Add intrinsics for integer minimum and maximum - #161081

scottmcm · 2026-08-14T05:52:53Z

I got inspired to do this when looking at SliceOrd::compare where I was reminded that if a < b { a } else { b } isn't great in MIR since it takes 4 BBs. Looking at the codegen side, it turns out we currently emit 42 lines of LLVM-IR including 4 allocas for u16::max (pre-optimization), which is also unnecessarily bad†.

But both LLVM and Cranelift have dedicated things for min & max:

so let's just use those directly!

This actually wouldn't have been worth doing originally, but a couple of things have happened to change that:

Back in 1.0 there was only cmp::min & cmp::max, so there was no place to actually do this at all, but in 2017 they were added to Ord as overridable things Tracking issue for Ord::{min, max} #25663 (comment)
LLVM originally used icmp+select for these, not a dedicated construct, but then added one and as of 2022 the intrinsic is fully usable https://www.npopov.com/2022/12/20/This-year-in-LLVM-2022.html#integer-minmax-intrinsics
Before we had intrinsic fallback this would have been more annoying to support everywhere -- GCC, 128-bit numbers on cg_clif, CTFE, anything out-of-tree -- but now that we can write the obvious fallback we don't need to worry about that.
The intrinsic would have helped less when it forced extra BBs and allocas in codegen anyway, but now we can keep the result in SSA without needing to make it a primitive.

† Admittedly we could clean up the gratuitous badness there without needing an intrinsic, but I like doing the intrinsic anyway because that's the only way to avoid it always being stuck in the non-SSA path from the multi-BB assignments. Even if we made it inlineable, GVN and such will still just give up on seeing the x = if a < b { a } else { b } because it's multiple assignments to the same Local, which is non-ideal for something primitive-like.

Done without LLMs.

scottmcm · 2026-08-14T05:54:10Z

+#[miri::fallback_is_spec]
+pub const fn integer_min<T: Copy + [const] PartialOrd>(a: T, b: T) -> T {
+    if a < b { a } else { b }
+}


cc @RalfJung because I said miri::fallback_is_spec

View changes since the review

Odd, why did the bot not ping me...

... ah, because it's a draft. :)

Yeah, because I know my odds of getting a working PR first push are about 2% on a good day 🙃

EDIT: oh, and also I managed to spell it wrong 🤦

LGTM apart from the comment nit. :)

rustbot · 2026-08-14T18:25:49Z

Some changes occurred to the intrinsics. Make sure the CTFE / Miri interpreter
gets adapted for the changes, if necessary.

cc @rust-lang/miri, @RalfJung, @oli-obk, @lcnr

rustc_codegen_cranelift is developed in its own repository. If possible, consider making this change to rust-lang/rustc_codegen_cranelift instead.

cc @bjorn3

Any special-casing of Miri in the standard library requires review.

cc @rust-lang/miri

⚠️ #[miri::intrinsic_fallback_is_spec] must only be used if the function actively checks for all UB cases,
and explores the possible non-determinism of the intrinsic.

cc @rust-lang/miri

rustbot · 2026-08-14T18:25:51Z

r? @clarfonthey

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scottmcm · 2026-08-14T21:18:19Z

Given that LLVM can collapse all the mess this probably won't show much of a difference, but might as well make sure it's at least not worse somehow
@bors try @rust-timer queue

Add intrinsics for integer minimum and maximum

RalfJung · 2026-08-14T21:35:54Z

+#[rustc_nounwind]
+#[rustc_intrinsic]
+#[miri::intrinsic_fallback_is_spec]
+pub const fn integer_min<T: Copy + [const] PartialOrd>(a: T, b: T) -> T {


The name says integer but the type signature does not. If there are constraints on the type that go beyond the signature, please spell them out in the doc comment.

View changes since the review

Do you have thoughts about doing it via a trait?

Like we have FloatPrimitive I could add IntegerPrimitive and bound it that way.

That exists for fallbacks more than for just type checking, though, so I don't know if it's worth doing here vs just documenting it.

Edit: Oh, actually, I think I might as well do that because then the Ord requirement can come from that trait instead of listing it out.

Done, with both a bound and a doc-comment update.

rust-bors · 2026-08-14T22:23:10Z

☀️ Try build successful (CI)
Build commit: 8764a9b (8764a9b9995264025b6234cf175d01db8c729849)
Base parent: d453bdd (d453bdd8f092d099bc336f0bda4163f809ad18e0)

clarfonthey · 2026-08-14T22:57:29Z

I'm happy with this and would be fine merging once you make whatever fallback changes you were mentioning. I'm not on the compiler team, but just from the perspective of optimising the giant pile of operations we have in libstd, adding more intrinsics for "obvious" primitives like this is fine, even if they're technically redundant.

For example, #161069 which also fell under my review recommended adding some potential additional reasoning for checked_next_power_of_two and I was already a bit worried that the complexity of the operations involved would not optimise correctly, which I haven't yet verified. If the codegen backends support it, I think we should go ahead with these, especially if there's some extra empirical justification as there seems to be here.

clarfonthey · 2026-08-14T22:58:37Z


        other => {
-            tcx.dcx().emit_err(UnrecognizedIntrinsicFunction { span, name: other });
+            tcx.dcx().emit_err(UnrecognizedIntrinsicFunction { span, name: other, file: file!() });


I can definitely tell that this was motivated by you forgetting which file this was when you added this intrinsic. :p

View changes since the review

Yeah, this is where span_bug! might be better than emit_err. ;)
I don't know why we bother with "pretty" errors for intrinsic misuse anyway...

scottmcm · 2026-08-14T23:26:55Z

Curious, the build didn't get queued
@rust-timer queue

clarfonthey · 2026-08-14T23:50:48Z

@rust-timer build 8764a9b

rust-timer · 2026-08-15T00:33:25Z

Finished benchmarking commit (8764a9b): comparison URL.

