Add test for stable hash uniqueness of adjacent field values #93193
Conversation
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
rustbot
added
the
T-compiler
rust-highfive
added
the
S-waiting-on-review
Kobzol
changed the title
This comment has been minimized.
This comment has been minimized.
Kobzol
force-pushed
the
stable-hash-permutation-test
branch
from
90449b9
to
1ffd043
Compare
|
@bors r+ rollup |
|
📌 Commit 1ffd043 has been approved by |
bors
added
S-waiting-on-bors
bors
added a commit
to rust-lang-ci/rust
that referenced
this pull request
Jan 27, 2022
…askrgr Rollup of 8 pull requests Successful merges: - rust-lang#91641 (Define c_char using cfg_if rather than repeating 40-line cfg) - rust-lang#92899 (Mention std::iter::zip in Iterator::zip docs) - rust-lang#93193 (Add test for stable hash uniqueness of adjacent field values) - rust-lang#93325 (Introduce a limit to Levenshtein distance computation) - rust-lang#93339 (rustdoc: add test case for multiple traits and erased names) - rust-lang#93357 (Clarify the `usage-of-qualified-ty` error message.) - rust-lang#93363 (`#[rustc_pass_by_value]` cleanup) - rust-lang#93365 (More arena cleanups) Failed merges: r? `@ghost` `@rustbot` modify labels: rollup
bors
added a commit
to rust-lang-ci/rust
that referenced
this pull request
Feb 3, 2022
…sion, r=the8472 Compress amount of hashed bytes for `isize` values in StableHasher This is another attempt to land rust-lang#92103, this time hopefully with a correct implementation w.r.t. stable hashing guarantees. The previous PR was [reverted](rust-lang#93014) because it could produce the [same hash](rust-lang#92103 (comment)) for different values even in quite simple situations. I have since added a basic [test](rust-lang#93193) that should guard against that situation, I also added a new test in this PR, specialised for this optimization. ## Why this optimization helps Since the original PR, I have tried to analyze why this optimization even helps (and why it especially helps for `clap`). I found that the vast majority of stable-hashing `i64` actually comes from hashing `isize` (which is converted to `i64` in the stable hasher). I only found a single place where is this datatype used directly in the compiler, and this place has also been showing up in traces that I used to find out when is `isize` being hashed. This place is `rustc_span::FileName::DocTest`, however, I suppose that isizes also come from other places, but they might not be so easy to find (there were some other entries in the trace). `clap` hashes about 8.5 million `isize`s, and all of them fit into a single byte, which is why this optimization has helped it [quite a lot](rust-lang#92103 (comment)). Now, I'm not sure if special casing `isize` is the correct solution here, maybe something could be done with that `isize` inside `DocTest` or in other places, but that's for another discussion I suppose. In this PR, instead of hardcoding a special case inside `SipHasher128`, I instead put it into `StableHasher`, and only used it for `isize` (I tested that for `i64` it doesn't help, or at least not for `clap` and other few benchmarks that I was testing). ## New approach Since the most common case is a single byte, I added a fast path for hashing `isize` values which positive value fits within a single byte, and a cold path for the rest of the values. To avoid the previous correctness problem, we need to make sure that each unique `isize` value will produce a unique hash stream to the hasher. By hash stream I mean a sequence of bytes that will be hashed (a different sequence should produce a different hash, but that is of course not guaranteed). We have to distinguish different values that produce the same bit pattern when we combine them. For example, if we just simply skipped the leading zero bytes for values that fit within a single byte, `(0xFF, 0xFFFFFFFFFFFFFFFF)` and `(0xFFFFFFFFFFFFFFFF, 0xFF)` would send the same hash stream to the hasher, which must not happen. To avoid this situation, values `[0, 0xFE]` are hashed as a single byte. When we hash a larger (treating `isize` as `u64`) value, we first hash an additional byte `0xFF`. Since `0xFF` cannot occur when we apply the single byte optimization, we guarantee that the hash streams will be unique when hashing two values `(a, b)` and `(b, a)` if `a != b`: 1) When both `a` and `b` are within `[0, 0xFE]`, their hash streams will be different. 2) When neither `a` and `b` are within `[0, 0xFE]`, their hash streams will be different. 3) When `a` is within `[0, 0xFE]` and `b` isn't, when we hash `(a, b)`, the hash stream will definitely not begin with `0xFF`. When we hash `(b, a)`, the hash stream will definitely begin with `0xFF`. Therefore the hash streams will be different. r? `@the8472`
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
This PR adds a simple test to check that stable hash will produce a different hash if the order of two values that have the same combined bit pattern changes.
r? @the8472