Add a stress test for uninit representations

[HeroicKatora]

Test ctfe that never initializes a large portion of its value representations. This might be optimized independent of other changes to ctfe. The potential usefulness of having these benchmarks to evaluate such changes was discussed in rust-lang/rust#62655

cc: @oli-obk @RalfJung @Mark-Simulacrum

[HeroicKatora]

Addressed both of the review comments.

[Mark-Simulacrum]

Will merge with CI passing.

[RalfJung]

@HeroicKatora how long does that test take to run on your system (and how long does ctfe-stress-2 take)?

[Mark-Simulacrum]

Hm, just copy/pasting the source code leads to a few compilation failures; looks like CI isn't catching it but https://travis-ci.com/rust-lang-nursery/rustc-perf/jobs/245574193#L1228 is a sample log

[HeroicKatora]

Turns out I am bad a copy&paste, the tests were mostly built from perf files I had flying around for my own testing and I didn't properly validated. Thanks CI.

After the import fix, current exection was ~4 seconds, so I've reduce the shift/size by 1 which brings it very close to a 2 seconds mark according to a comment in the other threads (clean build on nightly).

[Mark-Simulacrum]

Yes, the specific execution time is not really relevant (due to differences in CPU speed etc); you probably want to be somewhere around the time that ctfe-stress-2 takes to execute locally for you.

[HeroicKatora]

ctfe-stress-2 clean definitely takes longer, about a factor of 5-6. Should I bump it up to that time or is faster also okay? I'm not quite sure how incremental would behave since ctfe-stress-uninit actually creates static variables while ctfe-stress-2 does not.

[RalfJung]

you probably want to be somewhere around the time that ctfe-stress-2 takes to execute locally for you.

Wait what? No. This is like one of the tests in ctfe-stress-2, which are all geared to around 2s on my system (or rather they were when I originally submitted it).
There's like 10-15 tests in there, so your new test should be around 10-15 times faster than that.

That said, are these 4s with or without your PR? If that PR is going to cut the time down in half it may be worth factoring that in already.

[Mark-Simulacrum]

Ah, right, I forget that we had that many tests in there. In that case, yes; we will want just 1/10 or so. We can use timings from the PR rather than nightly, though.

[HeroicKatora]

There's like 10-15 tests in there, so your new test should be around 10-15 times faster than that.

That said, are these 4s with or without your PR? If that PR is going to cut the time down in half it may be worth factoring that in already.

This perf contains more than one case also, some that will be directly affected by the PR. Some other cases are not yet fully affected and included to ensure that future improvements don't need to add their own stress test crates, I'd say the ratio of directly affected code is about 50/50. SURROUND, NONE, SOME and all other cases that contain only some uninitialize memory are only partially improved. The PR improves the complete set of tests by about ~12% in the current state.

[HeroicKatora]

With the PR, the timing difference between ctfe-stress-2 and ctfe-stress-uninit is locally a factor of 7.2 (13.6s vs. 1.9s).

[nnethercote]

Can we just add this new code to ctfe-stress-2 and rename it as ctfe-stress-3? We previously had a bunch of separate CTFE benchmarks but merged them because they were bloating the output.

[HeroicKatora]

Which of the two options should I go with, then? Any differing opinions?

[Mark-Simulacrum]

I think in this case -3 is probably the best option as it sidesteps most of the concerns and is presumably fairly easily.

[RalfJung]

Sounds good to me.

[HeroicKatora]

Sure, easy enough.

[Mark-Simulacrum]

This should be updated.

[HeroicKatora]

Remove it or merge the two comments?

[Mark-Simulacrum]

There is no uninit benchmark anymore, I would expect a single entry with -3.

[Mark-Simulacrum]

Probably just dropping the uninit bit; that can be moved to a comment in the file.