[llvm-dev] Testing "normal" cross-compilers versus GPU backends

Thu Sep 3 17:56:56 PDT 2015

> -----Original Message-----
> From: Mehdi Amini [mailto:mehdi.amini at apple.com]
> Sent: Thursday, September 03, 2015 3:26 PM
> To: Robinson, Paul
> Cc: Tom Stellard; llvm-dev at lists.llvm.org; NAKAMURA Takumi
> Subject: Re: Testing "normal" cross-compilers versus GPU backends
> 
> 
> > On Sep 3, 2015, at 11:23 AM, Robinson, Paul
> <Paul_Robinson at playstation.sony.com> wrote:
> >
> >
> >
> >> -----Original Message-----
> >> From: Tom Stellard [mailto:tom at stellard.net]
> >> Sent: Thursday, September 03, 2015 7:31 AM
> >> To: Mehdi Amini
> >> Cc: Robinson, Paul; llvm-dev at lists.llvm.org; NAKAMURA Takumi
> >> Subject: Re: Testing "normal" cross-compilers versus GPU backends
> >>
> >> On Thu, Sep 03, 2015 at 02:07:54AM -0700, Mehdi Amini wrote:
> >>>
> >>>> On Sep 3, 2015, at 12:18 AM, Robinson, Paul
> >> <Paul_Robinson at playstation.sony.com> wrote:
> >>>>
> >>>>
> >>>>
> >>>>> -----Original Message-----
> >>>>> From: Mehdi Amini [mailto:mehdi.amini at apple.com]
> >>>>> Sent: Wednesday, September 02, 2015 7:10 PM
> >>>>> To: Robinson, Paul
> >>>>> Cc: llvm-dev at lists.llvm.org; tom at stellard.net; NAKAMURA Takumi
> >>>>> Subject: Re: Testing "normal" cross-compilers versus GPU backends
> >>>>>
> >>>>> Hi Paul,
> >>>>>
> >>>>> Thanks for the summary!
> >>>>>
> >>>>>> On Sep 2, 2015, at 5:44 PM, Robinson, Paul
> >>>>> <Paul_Robinson at playstation.sony.com> wrote:
> >>>>>>
> >>>>>> This note arose from
> >> https://urldefense.proofpoint.com/v2/url?u=http-
> >>>>> 3A__reviews.llvm.org_D12506&d=BQIFAg&c=eEvniauFctOgLOKGJOplqw&r=v-
> >>>>>
> >>
> ruWq0KCv2O3thJZiK6naxuXK8mQHZUmGq5FBtAmZ4&m=Wr0uOhkAp_10X4edWwxZQ9V8L97j8e
> >>>>> o6cR_1Ia-gMOw&s=OOTP9DnL-TWV1zvy9EcU0Z6yfTq5lBjhE-LvYlWMJ3Y&e=  but
> >> the
> >>>>> reviewers
> >>>>>> felt that we needed a broader audience, because the proposed patch
> >>>>>> really didn't solve the entire problem and we had no better ideas.
> >>>>>>
> >>>>>> Mehdi Amini needs to build LLVM with just a GPU backend, and still
> >> have
> >>>>>> "ninja check" Just Work.  Commits r243958-243960 tried to
> accomplish
> >>>>>> that; however they are too big a hammer, and cause much simpler
> >> cross
> >>>>>> environments (like mine) to exclude big chunks of very useful tests
> >>>>>> (including my favorite, DebugInfo).
> >>>>>>
> >>>>>> FYI, my main cross environment is building on X86 Windows but using
> >>>>>> a default target triple for PS4 (which is also X86).
> >>>>>>
> >>>>>> I experimented with building LLVM with just the ARM backend
> (running
> >> on
> >>>>>> an X86 workstation) and setting the default triple to some ARM
> >> value.
> >>>>>> "ninja check" worked fine (without Mehdi's series of commits), so
> >> the
> >>>>>> normal kind of cross-compiler environment seems to be happy with
> how
> >>>>>> things were set up originally.
> >>>>>>
> >>>>>> Mehdi reports building LLVM with the X86 and AMDGPU backends,
> >> setting
> >>>>>> the default triple to "amdgcn--amdhsa", and getting 200-some
> >> failures.
> >>>>>>
> >>>>>> (This does make me wonder about AMDGPU testing in general; how does
> >> that
> >>>>>> work?  The only places I see lit checks for AMDGPU are in the usual
> >>>>>> target-dependent places.)
> >>>>>
> >>>>> I don’t understand this interrogation about how do you do testing in
> >>>>> general. The same way you don’t process tests/CodeGen/X86/* with the
> >> ARM
> >>>>> backend, you can’t process any random IR through these backends.
> >>>>
> >>>> You said you had 200+ failures with AMDGPU.  Are the AMD folks simply
> >>>> tolerating the 200 failures, and you don't want to?  I should hope
> >> there
> >>>> is more to it than that.
> >>>
> >>> Well, I don’t know, they might just run `ninja check` with the default
> >> triple set to X86?
> >>> (which I would consider being working around a buggy test suite)
> >>>
> >>
> >> I always enable AMDGPU and X86 when I build, so I've never run into
> >> this problem.
> >>
> >> -Tom
> >
> > Tom, presumably your default target triple is X86-ish?  And the only
> > tests to exercise the AMDGPU backend are those that explicitly specify
> > a triple for AMDGPU?
> 
> This is how we used to do it as well, and I assume this is how most of
> backends out of the main CPUs are dealing with the situation.
> This is what I consider a bug and was trying to solve (admittedly not in
> an optimal way).
> 
> > Mehdi, assuming that's what Tom does, your stated goal was to be able to
> > run tests *without* including the X86 backend, so Tom's solution won't
> > work for you (restating just for confirmation).
> 
> Yes, or alternatively I expect the test suite to pass whatever the default
> triple is.

