October/146162.html
> (In short, if FileCheck --check-prefix or --check-prefixes specifies a
> prefix which does not occur in the test, FileCheck will error. Note: a
> `-NOT` pattern is also counted as an occurrence.)
>
> Mircea created a worksheet
> https://docs.google.com/spreadsheets/d/1o6q3XH1n3DDyyccnYZ_kVfhFbTDzC_S09e973_cwYuw/edit#gid=0 which
> may be a bit stale now but llvm-project has reached a state where all
> issues have been identified and fixed, or worked around (by opting in
> FileCheck --allow-unused-prefixes in some test directories, such as:
> clang/test/OpenMP, llvm/test/Transforms/Attributor and llvm/test/FileCheck).
>
> We can make a switch if the community thinks that not allowing unused
> prefixes is the better default: https://reviews.llvm.org/D95849
> For downstream projects using FileCheck and lit, it should be easy to
> restore the previous permissive behavior
>
> from lit.llvm.subst import ToolSubst
>
> fc = ToolSubst('FileCheck', unresolved='fatal')
> config.substitutions.insert(0, (fc.regex, 'FileCheck --allow-unused-prefixes'))
>
> # Note: if multiple --allow-unused-prefixes options are specified, the last wins.
>
>
> Personally I am strongly in favor of disallowing unused check prefixes by
> default. We have identified numerous issues:
>
> (1) typo. A misspelled check prefix does not test what it intends to test.
> (2) bitrot tests due to refactoring
> (3) unspecified `-NOT` patterns. Sometimes a test uses something like
> `--check-prefixes=COMMON,TRUE` and `--check-prefixes=COMMON,FALSE` to test
> both behaviors but they forget to include a `FALSE-NO:` pattern to test
> that some string does not appear.
>
> (1) and (2) are especially common. There are indeed tests where
> --allow-unused-prefixes is more suitable - but they are sufficiently few
> that I think the default should be --allow-unused-prefixes=false.
>
> So, what do folks think?
>

Just my 2c, but I think we should allow unused prefixes. This does catch
the occasional typo, but also has a cost: Historically, certain kinds of
tests simply used a certain boilerplate of check lines, because differences
are common, even if they don't occur for each test. For X86 vector tests,
it makes more sense to simply always include AVX1 and AVX2 test prefixes,
even if it so happens that for *this* particular test, codegen is identical
and only the AVX prefix ends up being used. This means that whenever
codegen changes in a minor way (e.g. due to a target-independent
SimplyDemandedBits change that has no direct relation to X86) and a
difference is introduced, you need to now figure out which new prefixes you
have to add. Or drop prefixes if a codegen difference goes away. Having to
manually adjust check prefixes takes away from the usual experience of
"Just run update_(llc_)test_checks".

At least I personally have found the gradual migration towards disallowing
unused prefixes to be more annoying than useful. I guess ergonomics could
be improved if update_test_checks automatically dropped unused prefixes,
but there's really no way to automatically add prefixes, without
domain-specific knowledge.

Regards,
Nikita
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20210203/cb403b6f/attachment.html>