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

[Robinson, Paul via llvm-dev]

> -----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.

> 
> 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.
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.  But the latter
is what your patch implemented, and it is preventing useful testing.
--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