Faster fmt::Display of 128-bit integers, without unsafe pointer #136594

Hey, I'm very sorry this PR kept falling off my todo list. I have now had a chance to read through thoroughly and it looks great. Left a few small comments, but with those, a rebase, and a final try job I think this should be good to go.

Any chance you could post local benchmarks as well?

library/core/src/fmt/num.rs

tgross35 · 2025-06-10T04:51:56Z

library/core/src/fmt/num.rs

-    } else {
-        n.widening_mul(FACTOR).1 >> 62
-    };
+    // These constant values are computed with the CHOOSE_MULTIPLIER procedure.


What is CHOOSE_MULTIPLIER referring to?

It is described in the paper.

Ah, annoyingly the paper can't be searched for "CHOOSE_MULTIPLIER" since it uses a weird underscore. I'm trying to check the algorithm - if d is 1_0000_0000_0000_0000, what values are prec and N?

Maybe I should use the weird underscore in the code too then? 🤔

N is the input argument and prec(ision) is 128-bit.
https://gist.github.com/pascaldekloe/df103c17ebf01920958053c76505aedf#file-main-go-L26

tgross35 · 2025-06-10T04:52:12Z

library/core/src/fmt/num.rs

 ///
-/// Integer division algorithm is based on the following paper:
+/// The integer division algorithm is based on the following paper:
 ///
 ///   T. Granlund and P. Montgomery, “Division by Invariant Integers Using Multiplication”
 ///   in Proc. of the SIGPLAN94 Conference on Programming Language Design and
 ///   Implementation, 1994, pp. 61–72


It seems like the implementation has changed quite a bit - is it still from the paper?

Yes the algorithm is an exact match of the paper now, including the names of each parameter involved.

library/core/src/fmt/num.rs

tgross35 · 2025-06-10T17:51:11Z

@bors try @rust-timer queue

bors · 2025-06-10T17:52:54Z

⌛ Trying commit 0483c93 with merge f791aa5...

Faster fmt::Display of 128-bit integers, without unsafe pointer In followup of #135265, hereby the 128-bit part. * Batches per 16 instead of 19 digits * Buffer access as array insteaf of unsafe pointer * Added test coverage for i128 and u128 r? tgross35 ChrisDenton

pascaldekloe · 2025-06-10T18:24:08Z

No problem @tgross35. This is no priority patch. Happy with these feedback loops. Getting to learn the language. Local benchmark will follow in an hour or so...

bors · 2025-06-10T20:25:42Z

☀️ Try build successful - checks-actions
Build commit: f791aa5 (f791aa5b7459654ec84452793d1bea1ddf0e6b32)

rust-timer · 2025-06-10T22:27:50Z

Finished benchmarking commit (f791aa5): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

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

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.4%	[-0.4%, -0.4%]	1
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (secondary 0.8%)

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)	8.5%	[8.5%, 8.5%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.8%	[-2.9%, -1.0%]	3
All ❌✅ (primary)	-	-	0

Cycles

Results (secondary 2.7%)

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)	2.7%	[2.7%, 2.7%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

Results (primary -0.0%)

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)	-0.0%	[-0.0%, -0.0%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.0%	[-0.0%, -0.0%]	1

Bootstrap: 754.321s -> 755.157s (0.11%)
Artifact size: 372.15 MiB -> 372.14 MiB (-0.00%)

rustbot assigned tgross35 Feb 5, 2025

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 Feb 5, 2025

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pascaldekloe force-pushed the fmt-int128 branch from 658adc6 to 3a09315 Compare February 6, 2025 12:43

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rustbot added the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Apr 26, 2025

pascaldekloe force-pushed the fmt-int128 branch from 3a09315 to bfb09cf Compare April 28, 2025 13:48

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pascaldekloe force-pushed the fmt-int128 branch from bfb09cf to 1ce1b97 Compare April 29, 2025 20:16

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 30, 2025

tgross35 reviewed Jun 10, 2025

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pascaldekloe added 2 commits June 10, 2025 17:46

format integer tests regrouped, min/max coverage and 128-bit coverage

e9ebe25

faster fmt::Display of 128-bit integers, without unsafe pointer

0483c93

pascaldekloe force-pushed the fmt-int128 branch from 1ce1b97 to 0483c93 Compare June 10, 2025 17:31

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Faster fmt::Display of 128-bit integers, without unsafe pointer #136594

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Faster fmt::Display of 128-bit integers, without unsafe pointer #136594

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rust-timer commented Jun 10, 2025

Overall result: ✅ improvements - no action needed

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