Um, that may be asking a bit much...

> 
> > Krzysztof suggested much the same thing that I think you are currently
> > doing, which is deliberately configure a default triple but exclude the
> > corresponding backend.
> 
> You and Takumi were considering this as an unsupported configuration
> before, and I tend to agree with that (this is the configuration I’m using
> for our tests but it was not intentional to leave the default triple
> unset).

Right, intuitively it doesn't make sense.  Is it actually useful to build a 
GPU compiler that will crash unless you ask it to generate GPU code? Seems
to me it should default to producing GPU code.

> 
> > I expect we can detect that situation in lit.cfg
> > and exclude tests on that basis, rather than 'native'.  It would solve
> > the problem for my case (host triple != target triple, although the arch
> > parts of the triple do match) and the "normal" cross-compiler case (e.g.
> > host = X86, backend + target triple = ARM).
> >
> > I'm going to play around with that and see what I can do to make it
> work.
> >
> >>
> >>
> >>>>
> >>>>>
> >>>>> IMO, the problem is in general about tests that are written without
> >>>>> specifying a triple, that will be executed with the default triple.
> >>>>>
> >>>>> Most of these tests were written with X86 (or ARM) in mind, and
> there
> >> is
> >>>>> no guarantee that they will behave as intended with every possible
> >> triple.
> >>>>> The DataLayout for instance has to be the one from the target, and
> is
> >> not
> >>>>> portable.
> >>>>> I think a "portable backend test” is pretty rare in general.
> >>>>
> >>>> It depends on what the test is trying to do.  I'm sure it is quite
> >> common
> >>>> for IR tests to behave essentially the same way regardless of target.
> >>>
> >>> IR tests != backend test (I may miss your point here, it’s late…).
> >
> > Right, sorry, lost focus for a moment there... nevertheless it is still
> > the case that many tests exercise functionality that is not particularly
> > target-centric and these should be run for any target that actually
> > supports that functionality.  For example, the DebugInfo tests should
> > be run for any target that supports emitting debug info.
> 
> I’m not sure that “debug info” support is all or nothing.
> As an extreme example, I know targets that support debug info but do not
> support function calls, what if your "debug info” test involves these?

Then as part of getting the test suite to work for you, you would need to
disable that particular test for your target.  It sounds like this kind of
thing is exactly what the Hexagon folks did, and it seems quite reasonable.
(And in fact I see two DebugInfo tests marked XFAIL: hexagon.)

> 
> Also, I’m not a DebugInfo expert, but when a front-end generated them,
> aren’t they dependent on the DataLayout? Hence the target?

Not really. DebugInfo tests primarily care what the DWARF description
looks like, not so much what the generated code looks like, and there
are 100 or so DebugInfo tests that work across lots of targets.

Many DebugInfo tests are in target-dependent directories as artifacts of
how the tests are implemented, rather than because they are truly
target-dependent. The ones I've tripped over tend to be target-specific
because they generate assembler and the assembler syntax varies too much
across targets to be worth making the CHECKs tolerant enough.

There are fine details that would be DataLayout dependent, but 99% of
the DebugInfo tests aren't checking things that are at such a low level.

> It means that many DebugInfo test could fail with a backend that would
> have a significantly different DataLayout and won’t expect some stuff the
> way they are played down.
> 
> 
> >  Whether a
> > target supports debug info is orthogonal to its native-ness. As written
> > the top-level DebugInfo tests should work fine as long as the default
> > triple's backend is included, and that backend supports debug info.
> >
> > If your backend doesn't support debug info, then it's reasonable to
> > exclude those from your testing; but we can't do that at the cost of
> > excluding those from testing other backends that *do* support the
> feature,
> > even if that testing runs in a cross-compiler environment.
> >
> > In this particular example, we'd be setting things up so that DebugInfo
> > is excluded for the wrong reason (not based on some more abstract idea
> > of the feature-set of the target) but as Krzysztof observes, getting
> > a feature-oriented set of conditions would be a huge task.
> 
> Agree: the predicate I wrote is not correct, and I don’t see a trivial
> “good” solution. This is why any compromise or intermediate solution that
> is good enough to support whatever use-case people here have, including
> your, seems appropriate to implement.
> 
> Let me know I can help coding something.

