[llvm-dev] FileCheck idiom difficulties

[Robinson, Paul via llvm-dev]

I have to say, I’m not super excited about the answer always being “let’s add another FileCheck directive.”  FileCheck is basically a meta-grep, that knows how to execute some sub-searches in different orders.  Asking it to do things like implicit paren balancing is really out of scope for the tool.
--paulr

From: Reid Kleckner <rnk at google.com>
Sent: Wednesday, November 6, 2019 2:13 PM
To: James Henderson <jh7370.2008 at my.bristol.ac.uk>
Cc: llvm-dev <llvm-dev at lists.llvm.org>; George Rimar <grimar at accesssoftek.com>; Fāng-ruì Sòng <maskray at google.com>; Robinson, Paul <paul.robinson at sony.com>; thomasp at graphcore.ai
Subject: Re: [llvm-dev] FileCheck idiom difficulties

I think we should think about changing llvm-readobj to produce more filecheck friendly output.

This was a problem we used to have with LLVM debug info metadata: it wasn't very structured, and it was printed in some arbitrary order. Then Duncan (I think) added the DI* classes, which made it easier to match something semantic, like `DILocalVariable.*name: "foo"`, and standardized on a topological output ordering, so you could start from the variable, then match the type, and then find the type metadata number later.

If we printed `Section (.foo) {` for example, that would help some.

It still doesn't help establish delimited regions for properties printed across multiple lines for readability, though... Should we add some kind of ad-hoc delimiter balancing to FileCheck? Something like:

CHECK-SCOPE: Section {
CHECK: Name: asdf
CHECK: Name: asdf
CHECK-ENDSCOPE: }

A SCOPE directive line would have to end in a known delimiter, '(', '{', or '['. The ENDSCOPE directive would only match lines with delimiters that balance with the opening delimiter. It would kind of work for filechecking JSON, for example.

The scope idea here is pretty half-baked, but it's food for thought.

On Wed, Nov 6, 2019 at 4:18 AM James Henderson via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:

Hi all,

Many of our lit tests use FileCheck and a tool like llvm-readobj to check properties of a section header/symbol/etc. A typical (pseudoised for brevity) output to match against might be something like the following:

Symbols [
  Symbol {
    Name: foo
    Value: 0
    Type: Function
    Section: .foo (1)
  }
  Symbol {
    Name: bar
    Value: 1
    Type: Object
    Section: .foo (1)
  }
]

and your lit test might want to check the properties of the foo symbol like so:

# CHECK:      Name: foo
# CHECK-NEXT: Value: 0
# CHECK-NEXT: Type: Function
# CHECK-NEXT: Section: .foo (1)

This is fine. But what if you only care about the section of a symbol, and not the value or type etc? You could do the following:

# CHECK: Name: foo
# CHECK: Section: .foo (1)

Hopefully some of you will already notice the problem with this approach: if foo was in, say, the .baz section, the test will spuriously pass, because the Section line will match the Section line for .bar. One alternative to this is to explicitly match each field in between, using CHECK-NEXT:

# CHECK:      Name: foo
# CHECK-NEXT: Value:
# CHECK-NEXT: Type:
# CHECK-NEXT: Section: .foo (1)

This works, but somewhat hides what is really being tested by adding extra noise to the checks. In reality, there are actually other fields too that need to be listed, meaning the "interesting" parts of the test are even more hidden.

I recently started using yet another approach:

# CHECK: Name: foo
# CHECK: Section:
# CHECK-SAME:     .foo (1)

This works because the Section: matched will be the first one found, i.e. the one belonging to foo, and then .foo will be looked for on the same line. However, I noticed today that this pattern has its own problem, namely that there could be something between the Section tag and .foo. In other words, the above pattern would match "Section: .bar.foo". A couple of solutions to this are:

# CHECK: Section:
# CHECK-NOT: {{[:graph:]}}
# CHECK-SAME: .foo (1)

# CHECK: Section:
# CHECK-SAME: {{^}} .foo (1)

The first one ensures that there's no non-whitespace between the end of "Section:" and the start of ".foo (1)". The second ensures that the start of the CHECK-SAME match is the "start of line", and since the first half of the line has already been consumed, it means " .foo (1)" must immediately follow "Section:". However, the first is even less readable than the current CHECK-SAME approach, whilst the second is somewhat confusing if you don't realise that FileCheck effectively consumes the things it has matched already, so that they effectively don't exist any more.

Does anybody have any other suggestions/thoughts/comments? One idea I had was for a new directive something like "CHECK-IMMEDIATE" which is implicitly the same as the final approach I suggested above, but maybe adding a new directive to achieve this isn't the right approach?

James
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