Overall result: ❌✅ regressions and improvements - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@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)	0.9%	[0.9%, 0.9%]	1
Regressions ❌ (secondary)	1.5%	[1.5%, 1.5%]	1
Improvements ✅ (primary)	-0.5%	[-0.5%, -0.5%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.2%	[-0.5%, 0.9%]	2

Max RSS (memory usage)

Results (primary -0.9%, secondary -0.7%)

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

	mean	range	count
Regressions ❌ (primary)	2.6%	[1.5%, 3.6%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-4.3%	[-6.1%, -2.5%]	2
Improvements ✅ (secondary)	-0.7%	[-0.7%, -0.7%]	1
All ❌✅ (primary)	-0.9%	[-6.1%, 3.6%]	4

Cycles

Results (secondary 1.7%)

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)	6.7%	[1.7%, 16.2%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.4%	[-4.2%, -2.9%]	3
All ❌✅ (primary)	-	-	0

Binary size

Results (primary -0.0%, secondary -0.0%)

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

	mean	range	count
Regressions ❌ (primary)	0.2%	[0.1%, 0.3%]	7
Regressions ❌ (secondary)	0.5%	[0.5%, 0.5%]	1
Improvements ✅ (primary)	-0.1%	[-0.7%, -0.0%]	16
Improvements ✅ (secondary)	-0.1%	[-0.2%, -0.0%]	14
All ❌✅ (primary)	-0.0%	[-0.7%, 0.3%]	23

Bootstrap: 461.184s -> 456.796s (-0.95%)
Artifact size: 396.50 MiB -> 396.41 MiB (-0.02%)

rustbot · 2026-08-17T04:33:52Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

scottmcm · 2026-08-17T04:34:11Z

@bors try @rust-timer queue

Add intrinsics for integer minimum and maximum

rust-bors · 2026-08-17T05:32:36Z

☀️ Try build successful (CI)
Build commit: c92286c (c92286c8f9a943cfbc1adca426cf24e708a61c76)
Base parent: 2c39ff4 (2c39ff499469be916d4e45506d1afed69bbaddb7)

rust-timer · 2026-08-17T06:14:39Z

Finished benchmarking commit (c92286c): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

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.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never rustc-perf
@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)	0.6%	[0.3%, 0.9%]	2
Regressions ❌ (secondary)	1.5%	[1.5%, 1.5%]	1
Improvements ✅ (primary)	-0.7%	[-1.1%, -0.4%]	3
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.2%	[-1.1%, 0.9%]	5

Max RSS (memory usage)

Results (primary -0.9%, secondary -1.4%)

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

	mean	range	count
Regressions ❌ (primary)	1.3%	[1.3%, 1.3%]	1
Regressions ❌ (secondary)	2.6%	[2.4%, 2.9%]	2
Improvements ✅ (primary)	-1.2%	[-2.9%, -0.5%]	7
Improvements ✅ (secondary)	-2.1%	[-2.9%, -1.1%]	12
All ❌✅ (primary)	-0.9%	[-2.9%, 1.3%]	8

Cycles

Results (primary -2.2%, secondary 4.0%)

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)	4.0%	[1.8%, 6.1%]	2
Improvements ✅ (primary)	-2.2%	[-2.2%, -2.2%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.2%	[-2.2%, -2.2%]	1

Binary size

Results (primary -0.0%, secondary -0.0%)

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

	mean	range	count
Regressions ❌ (primary)	0.2%	[0.0%, 0.8%]	25
Regressions ❌ (secondary)	0.5%	[0.5%, 0.5%]	1
Improvements ✅ (primary)	-0.4%	[-3.4%, -0.0%]	12
Improvements ✅ (secondary)	-0.1%	[-0.2%, -0.0%]	10
All ❌✅ (primary)	-0.0%	[-3.4%, 0.8%]	37

Bootstrap: 457.537s -> 456s (-0.34%)
Artifact size: 398.93 MiB -> 398.97 MiB (0.01%)

scottmcm · 2026-08-17T19:42:13Z

Perf changes look mostly like the kind of thing that makes sense from better inlining: reduced is_mir_available calls, and permuted codegen schedules from different CGU partitioning that sometimes is better, sometimes worse.

Overall perf seems perhaps slightly better, and the bootstrap being green in both runs is also promising.
@rustbot label: +perf-regression-triaged

scottmcm commented Aug 14, 2026

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Auto merge of #161081 - scottmcm:min-max-intrinsics, r=<try>

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Add intrinsics for integer minimum and maximum

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scottmcm added I-lang-nominated Nominated for discussion during a lang team meeting. I-lang-easy-decision Issue: The decision needed by the team is conjectured to be easy; this does not imply nomination labels Aug 15, 2026

scottmcm mentioned this pull request Aug 16, 2026

Iterator::{min,max}(_by_key) should use overridden min/max/lt #160203

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Add intrinsics for integer minimum and maximum

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Auto merge of #161081 - scottmcm:min-max-intrinsics, r=<try>

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Add intrinsics for integer minimum and maximum

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rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Aug 17, 2026

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Overall result: ❌✅ regressions and improvements - no action needed

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Overall result: ❌✅ regressions and improvements - please read:

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