Likely unlikely fix by x17jiri · Pull Request #120370 · rust-lang/rust

x17jiri · 2024-01-26T10:54:14Z

RFC 1131 ( #26179 ) added likely/unlikely intrinsics, but they have been broken for a while: #96276 , #96275 , #88767 . This PR tries to fix them.

Changes:

added a new cold_path() intrinsic
likely() and unlikely() changed to regular functions implemented using cold_path()

rustbot · 2024-01-26T10:54:22Z

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @cjgillot (or someone else) soon.

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rustbot · 2024-01-26T10:54:24Z

This PR changes MIR

cc @oli-obk, @RalfJung, @JakobDegen, @davidtwco, @celinval, @vakaras

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo, @GuillaumeGomez

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

Some changes occurred to the CTFE / Miri engine

cc @rust-lang/miri

This PR changes Stable MIR

cc @oli-obk, @celinval, @spastorino, @ouz-a

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

RalfJung · 2024-01-26T10:56:19Z

+    True,  // condition is probably true
+    False, // condition is probably false
+    Unpredictable,


Please make these doc comments, and also explain the last variant.

RalfJung · 2024-01-26T10:57:50Z

+                    // 'then' branch weight
+                    self.cx.const_u32(if expect == ExpectKind::True { 2000 } else { 1 }),
+                    // 'else' branch weight
+                    self.cx.const_u32(if expect == ExpectKind::True { 1 } else { 2000 }),


This is what Clang emits. I will add a comment

RalfJung · 2024-01-26T10:58:28Z

-                }
-                sym::unlikely => {
-                    self.expect(args[0].immediate(), false)
-                }


There are similar match arms here that can be cleaned up now.

EDIT: Ah, you already did. :)

bjorn3 · 2024-01-26T11:13:39Z

                let op_val = self.codegen_operand(bx, op);
                bx.assume(op_val.immediate());
            }
+            mir::StatementKind::Intrinsic(box NonDivergingIntrinsic::Expect(..)) => {}


Could you please also add this at

rust/compiler/rustc_codegen_cranelift/src/base.rs

Line 817 in 69db514

NonDivergingIntrinsic::Assume(_) => {}

?

Sure. Normally, only the LLVM backend is build, right? Because I didn't get any compile time errors in GCC or Cranelift

./x.py check should check all backends. ./x.py build only builds backends other than LLVM if you tell the build system to with the rust.codegen-backends option in config.toml.

Thanks, I updated this and one more match in Cranelift

rustbot · 2024-01-26T12:59:48Z

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

cjgillot · 2024-01-26T22:47:02Z

+                    }
+                } else {
+                    ExpectKind::None
+                };


Nit: could this be a Option<mir::ExpectKind>?

Good point. Updated

cjgillot · 2024-01-26T22:47:45Z

            let lltrue = helper.llbb_with_cleanup(self, target);
            let llfalse = helper.llbb_with_cleanup(self, targets.otherwise());
            if switch_ty == bx.tcx().types.bool {
+                let expect = if let Some(x) = self.mir[bb].statements.last()


What if the Expect is not the last statement? This could happen because of MIR opts.

Could it also be moved to a different BB? Or is that something worth considering?

I was also thinking it could be useful to emit a warning if there is Expect which cannot be paired with a branch.

Updated to handle expect anywhere in a BB

cjgillot · 2024-01-26T22:53:12Z

    Assume(Operand<'tcx>),

+    // Denotes a call to the intrinsic functions `likely`, `unlikely` and `unpredictable`.
+    Expect(Operand<'tcx>, ExpectKind),


For MIR opts:

SimplifyLocals: should this statement be removed when Operand is an unused local?

DeadStoreElimination: when the operand is a local that is never read again?

SimplifyConstCondition: when the operand if a constant?

Opt passes updated

cjgillot · 2024-01-26T22:53:51Z

+    HashStable,
+    TypeFoldable,
+    TypeVisitable
+)]


Nit: please split the derives in 2 lines.

Do you mean like this?

#[derive( Clone, TyEncodable, TyDecodable, Debug, PartialEq, Hash, HashStable, TypeFoldable, TypeVisitable )]

I mean:

#[derive(Clone, TyEncodable, TyDecodable, Debug, PartialEq)] #[derive(Hash, HashStable, TypeFoldable, TypeVisitable)]

To avoid rustfmt using a dozen lines.

x17jiri · 2024-01-29T13:06:18Z

@rustbot ready

rustbot · 2024-02-07T14:27:19Z

There are merge commits (commits with multiple parents) in your changes. We have a no merge policy so these commits will need to be removed for this pull request to be merged.

You can start a rebase with the following commands:

$ # rebase
$ git rebase -i master
$ # delete any merge commits in the editor that appears
$ git push --force-with-lease

The following commits are merge commits:

12a1bb3

x17jiri · 2024-02-08T16:53:49Z

@cjgillot I found I introduced a bug in simplify_branches.rs 🤦 It should be fixed now.

