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core: Fix overflow in int::mod_euc when self < 0 && rhs == MIN #50185

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merged 2 commits into from
Apr 24, 2018

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tesaguri
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This commit removes usage of abs, which overflows when self == MIN.

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r? @Kimundi

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@rust-highfive rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Apr 23, 2018
@@ -1765,7 +1765,11 @@ assert_eq!((-a).mod_euc(-b), 1);
pub fn mod_euc(self, rhs: Self) -> Self {
let r = self % rhs;
if r < 0 {
r + rhs.abs()
if rhs.is_negative() {
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This line and the previous might as well be consistent (whichever of < 0 or is_negative you prefer).

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This was my fault for not paying close enough attention during review! Thanks for catching it!

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kennytm commented Apr 24, 2018

@bors r+ rollup

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bors commented Apr 24, 2018

📌 Commit 104c64d has been approved by kennytm

@bors 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 Apr 24, 2018
kennytm added a commit to kennytm/rust that referenced this pull request Apr 24, 2018
core: Fix overflow in `int::mod_euc` when `self < 0 && rhs == MIN`

This commit removes usage of `abs`, which overflows when `self == MIN`.
bors added a commit that referenced this pull request Apr 24, 2018
Rollup of 11 pull requests

Successful merges:

 - #49461 (std: Child::kill() returns error if process has already exited)
 - #49727 (Add Cell::update)
 - #49812 (Fix revision support for UI tests.)
 - #49829 (Add doc links to `std::os` extension traits)
 - #49906 (Stabilize `std::hint::unreachable_unchecked`.)
 - #49970 (Deprecate Read::chars and char::decode_utf8)
 - #49985 (don't see issue #0)
 - #50118 (fix search bar bug)
 - #50139 (encourage descriptive issue titles)
 - #50174 (Use FxHashMap in syntax_pos::symbol::Interner::intern.)
 - #50185 (core: Fix overflow in `int::mod_euc` when `self < 0 && rhs == MIN`)

Failed merges:
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bors commented Apr 24, 2018

☔ The latest upstream changes (presumably #50191) made this pull request unmergeable. Please resolve the merge conflicts.

@bors bors added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Apr 24, 2018
@bors bors merged commit 104c64d into rust-lang:master Apr 24, 2018
@tesaguri tesaguri deleted the mod_euc-fix-overflow branch April 24, 2018 11:16
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Boscop commented May 3, 2018

I'd prefer mod_euc to have no branches, because it often occurs in very hot loops in my code.
Can we change it to remove the branches? So instead of this:

pub fn mod_euc(self, rhs: Self) -> Self {
	let r = self % rhs;
	if r < 0 {
		if rhs < 0 {
			r - rhs
		} else {
			r + rhs
		}
	} else {
		r
	}
}

Something like this:

pub fn mod_euc(self, rhs: Self) -> Self {
	let r = self % rhs;
	let r_pos = [r + rhs, r - rhs][(rhs < 0) as usize];
	[r, r_pos][(r < 0) as usize)]
}

The compiler should be able to remove the array bounds checks, right?

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varkor commented May 3, 2018

@Boscop: I'm curious to know how differently this performs — have you written a benchmark to see? (Also, maybe it would be better to continue this on the tracking issue where it's more discoverable!)

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Boscop commented May 4, 2018

@varkor
Nvm, it seems that the current version is the fastest (tested on x64 Win 8.1 with i64 ints only):

test tests::bench_mod_euc_branching    ... bench:  33,072,518 ns/iter (+/- 1,355,002)
test tests::bench_mod_euc_branchless   ... bench:  34,965,845 ns/iter (+/- 1,265,057)
test tests::bench_mod_euc_branchless_2 ... bench:  35,493,871 ns/iter (+/- 838,259)
test tests::bench_mod_euc_branchless_u ... bench:  35,033,348 ns/iter (+/- 1,717,562)

Is this the correct way to write a benchmark?

#[cfg(test)]
mod tests {
	use test::{Bencher, black_box};

	pub fn mod_euc_branching(slf: i64, rhs: i64) -> i64 {
		let r = slf % rhs;
		if r < 0 {
			if rhs < 0 {
				r - rhs
			} else {
				r + rhs
			}
		} else {
			r
		}
	}

	pub fn mod_euc_branchless(slf: i64, rhs: i64) -> i64 {
		let r = slf % rhs;
		let r_pos = [r + rhs, r - rhs][(rhs < 0) as usize];
		[r, r_pos][(r < 0) as usize]
	}

	pub fn mod_euc_branchless_u(slf: i64, rhs: i64) -> i64 {
		let r = slf % rhs;
		unsafe {
			let r_pos = *[r + rhs, r - rhs].get_unchecked((rhs < 0) as usize);
			*[r, r_pos].get_unchecked((r < 0) as usize)
		}
	}

	pub fn mod_euc_branchless_2(slf: i64, rhs: i64) -> i64 {
		let r = slf % rhs;
		let r_pos = r + rhs - 2 * rhs * (rhs < 0) as i64;
		r + (r_pos - r) * (r < 0) as i64
	}

	#[bench]
	fn bench_mod_euc_branching(b: &mut Bencher) {
		b.iter(|| {
			for a in -1000..1000 {
				for b in -1000..1000 {
					if b == 0 { continue; }
					black_box(mod_euc_branching(a, b));
				}
			}
		});
	}

	#[bench]
	fn bench_mod_euc_branchless(b: &mut Bencher) {
		b.iter(|| {
			for a in -1000..1000 {
				for b in -1000..1000 {
					if b == 0 { continue; }
					black_box(mod_euc_branchless(a, b));
				}
			}
		});
	}

	#[bench]
	fn bench_mod_euc_branchless_u(b: &mut Bencher) {
		b.iter(|| {
			for a in -1000..1000 {
				for b in -1000..1000 {
					if b == 0 { continue; }
					black_box(mod_euc_branchless_u(a, b));
				}
			}
		});
	}

	#[bench]
	fn bench_mod_euc_branchless_2(b: &mut Bencher) {
		b.iter(|| {
			for a in -1000..1000 {
				for b in -1000..1000 {
					if b == 0 { continue; }
					black_box(mod_euc_branchless_2(a, b));
				}
			}
		});
	}
}

@varkor
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varkor commented May 4, 2018

@Boscop: yeah, that looks fine to me. Thanks for checking it out!

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7 participants