Add a benchmark of the hang-on-test-failure code path
This is the code pattern that produces the performance problem in https://github.com/rust-lang/miri/issues/2273
I figured out what I was stuck on in https://github.com/rust-lang/miri/pull/2315#discussion_r916387919. For a while I was just doing `let x: &[u8] = &[0u8; 4096];` but that doesn't produce the runtime inside `Stack::item_popped` that I was looking for, I think because this allocation is never deallocated. But with `Vec`, I get the profile I'm looking for.
Optimizing Stacked Borrows (part 2): Shrink Item
This moves protectors out of `Item`, storing them both in a global `HashSet` which contains all currently-protected tags as well as a `Vec<SbTag>` on each `Frame` so that when we return from a function we know which tags to remove from the protected set.
This also bit-packs the 64-bit tag and the 2-bit permission together when they are stored in memory. This means we theoretically run out of tags sooner, but I doubt that limit will ever be hit.
Together these optimizations reduce the memory footprint of Miri when executing programs which stress Stacked Borrows by ~66%. For example, running a test with isolation off which only panics currently peaks at ~19 GB, with this PR it peaks at ~6.2 GB.
To-do
- [x] Enforce the 62-bit limit
- [x] Decide if there is a better order to pack the tag and permission in
- [x] Wait for `UnsafeCell` to become infectious, or express offsets + tags in the global protector set
Benchmarks before:
```
Benchmark 1: cargo +miri miri run --manifest-path bench-cargo-miri/backtraces/Cargo.toml
Time (mean ± σ): 8.948 s ± 0.253 s [User: 8.752 s, System: 0.158 s]
Range (min … max): 8.619 s … 9.279 s 5 runs
Benchmark 1: cargo +miri miri run --manifest-path bench-cargo-miri/mse/Cargo.toml
Time (mean ± σ): 2.129 s ± 0.037 s [User: 1.849 s, System: 0.248 s]
Range (min … max): 2.086 s … 2.176 s 5 runs
Benchmark 1: cargo +miri miri run --manifest-path bench-cargo-miri/serde1/Cargo.toml
Time (mean ± σ): 3.334 s ± 0.017 s [User: 3.211 s, System: 0.103 s]
Range (min … max): 3.315 s … 3.352 s 5 runs
Benchmark 1: cargo +miri miri run --manifest-path bench-cargo-miri/serde2/Cargo.toml
Time (mean ± σ): 3.316 s ± 0.038 s [User: 3.207 s, System: 0.095 s]
Range (min … max): 3.282 s … 3.375 s 5 runs
Benchmark 1: cargo +miri miri run --manifest-path bench-cargo-miri/unicode/Cargo.toml
Time (mean ± σ): 6.391 s ± 0.323 s [User: 5.928 s, System: 0.412 s]
Range (min … max): 6.090 s … 6.917 s 5 runs
```
After:
```
Benchmark 1: cargo +miri miri run --manifest-path bench-cargo-miri/backtraces/Cargo.toml
Time (mean ± σ): 6.955 s ± 0.051 s [User: 6.807 s, System: 0.132 s]
Range (min … max): 6.900 s … 7.038 s 5 runs
Benchmark 1: cargo +miri miri run --manifest-path bench-cargo-miri/mse/Cargo.toml
Time (mean ± σ): 1.784 s ± 0.012 s [User: 1.627 s, System: 0.156 s]
Range (min … max): 1.772 s … 1.797 s 5 runs
Benchmark 1: cargo +miri miri run --manifest-path bench-cargo-miri/serde1/Cargo.toml
Time (mean ± σ): 2.505 s ± 0.095 s [User: 2.311 s, System: 0.096 s]
Range (min … max): 2.405 s … 2.603 s 5 runs
Benchmark 1: cargo +miri miri run --manifest-path bench-cargo-miri/serde2/Cargo.toml
Time (mean ± σ): 2.449 s ± 0.031 s [User: 2.306 s, System: 0.100 s]
Range (min … max): 2.395 s … 2.467 s 5 runs
Benchmark 1: cargo +miri miri run --manifest-path bench-cargo-miri/unicode/Cargo.toml
Time (mean ± σ): 3.667 s ± 0.110 s [User: 3.498 s, System: 0.140 s]
Range (min … max): 3.564 s … 3.814 s 5 runs
```
The decrease in system time is probably due to spending less time in the page fault handler.
stacked_borrow now has an item module, and its own FrameExtra. These
serve to protect the implementation of Item (which is a bunch of
bit-packing tricks) from the primary logic of Stacked Borrows, and the
FrameExtra we have separates Stacked Borrows more cleanly from the
interpreter itself.
The new strategy for checking protectors also makes some subtle
performance tradeoffs, so they are now documented in Stack::item_popped
because that function primarily benefits from them, and it also touches
every aspect of them.
Also separating the actual CallId that is protecting a Tag from the Tag
makes it inconvienent to reproduce exactly the same protector errors, so
this also takes the opportunity to use some slightly cleaner English in
those errors. We need to make some change, might as well make it good.
Previously, Item was a struct of a NonZeroU64, an Option which was
usually unset or irrelevant, and a 4-variant enum. So collectively, the
size of an Item was 24 bytes, but only 8 bytes were used for the most
part.
So this takes advantage of the fact that it is probably impossible to
exhaust the total space of SbTags, and steals 3 bits from it to pack the
whole struct into a single u64. This bit-packing means that we reduce
peak memory usage when Miri goes memory-bound by ~3x. We also get CPU
performance improvements of varying size, because not only are we simply
accessing less memory, we can now compare a Vec<Item> using a memcmp
because it does not have any padding.
test that we also find bad uses of mem::uninitialized
Also we really don't need to separately test signed and unsigned integers... our test suite is big enough as it is. ;)
ui_test: require colon after command
Turns out we already use colons everywhere properly.
Also rename ui_test::comments to parser. I think that better describes its contents. :)
More robust comment parsing
fixes#2170
I haven't ported the entire test suite yet. Once we've done that, I will remove the old parsing system (or in fact, turn them into errors so that accidental usage of old-style comments will be detected)
