Implement Power ABI

[ViralBShah]

As the power port is advancing rapidly, the biggest remaining hurdle is now the C ABI. Opening this issue to track work towards getting the ABI working on Power.

[edelsohn]

This involves implementation of the PPC64 ELFv2 ABI in the Julia front-end for LLVM -- matching the definition in the LLVM backend for PPC64, and addressing lingering testsuite failures.

[ViralBShah]

The Vector ABI remains now. @edelsohn Would be great if someone on your side can test this out.

[edelsohn]

We have done some more testing, but it is unclear which tests should succeed without vector ABI support, i.e., the expected status of the testsuite is confusing.

[yuyichao]

I dont think any part of the test should depend on vector abi on power

[edelsohn]

We still see a number of errors with "make check". Are those present in your tests? Have they been traced to Julia code or LLVM code?

[yuyichao]

(I dont have the hardware but) I think it would be better to just paste the error.

[StefanKarpinski]

Or send to @ViralBShah if preferred.

[Keno]

What version of LLVM did you try with? I know @vtjnash had to fix a couple LLVM bugs. I believe they all should be in the build system if you're using the default configuration, but you may not have them yet if you're using a custom LLVM.

[vtjnash]

I ran into a number of openblas bugs, so I had to run the testsuite against the system reference blas. The result was that all tests passed for me (for reference, the last time I've run the whole test-suite was merging #16677).

I started writing tests for the POWER vector ABI: 33209f3, but they aren't yet part of the regular test-suite.

[edelsohn]

Have you reported any of the OpenBLAS bugs?

[vtjnash]

no, I hadn't worked on isolating them

[edelsohn]

Would you please reach out to Xianyi about the testsuite failures that are tied to OpenBLAS?

[Keno]

Vector ABI is merged, this should be complete.

[ViralBShah]

cc @xianyi