[llvm-dev] FileCheck: combining -DAG and -NOT

[Liao, Michael via llvm-dev]

On Fri, 2016-03-11 at 17:36 +0000, Robinson, Paul wrote:
> +Michael Liao, who originally implemented the –DAG feature AFAICT.
> Hopefully the address I found by web searching is correct.
>  
> If you want to try turning it into an error, you could find all the
> places we do use interspersed -DAG and -NOT and have a discussion
> about whether they're more buggy than they are useful/correct.
>  
> I errored on –NOT followed by –DAG, which caught 44 tests (two of
> which are in FileCheck's own tests, but those are not elaborate
> enough to tell what the intent was).  As a side project this will
> take a little time to work through, especially as about half are in
> CodeGen tests for unfamiliar targets.
>  
> Commentary in FileCheck itself can easily be interpreted to mean the
> intent was that –NOT would scan the region between the points defined
> by the last match of the preceding DAG group (which the code gets
> right) and the first match of the following DAG group (which the code
> does not get right). But the commentary is not really that clear.

That's the intention of the original design. CHECK-NOT never occurs
before we find the start point (the start of file by default) and end
point (the end of file by default.) All other points are through other
CHECKs, including CHECK-DAG but excluding CHECK-NOT.  So that, if you
use CHECK-NOT, you need to be aware of how that range is defined. As
CHECK-DAG pattern matches a group of pattern in any order, the match
point of that group of CHECK-DAG (a consecutive CHECK-DAGs without any
other CHECKs interleaved) is always the point where one of that pgroup
is matched. If one CHECK-DAG is separated by any other CHECKs
(including CHECK-NOT) from preceding CHECK-DAGs, it is not in the
preceding group of CHECK-DAG. That's way how we could check the order
where a group of patterns should never occur before another group of
patterns.

Thanks
- Michael

>  
> --paulr
>  
> From: David Blaikie [mailto:dblaikie at gmail.com] 
> Sent: Monday, March 07, 2016 10:59 PM
> To: Robinson, Paul
> Cc: llvm-dev at lists.llvm.org
> Subject: Re: [llvm-dev] FileCheck: combining -DAG and -NOT
>  
> Yeah, fair - thanks for the explanation/correction/etc. 
> 
> That does seem a bit odd. Dunno.
>  
> On Mon, Mar 7, 2016 at 10:54 PM, Robinson, Paul <Paul_Robinson at playst
> ation.sony.com> wrote:
> Nope.  Sorry, your expectation is incorrect.
> "BETWEEN BEFORE AFTER" would be accepted, because: (1) the first –DAG
> matches BEFORE; (2) the –NOT range starts at the preceding match-
> point, i.e. the (end of the) BEFORE, thus does not find BETWEEN; (3)
> the second –DAG starts at the same point as the –NOT.
> That is, the first –DAG and the following –NOT *are* ordered; the
> –NOT and the subsequent –DAG are *not* ordered.
> You most certainly cannot intermix them freely and expect them all to
> look at the same range; that is explicitly not the documented (or
> actual) behavior.
> --paulr
>  
> From: David Blaikie [mailto:dblaikie at gmail.com] 
> Sent: Monday, March 07, 2016 8:43 PM
> To: Robinson, Paul
> Cc: llvm-dev at lists.llvm.org
> Subject: Re: [llvm-dev] FileCheck: combining -DAG and -NOT
>  
> I'd probably have expected the behavior it has - that -DAG and -NOT
> are not ordered with respect to each other, and form a bag of things
> ordered with respect to enclosing CHECK:s.
> 
> If you want to try turning it into an error, you could find all the
> places we do use interspersed -DAG and -NOT and have a discussion
> about whether they're more buggy than they are useful/correct.
>  
> On Mon, Mar 7, 2016 at 5:39 PM, Robinson, Paul via llvm-dev <llvm-dev
> @lists.llvm.org> wrote:
> If you look at the FileCheck documentation page:
> http://llvm.org/docs/CommandGuide/FileCheck.html
> you'll find this intriguing example of combining -DAG with -NOT
> (slightly amended to avoid some potential confusion):
> 
> ; CHECK-DAG: BEFORE
> ; CHECK-NOT: BETWEEN
> ; CHECK-DAG: AFTER
> 
> The page says this will reject the sequence "AFTER BEFORE", which is
> correct.  It's intuitively obvious that it will also reject the text
> "BEFORE BETWEEN AFTER" as well.
> 
> Pop Quiz:  Will it accept or reject "BEFORE AFTER BETWEEN"?
> No, don't go try it, that's cheating; what do you *think* will
> happen?
> Take a minute, I won't mind.
> ---
> 
> So, here's the story.
> 
> Normally a -NOT line will scan the text between the points where the
> preceding and following CHECK lines match.  By that mental model,
> you'd
> expect the first -DAG to find BEFORE, the second -DAG to find AFTER,
> and
> then -NOT would verify the absence of "BETWEEN" between those two
> points.
> All very intuitive, and I've certainly seen tests written to expect
> that.
> 
> However, things get a little funky when you have -NOT followed by
> -DAG.
> What's the endpoint of the -NOT search?  The thing that follows isn't
> a CHECK, it's a CHECK-DAG, except that -NOT is already kind of DAG-
> ish
> so you have two DAG-ish groups staring at each other wondering who
> will
> go first.
> 
> In this case, turns out, it's the -NOT who blinks first.  The
> endpoint
> of the search is implicitly the end-of-input.  The Pop Quiz answer
> is:
> *Rejected.*  The BETWEEN occurs after BEFORE, and before the end-of-
> input.
> 
> Here's the real question:  *Should* FileCheck run the second -DAG
> group
> before it runs the -NOT group?  Then the range for the -NOT would be
> bounded by the matching points for the surrounding -DAG lines, which
> probably matches what basically everybody expects to happen.
> 
> Or, leave things as they are, and add a cautionary tale to the
> FileCheck
> documentation page?  Given there's an actual documentation example,
> it
> would seem inappropriate to make it an *error* if a -NOT is followed
> by a -DAG!
> 
> Thanks,
> --paulr
> 
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>  
-- 
Thanks
- Michael