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

[Robinson, Paul via llvm-dev]

This note arose from http://reviews.llvm.org/D12506 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.)


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."(**)
- Make piles of tests that seemed inapplicable to GPUs depend on the
  "native" feature (through REQUIRES: or in the lit.local.cfg).
- 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.

So, we need some solution that Mehdi can use in a GPU-backend-testing 
workflow, but that does not so drastically interfere with normal 
cross-compiler testing.  Does anybody have any suggestions?

Thanks,
--paulr


(*) This configuration seems inherently wrong; the default target triple
ought to require having the corresponding backend included, I think.
(**) Which makes this modification superfluous.