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

Thu Sep 3 11:23:23 PDT 2015

> -----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?

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).

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.  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.  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.

> >
> >
> > > 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
> >