Improved assumptions relating to isqrt by Apersoma · Pull Request #155265 · rust-lang/rust

Apersoma · 2026-04-13T22:25:25Z

Improved various assumptions relating to values yielded by isqrt.
Does not solve but does improve #132763.

Re-openeing of #154115

Added assumptions are:

if x is nonzero then x.isqrt() is nonzero
x.isqrt() <= x
x.isqrt() * x.isqrt() <= x

rustbot · 2026-04-13T22:25:31Z

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tgross35 · 2026-04-14T03:49:37Z

r? tgross35

tgross35 · 2026-04-14T03:51:51Z

Could you include a godbolt link or before+after asm outputs indicating what all improves here?

Apersoma · 2026-04-14T17:03:30Z

Could you include a godbolt link or before+after asm outputs indicating what all improves here?

examples

Without overflow checks the difference isn't nearly as large but it is an improvement.

@tgross35

tgross35

Accidentally reviewed at the wrong PR, copied the comments over from #154115 (comment).

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tgross35 · 2026-04-14T22:57:06Z

+            crate::hint::assert_unchecked(result * result <= $n);
+            crate::hint::assert_unchecked(result <= $n);


Aren't these two redundant? If not, could you show an asm case indicating that?

fn foo(x: u32) -> (u32, u32) { let sqrt = smart_sqrt(x); let a = x - sqrt; let b = x - sqrt*sqrt; (a, b) }

Overflow checks will be emitted when computing a regardless of crate::hint::assert_unchecked(result * result <= $n) and overflow checks will be emitted when computing b regardless of crate::hint::assert_unchecked(result <= $n) so both are necessary.

rustbot · 2026-04-14T22:57:34Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

Apersoma · 2026-04-15T20:57:20Z

@rustbot ready

Apersoma · 2026-04-15T22:33:07Z

i have no idea how my computer missed the missing e but the tidy should pass now

jhpratt · 2026-04-21T06:11:43Z

@bors try @rust-timer queue

Improved assumptions relating to isqrt

rust-bors · 2026-04-21T08:19:49Z

☀️ Try build successful (CI)
Build commit: c201a78 (c201a78525f5ec530e7598347c60d8a87df77f88, parent: 84c11900724736a2b9afac4822031d78a9ed8e86)

rust-timer · 2026-04-21T09:00:04Z

Finished benchmarking commit (c201a78): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

@bors rollup=never
@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

This perf run didn't have relevant results for this metric.

Max RSS (memory usage)

Results (secondary 2.6%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.6%	[2.4%, 2.8%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

Results (primary 2.3%, secondary -3.5%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.3%	[2.0%, 2.5%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.5%	[-4.0%, -2.3%]	7
All ❌✅ (primary)	2.3%	[2.0%, 2.5%]	2

Binary size

Results (primary -0.1%, secondary -0.1%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.1%	[-0.1%, -0.0%]	55
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.0%]	24
All ❌✅ (primary)	-0.1%	[-0.1%, -0.0%]	55

Bootstrap: 490.276s -> 490.941s (0.14%)
Artifact size: 394.36 MiB -> 394.31 MiB (-0.01%)

jhpratt · 2026-04-21T09:01:34Z

@bors r+

rust-bors · 2026-04-21T09:01:37Z

📌 Commit 7b59818 has been approved by jhpratt

It is now in the queue for this repository.

tgross35

Sorry sorry, I had some thoughts that I meant to post. Thank you Jacob for taking a look!

Also, please squash!

View changes since this review

tgross35 · 2026-04-21T09:50:26Z

@bors r-

rustbot · 2026-04-22T19:51:13Z

⚠️ Warning ⚠️

This PR is based on an upstream commit that is older than 28 days.

It's recommended to update your branch according to the rustc-dev-guide.

Apersoma · 2026-04-22T23:18:07Z

@rustbot ready

tgross35

Looks great, thank you!

