Add an optional `llvm = ` argument to rustc_intrinsic. by veluca93 · Pull Request #164544 · rust-lang/rust

veluca93 · 2026-10-18T23:00:40Z

This is meant for usage in core::arch, to define the platform-specific LLVM intrinsics that correspond to each intrinsic.

LLM usage: I used an LLM to point me at places in the code from which to start the implementation, but I did not generate any code. The main non-trivial input from the LLM was to prevent inlining of callees annotated with rustc_llvm_intrinsic. Code has been since re-designed following @bjorn3's and @RalfJung's proposals so there's no trace of any LLM input left :-)

r? RalfJ

rustbot · 2026-10-18T23:00:43Z

Some changes occurred in compiler/rustc_attr_ir

cc @jdonszelmann, @JonathanBrouwer

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann, @JonathanBrouwer

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

Some changes occurred in compiler/rustc_passes/src/check_attr.rs

cc @jdonszelmann, @JonathanBrouwer

rustbot · 2026-10-18T23:00:46Z

Error: Failed to set assignee to RalfJ: invalid assignee

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veluca93 · 2026-10-18T23:01:28Z

r? RalfJung

rustbot · 2026-10-18T23:01:32Z

RalfJung is not on the review rotation at the moment.
They may take a while to respond.

tgross35 · 2026-10-18T23:21:04Z

Cc @folkertdev

bjorn3 · 2026-10-19T08:19:04Z

Can you check that all places in the compiler that check for extern "llvm-intrinsic" have been adjusted as appropriate?

veluca93 · 2026-10-19T22:27:46Z

Can you check that all places in the compiler that check for extern "llvm-intrinsic" have been adjusted as appropriate?

I gave a look, and I think so, but I am definitely not knowledgeable enough of compiler internals to be sure.

rustbot · 2026-10-28T12:15:53Z

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

Some changes occurred to constck

cc @fee1-dead

Some changes occurred to the CTFE machinery

cc @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

rustbot · 2026-10-28T19:45:14Z

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

cc @rust-lang/clippy

rustbot · 2026-11-06T08:05:35Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

veluca93 · 2026-11-06T10:18:14Z

@rustbot ready

RalfJung · 2026-11-06T11:21:38Z

+#[rustc_intrinsic(llvm = "llvm.fshl.i64")]
+fn fshl(a: i64, b: i64, sh: i64) -> i64 {
+    (((a as i128) << 64) | (b as i128) >> sh) as i64
+}
+
+fn main() {
+    const { fshl(1, 2, 3) };
+    //~^ ERROR: cannot call non-const function `fshl` in constants [E0015]
+}


Not sure if this is worth a test, it's just standard const checking. Would be very surprising if we allowed calling a function that's not marked as const.

More interesting is ensuring that const stability checks work properly. You could extend tests/ui/consts/const-unstable-intrinsic.rs to also check backend intrinsics.

View changes since the review

Removed, and added more tests to the other file.

RalfJung · 2026-11-06T14:15:49Z

Btw the PR title and description seem to still refer to an earlier version of this PR.

rustbot · 2026-11-06T14:34:38Z

miri is developed in its own repository. If the Miri part of this change can be broken out, consider making this change to rust-lang/miri instead. However, if Miri needs adjusting for rustc changes, just ignore this message.

cc @rust-lang/miri

rust-bors · 2026-11-07T16:04:16Z

☔ The latest upstream changes (presumably #165588) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.

rustbot assigned RalfJung Oct 18, 2026

tgross35 reviewed Oct 18, 2026

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Comment thread tests/ui/intrinsics/rustc-llvm-intrinsic.rs Resolved

veluca93 force-pushed the rustc_llvm_intrinsic branch from 39047e6 to a0ad71b Compare October 19, 2026 07:09

bjorn3 reviewed Oct 19, 2026

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Comment thread compiler/rustc_codegen_ssa/src/mir/block.rs Resolved

veluca93 marked this pull request as draft October 23, 2026 15:39

rustbot 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 Oct 23, 2026

veluca93 force-pushed the rustc_llvm_intrinsic branch 2 times, most recently from ebce41f to 90416b5 Compare October 28, 2026 12:15

veluca93 marked this pull request as ready for review October 28, 2026 12:15

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Oct 28, 2026

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bjorn3 reviewed Oct 28, 2026

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Comment thread compiler/rustc_public_bridge/src/context/impls.rs Resolved

veluca93 force-pushed the rustc_llvm_intrinsic branch from 90416b5 to c906c98 Compare October 28, 2026 19:45

rustbot added the T-clippy Relevant to the Clippy team. label Oct 28, 2026

tgross35 reviewed Oct 28, 2026

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Comment thread tests/ui/intrinsics/rustc-llvm-intrinsic.rs Resolved

rustbot unassigned folkertdev Nov 6, 2026

rustbot 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 Nov 6, 2026

veluca93 force-pushed the rustc_llvm_intrinsic branch from e294fce to e18a91c Compare November 6, 2026 10:17

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Nov 6, 2026

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veluca93 force-pushed the rustc_llvm_intrinsic branch from e18a91c to f28292b Compare November 6, 2026 10:26

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veluca93 force-pushed the rustc_llvm_intrinsic branch from f28292b to 40c3e33 Compare November 6, 2026 10:35

RalfJung reviewed Nov 6, 2026

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Comment thread tests/ui/intrinsics/rustc-llvm-intrinsic.rs Resolved

Comment thread tests/ui/intrinsics/rustc-llvm-intrinsic.rs Resolved

veluca93 force-pushed the rustc_llvm_intrinsic branch from 40c3e33 to 62cfa98 Compare November 6, 2026 14:34

veluca93 changed the title ~~Add a rustc_llvm_intrinsic attribute.~~ Add an optional llvm = argument to rustc_intrinsic. Nov 6, 2026

veluca93 force-pushed the rustc_llvm_intrinsic branch from 62cfa98 to e6d0471 Compare November 6, 2026 15:06