add `-Zforce-intrinsic-fallback` flag by folkertdev · Pull Request #158377 · rust-lang/rust

folkertdev · 2026-06-24T19:39:53Z

Add a flag that forces the use of the fallback body (if one exists), so that we can test that these fallback implementations actually work.

cc #150946
cc #t-compiler > testing intrinsic fallback bodies
cc #t-infra > CI for -Zforce-intrinsic-fallback

rustbot · 2026-06-24T19:39:56Z

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

cc @bjorn3

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

cc @antoyo, @GuillaumeGomez

rustbot · 2026-06-24T19:39:59Z

r? @adwinwhite

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adwinwhite · 2026-06-27T08:37:06Z

r? codegen

scottmcm · 2026-06-28T20:21:34Z

@@ -0,0 +1,71 @@
+//@ add-minicore
+//@ assembly-output: emit-asm


I'm kinda puzzled here: why does this need assembly output? If it wants to be checking assembly should it be in assembly-llvm instead? For that matter, why does it need minicore?

Could this just be a normal codegen test that calls std::intrinsics::sqrtf32? Looking at llvm-ir should be pretty portable for it, no? (Especially with -C no-prepopulate-passes so it just sees what we emit, not post-optimization IR.)

View changes since the review

Right, this was an assembly test originally, but LLVM made more sense.

I've made it generic now, with -C no-prepopulate-passes -Copt-level=3 so hopefully the IR should be stable enough.

Also given that you've now looked at this code a bunch, maybe you can review it?

saethlin · 2026-07-03T03:16:39Z

I got here eventually!

@bors r+

rust-bors · 2026-07-03T03:16:41Z

📌 Commit 69d7670 has been approved by saethlin

It is now in the queue for this repository.

🌲 The tree is currently closed for pull requests below priority 10. This pull request will be tested once the tree is reopened.

…k, r=saethlin add `-Zforce-intrinsic-fallback` flag Add a flag that forces the use of the fallback body (if one exists), so that we can test that these fallback implementations actually work. cc rust-lang#150946 cc [#t-compiler > testing intrinsic fallback bodies](https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/testing.20intrinsic.20fallback.20bodies/with/606299558)

Rollup of 18 pull requests Successful merges: - #158692 (Add release notes for 1.96.1) - #134021 (Implement `IntoIterator` for `[&[mut]] Box<[T; N], A>`) - #152860 (Port the `without_debuginfo` test from `backtrace-rs` to the testsuite) - #155932 (MIR Call terminator: evaluate destination place before arguments) - #156777 (Add -Zautodiff_post_passes flag to limit which llvm passes to run after enzyme to make autodiff tests more robust) - #157151 (JSON target specs: remove 'x86-softfloat' compatibility alias) - #157835 (expand free alias types in the auto-trait orphan check) - #158377 (add `-Zforce-intrinsic-fallback` flag) - #158434 (delegation: refactor AST -> HIR lowering) - #158552 (make some tidy errors around python easier to understand) - #158624 (borrowck: Introduce BlameConstraint::to_obligation_cause_from_path()) - #158704 (Optimize `ArrayChunks::try_rfold` with `DoubleEndedIterator::next_chunk_back`) - #158711 (library: Comment on libtest's dicey internal soundness) - #158539 (Move `SizeHint` and `IoHandle` to `core::io`) - #158659 (refactor the normalization in `coerce_shared_info`) - #158689 (resolver: don't use `Finalize` when resolving visibilities during AST expansion) - #158698 (Update TypeVisitable implementation) - #158706 (Tweaks to MIR building scope API)

…k, r=saethlin add `-Zforce-intrinsic-fallback` flag Add a flag that forces the use of the fallback body (if one exists), so that we can test that these fallback implementations actually work. cc rust-lang#150946 cc [#t-compiler > testing intrinsic fallback bodies](https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/testing.20intrinsic.20fallback.20bodies/with/606299558) cc [#t-infra > CI for -Zforce-intrinsic-fallback](https://rust-lang.zulipchat.com/#narrow/channel/242791-t-infra/topic/CI.20for.20-Zforce-intrinsic-fallback/with/608129693)