@bors r=jhpratt,tgross35

View changes since this review

rust-bors · 2026-04-23T06:49:38Z

📌 Commit e722ba5 has been approved by jhpratt,tgross35

It is now in the queue for this repository.

tgross35 · 2026-04-23T06:50:44Z

Since perf didn't seem to report anything,
@bors rollup=maybe

…uwer Rollup of 10 pull requests Successful merges: - #146544 (mir-opt: Remove the workaround in UnreachableEnumBranching) - #154819 (Fix ICE for inherent associated type mismatches) - #155265 (Improved assumptions relating to isqrt) - #152576 (c-variadic: use `emit_ptr_va_arg` for mips) - #154481 (Mark a function only used in nightly as nightly only) - #155614 (c-variadic: rename `VaList::arg` to `VaList::next_arg`) - #155630 (Make `//@ skip-filecheck` a normal compiletest directive) - #155641 (Remove non-working code for "running" mir-opt tests) - #155652 (Expand `Path::is_empty` docs) - #155656 (rustc_llvm: update opt-level handling for LLVM 23)

Rollup merge of #155265 - Apersoma:isqrt-smarter, r=jhpratt,tgross35 Improved assumptions relating to isqrt Improved various assumptions relating to values yielded by `isqrt`. Does not solve but does improve #132763. Re-openeing of #154115 Added assumptions are: * if `x` is nonzero then `x.isqrt()` is nonzero * `x.isqrt() <= x` * `x.isqrt() * x.isqrt() <= x`

…uwer Rollup of 10 pull requests Successful merges: - rust-lang/rust#146544 (mir-opt: Remove the workaround in UnreachableEnumBranching) - rust-lang/rust#154819 (Fix ICE for inherent associated type mismatches) - rust-lang/rust#155265 (Improved assumptions relating to isqrt) - rust-lang/rust#152576 (c-variadic: use `emit_ptr_va_arg` for mips) - rust-lang/rust#154481 (Mark a function only used in nightly as nightly only) - rust-lang/rust#155614 (c-variadic: rename `VaList::arg` to `VaList::next_arg`) - rust-lang/rust#155630 (Make `//@ skip-filecheck` a normal compiletest directive) - rust-lang/rust#155641 (Remove non-working code for "running" mir-opt tests) - rust-lang/rust#155652 (Expand `Path::is_empty` docs) - rust-lang/rust#155656 (rustc_llvm: update opt-level handling for LLVM 23)

rustbot assigned jhpratt Apr 13, 2026

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Apr 13, 2026

Apersoma mentioned this pull request Apr 13, 2026

isqrt treated as a black_box #132763

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rustbot assigned tgross35 and unassigned jhpratt Apr 14, 2026

Rudxain reviewed Apr 14, 2026

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Comment thread library/core/src/num/imp/int_sqrt.rs Outdated

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Apr 21, 2026

rust-bors Bot pushed a commit that referenced this pull request Apr 21, 2026

Auto merge of #155265 - Apersoma:isqrt-smarter, r=<try>

c201a78

Improved assumptions relating to isqrt

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Apersoma force-pushed the isqrt-smarter branch from 0f7faa0 to 8640c9e Compare April 22, 2026 19:59

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improved assumptions relating isqrt (squashed)

e722ba5

Apersoma force-pushed the isqrt-smarter branch from 8640c9e to e722ba5 Compare April 22, 2026 21:59

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Apr 22, 2026

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JonathanBrouwer mentioned this pull request Apr 23, 2026

Rollup of 10 pull requests #155674

Merged

rust-bors Bot merged commit 04bcb39 into rust-lang:main Apr 23, 2026
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rustbot added this to the 1.97.0 milestone Apr 23, 2026

Apersoma deleted the isqrt-smarter branch April 24, 2026 21:02

		crate::hint::assert_unchecked(result * result <= $n);
		crate::hint::assert_unchecked(result <= $n);

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rust-timer commented Apr 21, 2026

Overall result: no relevant changes - no action needed

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