Support using const pointers in asm `const` operand by nbdd0121 · Pull Request #138618 · rust-lang/rust

nbdd0121 · 2025-03-17T18:07:31Z

Implements RFC#3848 with tracking issue #128464

This adds support of const pointers for asm const in addition to plain integers.

The inline asm! support is implemented using i constraint, and the global_asm! and naked_asm! support is implemented by inserting symbol + offset and make symbol compiler-used. For unnamed consts, it will create additional internal & hidden symbols so that they can be referenced by global_asm.

~~The feature is also implemented for GCC backend but it's untested.~~ Tested now.

fmease · 2025-03-17T18:09:34Z

r? codegen

compiler-errors · 2025-03-18T04:25:43Z

r? compiler-errors

rustbot · 2025-04-10T14:59:14Z

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo, @GuillaumeGomez

bors · 2025-04-29T06:25:48Z

☔ The latest upstream changes (presumably #140415) made this pull request unmergeable. Please resolve the merge conflicts.

traviscross · 2025-05-04T09:26:42Z

@rustbot author

rustbot · 2025-05-04T09:26:46Z

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

rustbot · 2025-05-13T10:00:16Z

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

bors · 2025-05-28T05:09:25Z

☔ The latest upstream changes (presumably #141668) made this pull request unmergeable. Please resolve the merge conflicts.

bjorn3 · 2026-04-30T13:14:18Z

+                                        // current codegen unit
+                                        tcx.symbol_name(instance)
+                                    }
+                                    GlobalAlloc::Static(def_id) => {


What is the difference between GlobalAsmOperandRef::Const with a static and GlobalAsmOperandRef::SymStatic?

View changes since the review

See #138618 (comment)

#[thread_local] works with sym STATIC today, but it cannot be referenced in CTFE. I would actually be in favour of removing that feature.

I would tend to agree with removing #[thread_local] support for sym STATIC. On some platforms (cough Windows cough) #[thread_local] accesses go through a function shim to ensure cross-dylib accesses work, so sym STATIC would probably already be broken for those platforms.

Rename GlobalAsmOperandRef::SymStatic to SymThreadLocalStatic

nbdd0121 · 2026-06-10T15:21:52Z

I think the approach I use in rustc_codegen_gcc to avoid symbol name collision when a const is generated multiple times doesn't work if multiple CGUs decide to codegen the same symbol for a promoted constant.

The proper fix I think is to let backend mint new symbol names, rather than construct a fake Instance and use its symbol name like the current approach. This does mean that I need to create a new mechanism for backend to mint new global symbol names.

rustbot · 2026-06-16T12:01:18Z

These commits modify the Cargo.lock file. Unintentional changes to Cargo.lock can be introduced when switching branches and rebasing PRs.

If this was unintentional then you should revert the changes before this PR is merged.
Otherwise, you can ignore this comment.

This is currently a no-op, but will be useful when const in `global_asm!` can be pointers.

Currently global_asm already have symbol names when using v0 scheme, this makes them obtain symbols with legacy scheme too.

This gives the asm-const code the basic ability to deal wiht pointer and provenances, which lays the ground work for asm_const_ptr. Note that `SymStatic` is not fully removed, a specialized is kept and renamed as `SymThreadLocalStatic`, for `#[thread_local]` statics where CTFE does not support naming. The `#[thread_local]` is unstable feature and it's not clear if we want to support this in `sym`, but removal of it should be a separate PR.

With the previous commit, now we can see there are some code duplication for the handling of `GlobalAlloc` inside backends. Do some clean up to unify them.

CTFE pointers created via type ID, `without_provenance` or pointers to const ZSTs can now be codegenned with all 3 backends. These pointers are generated in the same way as integers.

The backend now fully supports codegen of const pointers, remove the block inside typeck behind a new feature gate. Tests are also added.

rustbot · 2026-06-16T12:24:10Z

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.

nbdd0121 · 2026-06-16T12:24:39Z

I've reworked how symbol name is generated and stop using the mangled name of the const, due to the outlined symbol name conflict issue above. This also resolves the concern #138618 (comment).

The new approach generates an unique symbol name prefix for each CGU, and then the codegen backends can use this symbol prefix to create helper symbols that they need internally. I think this can be useful longer term as well. The generated prefix is demangled to something like crate_name::{shim:cgu#1} for non-incremental build and crate_name::{shim:cgu#0}::<"incremental CGU name"> for incremental build.

EDIT: Due to two pushes the range-diff above is not the full picture, this is the full range diff.

Darksonn · 2026-06-16T12:28:15Z

Is the cgu index still included in the symbol name when the number of cgus is 1?

nbdd0121 · 2026-06-16T12:32:53Z

It'll still demangle to things like crate_name::{shim:cgu#0} even with one CGU. However, v0 mangling will omit disambiguator completely if it is of value 0, so the resulting symbol name is just going to be something like _RNSCs<stable crate ID>_10crate_name3cgu

nbdd0121 · 2026-06-16T16:32:15Z

@rustbot ready

rustbot assigned fmease Mar 17, 2025

This comment was marked as outdated.

Sign in to view

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. labels Mar 17, 2025

rustbot assigned workingjubilee and unassigned fmease Mar 17, 2025

This comment has been minimized.