I have experimented with implementing the thing Takumi and I think should
be a configuration error. :-)  Basically it takes the same kind of approach
that I did in D12506, except it checks for the existence of the target that
matches the default triple. If that target exists then 'llc' with no triple
will succeed, and it looks like the bulk of the tests that you disabled are
in that category.  I'm not especially happy about this tactic, though.

The Hexagon precedent is interesting; Krzysztof said they set the default
triple, and didn't have to xfail all that much stuff.  Searching the tree,
I find exactly 7 individual tests marked XFAIL: hexagon, plus it disables 
all of ExecutionEngine, and turns off the 'object-emission' feature.

I'm curious if you would try setting the default triple to match your
target, and see what /kinds/ of tests fail.  The raw number is much less
interesting than in the categories.

Thanks,
--paulr

> 
> —
> Mehdi
> 
> 
> >
> >>>
> >>>
> >>>> We have lots of tests (the ones you chose to mark "native") that had
> >> been
> >>>> working fine with ARM, X86, PPC, SPARC, and whatever.
> >>>>
> >>>> The fact that they don't work with your backend is different from
> >> saying
> >>>> those tests can't possibly work for any cross-compiler.
> >>>
> >>> I believe this is far from what I said, or at least from what I had in
> >> mind.
> >>>
> >>>> But the latter is what your patch implemented, and it is preventing
> >> useful testing.
> >>>
> >>> I don’t disagree with that statement, and I agree that it should be
> >> fixed.
> >>> It doesn’t mean that I think the previous situation was better though.
> >>> The current over-conservative state seems at least more correct than
> >> having only a subset of the targets that can pass `ninja check`.
> >>>
> >>> Are you attending the LLVM bay area social tonight by any chance?
> >
> > Unfortunately no, I have a prior commitment.
> > --paulr
> >
> >>>
> >>> Best,
> >>>
> >>> —
> >>> Mehdi
> >>>
> >>>
> >>>
> >>>> --paulr
> >>>>
> >>>>>
> >>>>> You can run all the tests by setting the default triple to X86 and
> >>>>> compiling in the X86 backend, but that’s just a trick to make the
> >> tests
> >>>>> happy.
> >>>>> Alternatively, and this is what I tried to do, blacklisting these
> >> tests
> >>>>> that “pretends” to be “portable” but are not.
> >>>>>
> >>>>>
> >>>>>> Mehdi's solution was:
> >>>>>> - In lit.cfg, change the existing "native" feature definition from
> >>>>>> "host-triple == target-triple" to also check "and the corresponding
> >>>>>> backend is included."(**)
> >>>>>
> >>>>> I agree that we could remove the condition on the host backend
> >> included
> >>>>> and deemed it a unsupported, but the build system needs to reject
> >> this
> >>>>> configuration.
> >>>>>
> >>>>>
> >>>>>> - Make piles of tests that seemed inapplicable to GPUs depend on
> the
> >>>>>> "native" feature (through REQUIRES: or in the lit.local.cfg).
> >>>>>
> >>>>> Nitpick: the GPU is just an example, any other backends can be
> >> affected.
> >>>>> It seems that these test are “lying” about the target they will be
> >> able to
> >>>>> run on (like if they would run on “anything”).
> >>>>>
> >>>>>
> >>>>>> - Build LLVM with just the GPU backend, and not set a target triple
> >>>>>> (i.e., it's set to the host triple, typically an X86-something).(*)
> >>>>>> Thus lit.cfg sees matching host and target triples, but the X86
> >>>>>> backend is missing, and so the "native" feature is not set.
> >>>>>>
> >>>>>> [The "native" feature was invented to identify environments where
> >>>>>> JIT would work properly. The "host-triple == target-triple"
> >> condition
> >>>>>> isn't exactly right, but it works well enough.]
> >>>>>>
> >>>>>> The major problem is that these new "native" dependencies are
> >> incorrect.
> >>>>>> For example the DebugInfo tests don't really require it; they work
> >> fine
> >>>>>> as long as the default triple has the corresponding backend
> >> included,
> >>>>>> as my ARM-on-X86 experiment demonstrated.
> >>>>>
> >>>>> Are they are guarantee’d to work with a default triple set to any of
> >> the
> >>>>> possible (in-tree) backend?
> >>>>> (I don’t know enough about these tests, which is definitively why I
> >>>>> included them in the “big hammer” solution)
> >>>>>
> >>>>> Thanks,
> >>>>>
> >>>>> —
> >>>>> Mehdi
> >>>