[llvm-dev] [RFC] Improving FileCheck

Fri Apr 3 11:05:15 PDT 2020

First of all, thanks for working on this. In my opinion, filecheck needs help, and anybody trying to make it better is a hero.

That said, from skimming the review thread, it seems to me that perhaps automatically detecting bad CHECKs might not be feasible. Perhaps better debugging tools might be a solution. If I could ask filecheck to list all directives in a file, I could easily find this stuff manually. If I used this hypothetical filecheck --list-directives, and I don’t see the one I just added, then I know it’s bad. This would be much easier to catch in code review in the future.

Such a debugging framework could be expanded in the future to do more fancy things. I’d love it if there were some sort of REPL like functionality where I can throw inputs at a directive and see what it finds.

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Jon Roelofs via llvm-dev
Sent: Friday, April 3, 2020 9:58 AM
To: llvm-dev at lists.llvm.org
Subject: [EXT] [llvm-dev] [RFC] Improving FileCheck

I'd like to (re)start a discussion on a few gotchas in FileCheck that I've noticed working on various tests in llvm's suites. This begain in a review [1], but I'll try to summarize here so it gets the right audience before decisions are made on it (so to speak).

1: https://reviews.llvm.org/D77227

The main sticking point is the abundance of checks in FileCheck tests that appear to be checking something, but are in fact silently hiding failures. The biggest class of this bug appears to be CHECK lines that omit the trailing colon, though there are a few others.

CHECK:            legitimate test
CHECK             gotcha A
CHECK :           gotcha B
CHECKNEXT:        gotcha C
CHECKDAG:         gotcha D
CHECK_NOT:        gotcha E
CHECK-LABEL-NOT:  ??? F
CHECK-SAME-DAG:   ??? G

Gotcha A
--------

CHECK  gotcha A

A lot of cases of (A) are benign, but buried in there are cases where we have tests that don't check what they intend to, which are broken when the missing colons are added [2]. Some grep analysis from paulr in [3] found some 178 tests across 72 test files that seem like likely mistakes, suggesting that having some automated tooling to catch this is probably not a bad idea.

In the review thread, a couple of issues surfaced with simply matching on `${CHECKNAME}\b`, making that less attractive as a remedy:

A1) There are quite a lot of RUN: lines that have CHECK names on them from their --check-prefix/--check-prefixes arguments, and we don't want tooling to match on those. This could be addressed with a script that quotes them all, but that would mean touching pretty much every test file, which is less than ideal.

A2) There are a few RUN lines with missing colons, though those seem infrequent enough to not worry about [5].

A3) There are quite a lot of mentions of CHECK names in comments that are clearly not meant to be tests [6]. Any solution to this, as far as I can tell, will likely need to reword many of those.

A4) We need some way to comment out CHECK lines that conveys intent better than removing the colon. This appears to be intentional in some testcases, but unintentional in the vast majority of them.

To address (A1), a number of rules were proposed in [1], the best of which seems to be that we look for lines matching `[#/;*!]\s*CHECK[ \t]`, and emit a diagnostic of some form to help correct it. This gave a pretty good false positive rate of 25% on the 186 tests it "broke" [7].

An open question here from jdenny is whether it makes sense to require all checks to follow that pattern (with the colon, of course) to make things less user-hostile [8]:

> Consider this example that has a well formed directive that doesn't follow the rule:
>
> // FIXME(201806L) CHECK: assert: 0
> Approach A (from a previous comment): FileCheck executes the directive. If the user later accidentally removes the :, FileCheck won't execute the directive and won't diagnose the error unless the user is wiling to endure false positives by opting into the more verbose mode Paul suggested.
>
> Approach B (from that some comment): FileCheck ignores the directive. That just makes things worse because the above otherwise well formed directive is then an undiagnosed malformed directive (unless the user opts into a more verbose mode).
>
> Approach C (new proposal): FileCheck reports the directive as an error (in any mode). The more verbose mode is still needed to catch the case that the : is missing here, but at least users are guaranteed to get a slap when they write them with :
2: llvm/test/Transforms/InstCombine/phi-preserve-ir-flags.ll
3: https://reviews.llvm.org/D77227#1955596
4: https://github.com/llvm/llvm-project/blob/56decd982dc03a74d1796d9d4dbd7d9e0cea98dc/llvm/lib/Support/FileCheck.cpp#L1141
5: llvm/test/CodeGen/AArch64/speculation-hardening.ll
6: llvm/test/MC/ARM/dwarf-asm-multiple-sections.s:88
7: https://reviews.llvm.org/differential/diff/254562
8: https://reviews.llvm.org/D77227#1958228

Gotcha B
--------

CHECK :  gotcha B

This pattern is a variant of (A) that also disables perfectly good tests, but in a way that isn't obvious that it doesn't work. jhenderson brings up some good points [9] in favor of extending FileCheck to make FileCheck do what the user intended here. Luckily, that doesn't seem to conflict with the rules proposed in (A).

9: https://reviews.llvm.org/D77227#1959041

Gotchas C, D, E
---------------

I believe these can be handled pretty simply in FileCheck itself, but I have not spent much time trying to estimate how many tests are affected by this class of bug.

??? F, G
--------

There are a number of check suffix combinations that are not explicitly supported (in the docs), but appear (maybe) useful. For these, there is some precedent on mitigating them within FileCheck itself [4], though the combinatorial explosion warrants being careful about how we go about detecting them (if at all).

--
Jon Roelofs
jroelofs at jroelofs.com<mailto:jroelofs at jroelofs.com>
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