Add intrinsics for integer minimum and maximum by scottmcm · Pull Request #161081 · rust-lang/rust

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-bors · 2026-08-30T20:49:04Z

Auto build was cancelled. Cancelled workflows:

https://github.com/rust-lang/rust/actions/runs/33334567339

The next pull request likely to be tested is #162028.

Add intrinsics for integer minimum and maximum 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 `alloca`s](https://rust.godbolt.org/z/ha4h4nT3r) for `u16::max` (pre-optimization), which is also unnecessarily bad†. But both LLVM and Cranelift have dedicated things for min & max: - https://llvm.org/docs/LangRef.html#llvm-umax-intrinsic - https://docs.rs/cranelift-codegen/latest/cranelift_codegen/ir/trait.InstBuilder.html#method.smin 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 #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](bytecodealliance/wasmtime#13790), 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 `alloca`s in codegen anyway, but [now](rust-lang/compiler-team#970) 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.

rust-bors · 2026-08-31T00:04:28Z

💔 Test for b59ba1f failed: CI. Failed job:

auto - x86_64-msvc-2 (web logs, enhanced plaintext logs)

rdrpenguin04 · 2026-08-31T00:05:58Z

(As an observer trying to understand the Rust build system, is this kind of failure known/expected to happen sometimes? Segfaulting on installing sccache seems like a very strange way to fail)

rust-log-analyzer · 2026-08-31T00:13:32Z

A job failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

  0      0   0      0   0      0      0      0                              0
100 19.08M 100 19.08M   0      0 147.3M      0                              0
100 19.08M 100 19.08M   0      0 147.1M      0                              0
100 19.08M 100 19.08M   0      0 146.9M      0                              0
src/ci/scripts/install-sccache.sh: line 19:  1910 Segmentation fault         curl -fo sccache/sccache.exe "${MIRRORS_BASE}/2025-02-24-sccache-v0.10.0-x86_64-pc-windows-msvc.exe"
##[error]Process completed with exit code 139.
##[group]Run echo "disk usage:"
echo "disk usage:"
df -h
shell: C:\Program Files\Git\bin\bash.EXE --noprofile --norc -e -o pipefail {0}

clarfonthey · 2026-08-31T00:17:14Z

There are lots of spurious issues with GitHub actions, unfortunately. Whenever they come up, we just retry the job and hope it doesn't happen again.

clarfonthey · 2026-08-31T00:19:54Z

@bors try jobs=x86_64-msvc-1

Add intrinsics for integer minimum and maximum try-job: x86_64-msvc-1

rust-bors · 2026-08-31T02:59:15Z

☀️ Try build successful (CI)
Build commit: 475f470 (475f4701ee52aacf44dbe78e603d6ce53fe49bf3)
Base parent: 4b7e3a7 (4b7e3a76d8df78960dc7c65cad43f5da1dac8ade)

scottmcm · 2026-08-31T03:46:19Z

@bors retry

rust-bors · 2026-08-31T07:54:36Z

☀️ Test successful - CI
Approved by: clarfonthey
Duration: 3h 12m 17s
Pushing be4b6a9 to main...

rust-log-analyzer · 2026-08-31T08:07:12Z

A job failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

   Compiling diff v0.1.13
   Compiling glob-match v0.2.1
   Compiling citool v0.1.0 (/home/runner/work/rust/rust/src/ci/citool)
    Finished `release` profile [optimized] target(s) in 48.10s
     Running `target/release/citool post-merge-report 5321a4f40c957cf3587c055e77461febc2ebc865 be4b6a99a3c605d27786c7d48772fec5bd2df76c`
Downloading metrics of job aarch64-gnu
Downloading metrics of job aarch64-gnu-debug
Downloading metrics of job arm-android
Downloading metrics of job armhf-gnu
Downloading metrics of job dist-aarch64-linux
Downloading metrics of job dist-android
Downloading metrics of job dist-arm-linux-gnueabi

rust-timer · 2026-08-31T09:10:11Z

Finished benchmarking commit (be4b6a9): comparison URL.

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

Our benchmarks found a performance regression caused by this PR.
This might be an actual regression, but it can also be just noise.

Next Steps:

If the regression was expected or you think it can be justified,
please write a comment with sufficient written justification, and add
@rustbot label: +perf-regression-triaged to it, to mark the regression as triaged.
If you think that you know of a way to resolve the regression, try to create
a new PR with a fix for the regression.
If you do not understand the regression or you think that it is just noise,
you can ask the @rust-lang/wg-compiler-performance working group for help (members of this group
were already notified of this PR).

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

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)	0.1%	[0.1%, 0.2%]	4
Regressions ❌ (secondary)	0.2%	[0.2%, 0.2%]	1
Improvements ✅ (primary)	-0.9%	[-0.9%, -0.9%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.1%	[-0.9%, 0.2%]	5

Max RSS (memory usage)

Results (primary 0.1%, secondary -1.0%)

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

	mean	range	count
Regressions ❌ (primary)	1.3%	[0.7%, 2.8%]	4
Regressions ❌ (secondary)	2.3%	[0.6%, 3.9%]	2
Improvements ✅ (primary)	-2.3%	[-3.3%, -1.2%]	2
Improvements ✅ (secondary)	-7.6%	[-7.6%, -7.6%]	1
All ❌✅ (primary)	0.1%	[-3.3%, 2.8%]	6

Cycles

Results (primary 0.1%, 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)	2.4%	[2.4%, 2.4%]	1
Regressions ❌ (secondary)	1.4%	[1.4%, 1.4%]	1
Improvements ✅ (primary)	-2.3%	[-2.3%, -2.3%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[-2.3%, 2.4%]	2

Binary size

Results (primary 0.2%, secondary 0.2%)

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

	mean	range	count
Regressions ❌ (primary)	0.9%	[0.0%, 7.6%]	11
Regressions ❌ (secondary)	7.6%	[7.6%, 7.6%]	1
Improvements ✅ (primary)	-0.3%	[-1.8%, -0.0%]	15
Improvements ✅ (secondary)	-0.4%	[-0.5%, -0.0%]	13
All ❌✅ (primary)	0.2%	[-1.8%, 7.6%]	26

Bootstrap: 477.635s -> 474.007s (-0.76%)
Artifact size: 402.71 MiB -> 402.61 MiB (-0.03%)

fbstj · 2026-08-31T11:00:10Z

+                            (sym::integer_max, true) => "llvm.smax",
+                            (sym::integer_min, false) => "llvm.umin",
+                            (sym::integer_min, true) => "llvm.smin",
+                            _ => bug!(),


is this another unreachable!()?

Yeah, the compiler uses it to display ICEs (internal compiler errors). But here unreachable!() would also work.

panstromek · 2026-08-31T15:50:45Z

perf triage:

Some compile time impact was expected and justfied in #161081 (comment)

The post-merge result doesn't match the pre-merge run, but I assume that's also somewhat expected based on that comment justification.

@rustbot label: +perf-regression-triaged

scottmcm commented Aug 14, 2026

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

8764a9b

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 try-job: x86_64-msvc-1

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Improve Session/CodegenBackend construction #161432

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Refactor core::cmp::{smallest, largest} & add mir-opt test #161275

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