…uwer Rollup of 20 pull requests Successful merges: - #158692 (Add release notes for 1.96.1) - #134021 (Implement `IntoIterator` for `[&[mut]] Box<[T; N], A>`) - #155932 (MIR Call terminator: evaluate destination place before arguments) - #155989 (Update `transmute_copy` to ub_checks and `?Sized`) - #156777 (Add -Zautodiff_post_passes flag to limit which llvm passes to run after enzyme to make autodiff tests more robust) - #157151 (JSON target specs: remove 'x86-softfloat' compatibility alias) - #157835 (expand free alias types in the auto-trait orphan check) - #157857 (Stabilize `#[my_macro] mod foo;` (part of `proc_macro_hygiene`)) - #158377 (add `-Zforce-intrinsic-fallback` flag) - #158434 (delegation: refactor AST -> HIR lowering) - #158552 (make some tidy errors around python easier to understand) - #158624 (borrowck: Introduce BlameConstraint::to_obligation_cause_from_path()) - #158704 (Optimize `ArrayChunks::try_rfold` with `DoubleEndedIterator::next_chunk_back`) - #158711 (library: Comment on libtest's dicey internal soundness) - #158751 (rustdoc: Fix crash when trying to inline foreign item which cannot have attributes) - #158539 (Move `SizeHint` and `IoHandle` to `core::io`) - #158659 (refactor the normalization in `coerce_shared_info`) - #158689 (resolver: don't use `Finalize` when resolving visibilities during AST expansion) - #158698 (Update TypeVisitable implementation) - #158706 (Tweaks to MIR building scope API)

JonathanBrouwer · 2026-07-05T12:11:14Z

💔 I suspect this PR failed tests as part of a rollup
@bors r-

After fixing the problem, consider running a try job for the failed job before re-approving.

Link to failure: #158798 (comment)

rust-bors · 2026-07-05T12:11:21Z

This pull request was unapproved.

This PR was contained in a rollup (#158798), which was unapproved.

View changes since this unapproval

rustbot · 2026-07-05T13:33:06Z

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.

folkertdev · 2026-07-05T14:56:31Z

@bors try jobs=dist-i586-gnu-i586-i686-musl

add `-Zforce-intrinsic-fallback` flag try-job: dist-i586-gnu-i586-i686-musl

rust-bors · 2026-07-05T16:37:00Z

☀️ Try build successful (CI)
Build commit: 65a628b (65a628b17e22314b8c2a6b1d1667246001df89a8)
Base parent: 4eb6253 (4eb62535fc12dd1a63fd43a4173e224e79313c4d)

folkertdev · 2026-07-05T17:01:53Z

My LLVM checks were a bit too strict apparently

@bors r=saethlin

rust-bors · 2026-07-05T17:01:56Z

📌 Commit 6810f13 has been approved by saethlin

It is now in the queue for this repository.

🌲 The tree is currently closed for pull requests below priority 2. This pull request will be tested once the tree is reopened.

rustbot assigned adwinwhite Jun 24, 2026

folkertdev changed the title ~~add -Zforce_intrinsic_fallback flag~~ add -Zforce-intrinsic-fallback flag Jun 24, 2026

folkertdev force-pushed the force-intrinsic-fallback branch from 5c1f7b7 to c9f5a33 Compare June 24, 2026 19:52

scottmcm reviewed Jun 24, 2026

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Comment thread compiler/rustc_codegen_cranelift/src/intrinsics/mod.rs Outdated

rustbot assigned saethlin and unassigned adwinwhite Jun 27, 2026

folkertdev force-pushed the force-intrinsic-fallback branch from c9f5a33 to 5da2144 Compare June 28, 2026 19:55

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scottmcm reviewed Jun 28, 2026

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folkertdev force-pushed the force-intrinsic-fallback branch 3 times, most recently from 76314f6 to 69d7670 Compare June 28, 2026 21:05

rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jul 3, 2026

jhpratt mentioned this pull request Jul 3, 2026

Rollup of 18 pull requests #158753

Closed

folkertdev mentioned this pull request Jul 4, 2026

intrinsics: Add a fallback for non-const libm float functions #150946

Open

RalfJung mentioned this pull request Jul 4, 2026

remove -Zmiri-force-intrinsic-fallback rust-lang/miri#5159

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JonathanBrouwer mentioned this pull request Jul 5, 2026

Rollup of 20 pull requests #158798

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add -Zforce-intrinsic-fallback flag

6810f13

folkertdev force-pushed the force-intrinsic-fallback branch from 69d7670 to 6810f13 Compare July 5, 2026 13:33

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rust-bors Bot pushed a commit that referenced this pull request Jul 5, 2026

Auto merge of #158377 - folkertdev:force-intrinsic-fallback, r=<try>

65a628b

add `-Zforce-intrinsic-fallback` flag try-job: dist-i586-gnu-i586-i686-musl

rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jul 5, 2026

		@@ -0,0 +1,71 @@
		//@ add-minicore
		//@ assembly-output: emit-asm

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