Replace Rvalue::NullaryOp by a variant in mir::Operand. #148766

cjgillot · 2025-11-09T20:19:43Z

This PR fully removes the MIR Rvalue::NullaryOp. After #148151, it was only useful for runtime checks like ub_checks, contract_checks and overflow_checks.

These are "runtime" checks, boolean constants that may only be true in codegen. It depends on a rustc flag passed to codegen, so we need to represent those flags cross-crate.

This PR replaces those runtime checks by special variants in MIR ConstValue. This allows code that expects constants to manipulate those as such, even if we may not always be able to evaluate them to actual scalars.

rustbot · 2025-11-09T20:19:47Z

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

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

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

⚠️ #[rustc_intrinsic_const_stable_indirect] controls whether intrinsics can be exposed to stable const
code; adding it needs t-lang approval.

cc @rust-lang/wg-const-eval

Some changes occurred in rustc_ty_utils::consts.rs

cc @BoxyUwU

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

This PR changes MIR

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

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

This PR changes rustc_public

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

Some changes occurred in match checking

cc @Nadrieril

Some changes occurred to constck

cc @fee1-dead

This PR changes a file inside tests/crashes. If a crash was fixed, please move into the corresponding ui subdir and add 'Fixes #' to the PR description to autoclose the issue upon merge.

rustbot · 2025-11-09T20:19:49Z

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RalfJung · 2025-11-10T10:56:06Z

compiler/rustc_middle/src/mir/consts.rs

+ /// Special constants whose value depends on the evaluation context. Their value depends on a
+ /// flag on the crate being codegenned.
+ RuntimeChecks(RuntimeChecks),


For const-eval execution, their value is fixed, right?

RalfJung · 2025-11-10T10:59:56Z

It seems slightly odd to call these things "constants" when they have a different value inside and outside of const blocks... I am not sure whether this actually breaks anything today, but it seems at least conceptually suboptimal and could lead to issues down the line. For instance, if a constant's body entirely consists just of one of these new constants, then replacing that outer constant by the inner one is not correct (or at least, it can change program behavior) -- so e.g. @saethlin's "trivial const" machinery might need a special case for this.

bors · 2025-11-11T20:36:08Z

☔ The latest upstream changes (presumably #148658) made this pull request unmergeable. Please resolve the merge conflicts.

cjgillot · 2025-11-11T21:39:01Z

I agree that's not entirely satisfactory.

Another design would put the variant inside mir::Const, but the conversion RuntimeChecks -> ConstValue would be done in mir::Const::eval, so without access to the interpreter hooks. This makes everything more complex, and I haven't managed to get the correct behavior yet.

RalfJung · 2025-11-11T21:44:16Z

That would be marginally better ("values" really should be unchanging), though still a footgun e.g. with the trivial const machinery I mentioned.

Making them a new variant of mir::Operand would be the conceptually cleanest option I think. Not sure how well it works in practice though.

cjgillot · 2025-11-17T23:29:16Z

Making them a new variant of mir::Operand would be the conceptually cleanest option I think. Not sure how well it works in practice though.

Latest commit does that. Cleaner than I expected.

RalfJung · 2025-11-18T08:23:45Z

compiler/rustc_const_eval/src/const_eval/dummy_machine.rs

+ // We can't look at `tcx.sess` here as that can differ across crates, which can lead to
+ // unsound differences in evaluating the same constant at different instantiation sites.


This comment doesn't really make sense in the dummy machine.

RalfJung · 2025-11-18T08:24:54Z

compiler/rustc_middle/src/mir/pretty.rs

 Constant(ref a) => write!(fmt, "{a:?}"),
 Copy(ref place) => write!(fmt, "copy {place:?}"),
 Move(ref place) => write!(fmt, "move {place:?}"),
+ RuntimeChecks(checks) => write!(fmt, "const {checks:?}"),


"const" is an odd choice, given that we moved it out of the constants

compiler/rustc_middle/src/mir/syntax.rs

compiler/rustc_middle/src/mir/statement.rs

cjgillot · 2025-11-19T01:24:32Z

@bors try @rust-timer queue

Replace Rvalue::NullaryOp by a variant in mir::Operand.

rust-bors · 2025-11-19T03:39:59Z

☀️ Try build successful (CI)
Build commit: f642b25 (f642b25044e7b22d14fdca3602c1a4aaf2b036f6, parent: 7c2c3c0ded2de378bfab2f5b55c387c66fbaf353)

rust-timer · 2025-11-19T05:03:15Z

Finished benchmarking commit (f642b25): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

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.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@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)	0.5%	[0.1%, 1.4%]	31
Regressions ❌ (secondary)	0.3%	[0.1%, 1.1%]	28
Improvements ✅ (primary)	-1.2%	[-4.1%, -0.2%]	10
Improvements ✅ (secondary)	-0.5%	[-1.6%, -0.1%]	13
All ❌✅ (primary)	0.1%	[-4.1%, 1.4%]	41