Anyway, the number of affected files is getting quite big. If there is anything I can do to make the review easier, please let me know

Noratrieb

small style nit (in case you're touching this code again, if you aren't it's fine to not implement it)

Noratrieb · 2024-02-15T18:16:18Z

+    /// condition is probably true
+    True,
+
+    /// condition is probably false
+    False,
+
+    /// condition is unpredictable by hardware
+    Unpredictable,


Suggested change

/// condition is probably true

True,

/// condition is probably false

False,

/// condition is unpredictable by hardware

Unpredictable,

/// Condition is probably true

True,

/// Condition is probably false

False,

/// Condition is unpredictable by hardware

Unpredictable,

bors · 2024-02-16T11:59:44Z

☔ The latest upstream changes (presumably #120500) made this pull request unmergeable. Please resolve the merge conflicts.

klensy · 2024-11-18T10:28:59Z

Someone used them to tune some perf and now they see that this actually only started working as expected, so less surprises.

RalfJung · 2024-11-18T10:29:50Z

Given that this is on nightly now, and relnotes will be put out in ~7 weeks, it also wouldn't make a lot of sense to attach this to the Rust 1.84 relnotes.

Kobzol · 2024-11-18T10:30:53Z

Right, I meant the detailed release notes, which also mention internal changes and interesting pieces of information like this, not the main blog post.

RalfJung · 2024-11-18T10:33:36Z

How often do these detailed release notes talk about nightly features? In my understand, not very often. But maybe I misremember.

klensy · 2024-11-18T10:34:43Z

Personally, i think that release notes also should contain block for nightly changes, i.e. new added\removed unstable features to help users discover them, but this not related to current PR.

RalfJung · 2024-11-18T10:36:42Z

That would announce such changes on average 9 weeks too late. I don't think it makes a lot of sense.

klensy · 2024-11-18T10:44:18Z

That would announce such changes on average 9 weeks too late. I don't think it makes a lot of sense.

With release cycle of 6 weeks this will be only <= 6 weeks if placed in nearest relnotes.

How currently users can discover new features (before stabilization), excluding ones that manually track this repo? This gives lot less possible testing for that features. Or i miss something?

RalfJung · 2024-11-18T10:50:31Z

Such updates can and often are posted to the tracking issues, and we expect interested users to follow those.

With release cycle of 6 weeks this will be only <= 6 weeks if placed in nearest relnotes.

That would be really strange, since the codebase that is released there doesn't contain these changes.

If we want to do something like this IMO it should be a completely separate "nightly feature release notes" announcement, not part of the regular release notes. It doesn't even have to use the same 6 week cadence.

rustbot assigned cjgillot Jan 26, 2024

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Jan 26, 2024

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cjgillot 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-review Status: Awaiting review from the assignee but also interested parties. labels Jan 27, 2024

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x17jiri mentioned this pull request Feb 3, 2024

#[cold] on match arms #120193

Closed

rustbot added has-merge-commits PR has merge commits, merge with caution. S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Feb 7, 2024

x17jiri force-pushed the likely_unlikely_fix branch from 12a1bb3 to 2d49c41 Compare February 7, 2024 14:35

rustbot removed has-merge-commits PR has merge commits, merge with caution. S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Feb 7, 2024

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Noratrieb reviewed Feb 15, 2024

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x17jiri force-pushed the likely_unlikely_fix branch from e4cc72b to d31b045 Compare February 17, 2024 07:36

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saethlin mentioned this pull request Nov 18, 2024

Meta tracking issue for branch hints (RFC 1131) #26179

Closed

x17jiri mentioned this pull request Dec 4, 2024

improve cold_path() #133852

Merged

x17jiri mentioned this pull request Dec 21, 2024

Export cold_path() in std::hint rust-lang/libs-team#510

Closed

This was referenced Feb 10, 2025

parallel: rustc_data_structures: sync: implement RwLock as enum { Sync(parking_lot::RwLock), NoSync(RefCell) } #136772

Closed

Full Compile-Time Processor Feature Detection BurntSushi/memchr#160

Open

tgross35 mentioned this pull request Jul 17, 2025

Tracking Issue for cold_path #136873

Closed

5 tasks

DaniPopes mentioned this pull request Jul 26, 2025

Re-implement likely/unlikely with #[cold] rust-lang/hashbrown#631

Merged

tgross35 mentioned this pull request Jan 24, 2026

Tracking Issue for likely_unlikely #151619

Open

5 tasks

This was referenced Feb 7, 2026

Rust unstable features needed for the kernel Rust-for-Linux/linux#2

Open

Rust unstable features needed for the kernel — Done Rust-for-Linux/linux#1223

Open

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