Sign in to view

rustbot assigned compiler-errors and unassigned workingjubilee Mar 18, 2025

nikomatsakis mentioned this pull request Mar 26, 2025

Getting Rust for Linux into stable Rust: language features rust-lang/rust-project-goals#116

Closed

53 tasks

ojeda mentioned this pull request Apr 9, 2025

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

Open

52 tasks

nbdd0121 force-pushed the asm_const_ptr branch from ffba8e8 to 92c0cc4 Compare April 10, 2025 10:05

nbdd0121 marked this pull request as ready for review April 10, 2025 14:59

tmiasko reviewed Apr 10, 2025

View reviewed changes

Comment thread compiler/rustc_monomorphize/src/collector.rs

compiler-errors reviewed Apr 13, 2025

View reviewed changes

Comment thread compiler/rustc_codegen_ssa/src/traits/asm.rs Outdated

Comment thread compiler/rustc_codegen_ssa/src/traits/asm.rs Outdated

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 May 4, 2025

nbdd0121 force-pushed the asm_const_ptr branch from 92c0cc4 to c7a2301 Compare May 13, 2025 08:27

This comment has been minimized.

Sign in to view

nbdd0121 force-pushed the asm_const_ptr branch from c7a2301 to 4240004 Compare May 13, 2025 10:00

workingjubilee reviewed Jun 4, 2025

View reviewed changes

Comment thread tests/ui/asm/const-refs-to-static.rs

Comment thread tests/assembly-llvm/asm/x86-types.rs Outdated

oli-obk reviewed Jul 9, 2025

View reviewed changes

Comment thread compiler/rustc_codegen_llvm/src/asm.rs Outdated

rustbot assigned bjorn3 and unassigned saethlin Apr 25, 2026

This comment has been minimized.

Sign in to view

bjorn3 requested changes Apr 30, 2026

View reviewed changes

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 Apr 30, 2026

nbdd0121 force-pushed the asm_const_ptr branch from 7c877c9 to 8a3bc3f Compare June 16, 2026 12:01

This comment has been minimized.

Sign in to view

nbdd0121 added 12 commits June 16, 2026 13:02

Collect constants within global_asm! in collector

c91f42f

This is currently a no-op, but will be useful when const in `global_asm!` can be pointers.

Add FnDef/Closure -> FnPtr coercion for inline asm const operand

2ae428c

Give global_asm symbol names

341828e

Currently global_asm already have symbol names when using v0 scheme, this makes them obtain symbols with legacy scheme too.

Delay stringification of const to backend

e325d63

Unify handling of GlobalAlloc inside backend

2c8952e

With the previous commit, now we can see there are some code duplication for the handling of `GlobalAlloc` inside backends. Do some clean up to unify them.

Handle pointers with offset for asm const

85bc9f8

Support codegen of asm const pointers without provenance

49ab24e

CTFE pointers created via type ID, `without_provenance` or pointers to const ZSTs can now be codegenned with all 3 backends. These pointers are generated in the same way as integers.

Give codegen units symbol names that backend can use

c280357

Generate unique symbol names if const pointers refer to promoted static

b7b5ea7

Implement asm_const_ptr feature

87ca5d0

The backend now fully supports codegen of const pointers, remove the block inside typeck behind a new feature gate. Tests are also added.

Add test if asm const are MIR inlined and monomorphized twice

ff3f3c0

nbdd0121 force-pushed the asm_const_ptr branch from 8a3bc3f to ff3f3c0 Compare June 16, 2026 12:24

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 Jun 16, 2026

Uh oh!

Conversation

nbdd0121 commented Mar 17, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

This comment was marked as outdated.

fmease commented Mar 17, 2025

Uh oh!

This comment has been minimized.

compiler-errors commented Mar 18, 2025

Uh oh!

rustbot commented Apr 10, 2025

Uh oh!

Uh oh!

Uh oh!

Uh oh!

bors commented Apr 29, 2025

Uh oh!

traviscross commented May 4, 2025

Uh oh!

rustbot commented May 4, 2025

Uh oh!

This comment has been minimized.

rustbot commented May 13, 2025

Uh oh!

bors commented May 28, 2025

Uh oh!

Uh oh!

Uh oh!

Uh oh!

This comment has been minimized.

Uh oh!

bjorn3 Apr 30, 2026 • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

nbdd0121 Apr 30, 2026

Choose a reason for hiding this comment

Uh oh!

bjorn3 Apr 30, 2026

Choose a reason for hiding this comment

Uh oh!

nbdd0121 Jun 16, 2026

Choose a reason for hiding this comment

Uh oh!

Uh oh!

nbdd0121 commented Jun 10, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

rustbot commented Jun 16, 2026

Uh oh!

This comment has been minimized.

rustbot commented Jun 16, 2026

Uh oh!

nbdd0121 commented Jun 16, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

Darksonn commented Jun 16, 2026

Uh oh!

nbdd0121 commented Jun 16, 2026

Uh oh!

nbdd0121 commented Jun 16, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

19 participants

nbdd0121 commented Mar 17, 2025 •

edited

Loading

bjorn3 Apr 30, 2026 •

edited by rustbot

Loading

nbdd0121 commented Jun 10, 2026 •

edited

Loading

nbdd0121 commented Jun 16, 2026 •

edited

Loading