Max RSS (memory usage)

Results (primary -0.1%, 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)	4.2%	[2.0%, 7.4%]	5
Regressions ❌ (secondary)	2.3%	[2.1%, 2.5%]	2
Improvements ✅ (primary)	-2.8%	[-6.1%, -1.5%]	8
Improvements ✅ (secondary)	-1.9%	[-2.3%, -1.2%]	3
All ❌✅ (primary)	-0.1%	[-6.1%, 7.4%]	13

Cycles

Results (primary -1.8%, secondary -1.3%)

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)	3.5%	[2.3%, 5.5%]	3
Improvements ✅ (primary)	-3.2%	[-5.0%, -2.2%]	3
Improvements ✅ (secondary)	-3.7%	[-8.0%, -2.1%]	6
All ❌✅ (primary)	-1.8%	[-5.0%, 2.4%]	4

Binary size

Results (primary -0.1%, secondary -0.4%)

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

	mean	range	count
Regressions ❌ (primary)	0.3%	[0.0%, 1.2%]	38
Regressions ❌ (secondary)	0.2%	[0.0%, 0.6%]	8
Improvements ✅ (primary)	-0.3%	[-5.6%, -0.0%]	57
Improvements ✅ (secondary)	-0.5%	[-5.6%, -0.0%]	58
All ❌✅ (primary)	-0.1%	[-5.6%, 1.2%]	95

Bootstrap: 474.558s -> 478.183s (0.76%)
Artifact size: 388.75 MiB -> 388.76 MiB (0.00%)

bors · 2025-11-23T21:49:11Z

☔ The latest upstream changes (presumably #147827) made this pull request unmergeable. Please resolve the merge conflicts.

rustbot · 2025-11-24T00:42:39Z

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.

lcnr · 2025-11-26T10:59:04Z

r? @RalfJung maybe? didn't even realize I was assigned

RalfJung · 2025-11-26T20:21:15Z

What do we do with these perf numbers? It's a mixed bag, but for primary benchmarks the median is red.

RalfJung

Everything MIR-related LGTM, modulo minor comments.

However, as noted before, I'm not sure how to judge whether those perf regressions are acceptable, or where they even come from. @saethlin do you have thoughts on that?

View changes since this review

RalfJung · 2025-12-01T21:45:12Z

compiler/rustc_codegen_cranelift/src/base.rs

 Operand::Constant(const_) => crate::constant::codegen_constant_operand(fx, const_),
+ Operand::RuntimeChecks(checks) => {
+ let int = checks.value(fx.tcx.sess);
+ let int = ScalarInt::try_from_uint(int, Size::from_bits(1)).unwrap();


ScalarInt implements From<bool> -- please use that instead of effectively inlining it. (Also, int is a confusing name for a boolean variable.)

RalfJung · 2025-12-01T21:45:24Z

compiler/rustc_codegen_cranelift/src/constant.rs

 match operand {
+ Operand::RuntimeChecks(checks) => {
+ let int = checks.value(fx.tcx.sess);
+ ScalarInt::try_from_uint(int, Size::from_bits(1))


Another place where the From<bool> impl could be used. (Also, int is a confusing name for a boolean variable.)

RalfJung · 2025-12-01T21:54:19Z

compiler/rustc_mir_transform/src/instsimplify.rs

+ let preserve_ub_checks =
+ attr::contains_name(tcx.hir_krate_attrs(), sym::rustc_preserve_ub_checks);
+ if !preserve_ub_checks {
+ SimplifyUbCheck { tcx }.visit_body(body);


Why is this a separate visitor now?

RalfJung · 2025-12-01T21:55:44Z

compiler/rustc_mir_transform/src/promote_consts.rs

 match operand {
 Operand::Copy(place) | Operand::Move(place) => self.validate_place(place.as_ref()),

+ // Promoting a runtime check would transform a runtime error into a compile-time error.


This isn't about promoting the actual check, but about promoting the value that determines whether there is a check... so the comment doesn't quite make sense to me.

Suggested change

// Promoting a runtime check would transform a runtime error into a compile-time error.

// `RuntimeChecks` behaves different in const-eval and runtime MIR, so we do not promote it.

RalfJung · 2025-12-01T21:56:36Z

compiler/rustc_public/src/mir/pretty.rs

 format!("move {mv:?}")
 }
 Operand::Constant(cnst) => pretty_mir_const(&cnst.const_),
+ Operand::RuntimeChecks(checks) => format!("const {checks:?}"),


This shouldn't say "const".

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-clippy Relevant to the Clippy team. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Nov 9, 2025

rustbot assigned lcnr Nov 9, 2025

rustbot added the T-libs Relevant to the library team, which will review and decide on the PR/issue. label Nov 9, 2025