transmutability: merge contiguous runs with a common destination #140509

tmiasko · 2025-04-30T11:08:20Z

The previous implementation was inconsistent about transitions that apply for an init byte. For example, when answering a query, an init byte could use corresponding init transition. Init byte could also use uninit transition, but only when the corresponding init transition was absent. This behaviour was incompatible with DFA union construction. Define an uninit transition to match an uninit byte only and update implementation accordingly. To describe that `Tree::uninit` is valid for any value, build an automaton that accepts any byte value. Additionally, represent byte ranges uniformly as a pair of integers to avoid special case for uninit byte.

joshlf · 2025-04-30T16:33:56Z

compiler/rustc_transmute/src/layout/dfa.rs

+            for (range, (x, y)) in union(xs, ys) {
+                let state = join(x, y);
+                match runs.last_mut() {
+                    // Merge contiguous runs with a common destination.


Awesome! Can you add a test for this behavior?

Note that if you've linked your stage1 build to cargo, you can cd into the rustc_transmute folder and test the crate independently of the rest of the compiler; e.g.:

RUSTC_BOOTSTRAP=1 cargo +stage1 test

It's quite a bit faster than using x.

There is an existing test for union operation which happens to exercise merging
functionality: instead of (d, 1, a), (d, 0, a) now we have (d, 0..=1, a).

rust/compiler/rustc_transmute/src/maybe_transmutable/tests.rs

Lines 301 to 324 in b9e0ecd

fn union() {

let [a, b, c, d] = [0, 1, 2, 3];

let s = Dfa::from_edges(a, d, &[(a, 0, b), (b, 0, d), (a, 1, c), (c, 1, d)]);

let t = Dfa::from_edges(a, c, &[(a, 1, b), (b, 0, c)]);

let mut ctr = 0;

let new_state = || {

let state = crate::layout::dfa::State(ctr);

ctr += 1;

state

};

let u = s.clone().union(t.clone(), new_state);

let expected_u =

Dfa::from_edges(b, a, &[(b, 0..=0, c), (b, 1..=1, d), (d, 0..=1, a), (c, 0..=0, a)]);

assert_eq!(u, expected_u);

assert_eq!(is_transmutable(&s, &u, Assume::default()), Answer::Yes);

assert_eq!(is_transmutable(&t, &u, Assume::default()), Answer::Yes);

}

}

jswrenn · 2025-05-01T14:24:27Z

@bors r+

bors · 2025-05-01T14:24:29Z

📌 Commit b9e0ecd has been approved by jswrenn

It is now in the queue for this repository.

…=jswrenn transmutability: merge contiguous runs with a common destination Based on rust-lang#140380. r? `@jswrenn` `@joshlf`

rustbot assigned jswrenn Apr 30, 2025

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Apr 30, 2025

tmiasko force-pushed the merge-contiguous-ranges branch from 0368f55 to eaa8f06 Compare April 30, 2025 11:10

tmiasko changed the title ~~transmutability: merge contiguous ranges with a common destination~~ transmutability: merge contiguous runs with a common destination Apr 30, 2025

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transmutability: merge contiguous runs with a common destination

b9e0ecd

tmiasko force-pushed the merge-contiguous-ranges branch from eaa8f06 to b9e0ecd Compare April 30, 2025 12:35

joshlf suggested changes Apr 30, 2025

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bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels May 1, 2025

matthiaskrgr mentioned this pull request May 2, 2025

Rollup of 9 pull requests #140596

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transmutability: merge contiguous runs with a common destination #140509

transmutability: merge contiguous runs with a common destination #140509

tmiasko commented Apr 30, 2025

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joshlf Apr 30, 2025

jswrenn Apr 30, 2025 •

edited

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tmiasko May 1, 2025

jswrenn commented May 1, 2025

bors commented May 1, 2025

	fn union() {
	let [a, b, c, d] = [0, 1, 2, 3];
	let s = Dfa::from_edges(a, d, &[(a, 0, b), (b, 0, d), (a, 1, c), (c, 1, d)]);

	let t = Dfa::from_edges(a, c, &[(a, 1, b), (b, 0, c)]);

	let mut ctr = 0;
	let new_state = \|\| {
	let state = crate::layout::dfa::State(ctr);
	ctr += 1;
	state
	};

	let u = s.clone().union(t.clone(), new_state);

	let expected_u =
	Dfa::from_edges(b, a, &[(b, 0..=0, c), (b, 1..=1, d), (d, 0..=1, a), (c, 0..=0, a)]);

	assert_eq!(u, expected_u);

	assert_eq!(is_transmutable(&s, &u, Assume::default()), Answer::Yes);
	assert_eq!(is_transmutable(&t, &u, Assume::default()), Answer::Yes);
	}
	}

transmutability: merge contiguous runs with a common destination #140509

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jswrenn Apr 30, 2025 •

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