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

[David Blaikie via llvm-dev]

On Wed, Feb 3, 2021 at 11:46 AM Nikita Popov <nikita.ppv at gmail.com> wrote:
>
> 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.

Is it particularly burdensome for those tests in particular to opt-in
to allowing unused prefixes on the RUN/FileCheck line? (if that's the
right tradeoff/makes things smoother compared to finding/adding the
prefixes only when they become necessary)

> 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