[llvm] [VPlan] Add transformation to narrow interleave groups. (PR #106441)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

> @mstorsjo thanks, I recommitted with a fix, limiting this to loops w/o interleaving for now: [577631f](https://github.com/llvm/llvm-project/commit/577631f0a52884384786bf899951e3094ad7f69c)

Great, thanks! I built the latest clang and reran the full ffmpeg testsuite now, and it seems to pass flawlessly now.

> I ran the ffmpeg tests from the test-suite, but it sounds like the test-suite coverage could be improved by running various encoders with different inputs and checking the output? Would the input files you are using for your testing be suitable for adding to the test-suite and define tests using them?

That would be great, but unfortunately it'd be a fairly massive effort... Upstream ffmpeg has got like over 5000 tests, defined in a combination of shell script and makefiles. Some tests decode test samples from a separately synced repo of test inputs (currently a 1.5 GB bunch of inputs) - some of them check the hash of the decoded output, expecting it to be bitexact. Some decode and check that the output is close enough to a reference output (e.g. for decoders that use floats internally). Some tests generate a synthetic test input, then try encode it into various formats, and decode it back again, and do some comparison on it, etc. (This particular test that I isolated was this kind, I think.)

So while it's probably not hard to extract a handful of testcases and set them up in llvm-test-suite, it probably doesn't give very much meaningful test coverage if it's only a few. (Chances that you'd break the exact same case again is pretty small.) ;-) And doing all of them would both be a quite massive effort, and it would also probably be a huge pain to maintain in sync (if we'd want to update targeting a newer release). This, compared with the cmake wrapping which is fairly low effort to update to a newer release if we want to do that.

If you do want to try to run the full upstream ffmpeg testsuite, it's not very hard:
```
$ mkdir ffmpeg-build
$ cd ffmpeg-build
$ ../path/to/ffmpeg/configure --cc=clang --samples=$(pwd)/../samples
$ make fate-rsync # download test samples into the path specified by --samples
$ make -j$(nproc)
$ make -j$(nproc) fate # run all tests
```

https://github.com/llvm/llvm-project/pull/106441