Support using const pointers in asm `const` operand #138618

nbdd0121 · 2025-03-17T18:07:31Z

Implements #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.

rustbot · 2025-03-17T18:07:35Z

r? @fmease

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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

compiler/rustc_monomorphize/src/collector.rs

compiler-errors · 2025-04-13T16:46:50Z

compiler/rustc_codegen_ssa/src/traits/asm.rs

@@ -25,9 +26,12 @@ pub enum InlineAsmOperandRef<'tcx, B: BackendTypes + ?Sized> {
        in_value: OperandRef<'tcx, B::Value>,
        out_place: Option<PlaceRef<'tcx, B::Value>>,
    },
-    Const {
+    Interpolate {


Please add more doc comment explaining the significance of the difference between Interpolate and Const for backends. It begs the question for why we ever turn const operands into strings, for example.

Added some explaination. Technically we should be able to use Const for integers, although I am a bit conservative here and don't want to change how existing things work.

This might also be useful to have in the future if we decided to add interpolate "some CTFE string to be interpolated" new type of operand.

compiler/rustc_codegen_ssa/src/traits/asm.rs

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.

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

workingjubilee · 2025-03-17T20:12:55Z

tests/assembly/asm/x86-types.rs

+// and on x86 PIC symbol can't be constant.
+// x86_64-LABEL: const_ptr:
+// x86_64: #APP
+// x86_64: mov al, byte ptr [{{.*}}anon{{.*}}]


// x86_64: mov al, byte ptr [{{.*}}anon{{.*}}]

What was the expansion you observed?

mov al, byte ptr [.Lanon.2e8c0013b01edcd3779e8174c2338a00.0]

compiler/rustc_codegen_llvm/src/asm.rs

compiler-errors · 2025-07-10T21:10:04Z

LGTM

@bors r+

bors · 2025-07-10T21:10:07Z

📌 Commit 81a4b3e has been approved by compiler-errors

It is now in the queue for this repository.

…-errors Support using const pointers in asm `const` operand Implements rust-lang#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.

tgross35 · 2025-07-10T21:41:30Z

@bors r-

486ffda changed the signature for const_alloc_to_llvm, see the failure at https://github.com/rust-lang/rust/actions/runs/16206384671/job/45757598329?pr=143753

bors · 2025-07-10T23:31:17Z

☔ The latest upstream changes (presumably #143746) made this pull request unmergeable. Please resolve the merge conflicts.

This is used for string interpolation currently, so rename it as so. The name `Const` will be used to denote a general CTFE constant that can either be integer or pointer.

This is intended for supporting passing arbitrary CTFE const into inline assembly.

`global_asm!` themselves don't need symbol names; they currently only have a symbol name during mono collecion to identify them to partitioning algorithm. However it will have shims generated under it which will need unique symbol names. The name themselves ultimately doesn't matter, so they're generated like other shim instances.

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

compiler-errors · 2025-07-11T15:26:35Z

@bors r+ rollup

bors · 2025-07-11T15:26:37Z

📌 Commit 8da54fc has been approved by compiler-errors

It is now in the queue for this repository.

compiler-errors · 2025-07-11T15:26:45Z

@bors rollup-

bors · 2025-07-11T17:42:06Z

⌛ Testing commit 8da54fc with merge 8e7515b...

Support using const pointers in asm `const` operand Implements #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.

rust-log-analyzer · 2025-07-11T18:36:33Z

The job dist-i586-gnu-i586-i686-musl failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

---- [ui] tests/ui/asm/const-refs-to-static.rs stdout ----

error: test compilation failed although it shouldn't!
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/asm/const-refs-to-static.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2" "--target=i586-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/asm/const-refs-to-static" "-A" "unused" "-A" "internal_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/i586-unknown-linux-gnu/native/rust-test-helpers" "-Clinker=i586-unknown-linux-gnu-gcc"
stdout: none
--- stderr -------------------------------
error: invalid operand for inline asm constraint 'i'
##[error]  --> /checkout/tests/ui/asm/const-refs-to-static.rs:17:20
   |
LL |     unsafe { asm!("/* {} */", const addr_of!(FOO)) };
   |                    ^^^^^^^^

error: aborting due to 1 previous error
------------------------------------------

bors · 2025-07-11T18:37:10Z

💔 Test failed - checks-actions

nbdd0121 · 2025-07-11T20:46:58Z

Looks like the i constraint doesn't work for GOT-rebased relocation, which x86 use for PIE and we enable PIE by default. We need to either:

Say this works only with static relocation-model on x86, or
Generate symbol + offset ourselves, like global_asm implementation.

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Stabilize tooling needed by Rust for Linux rust-lang/rust-project-goals#116

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

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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

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Rollup of 8 pull requests #143753

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bors 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-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Jul 10, 2025

nbdd0121 added 6 commits July 11, 2025 14:04

Rename *AsmOperandRef::Const to Interpolate

c5fc208

This is used for string interpolation currently, so rename it as so. The name `Const` will be used to denote a general CTFE constant that can either be integer or pointer.

Add InlineAsmOperandRef::Const

34658b8

This is intended for supporting passing arbitrary CTFE const into inline assembly.

Implement asm_const_ptr for inline asm

83e7f93

Collect constants within global_asm! in collector

d7235e6

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

Implement asm_const_ptr for global_asm and naked_asm

8da54fc

nbdd0121 force-pushed the asm_const_ptr branch from 81a4b3e to 8da54fc Compare July 11, 2025 13:24

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bors mentioned this pull request Jul 11, 2025

Rollup of 8 pull requests #143766

Merged

bors added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Jul 11, 2025

Support using const pointers in asm const operand #138618

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Support using const pointers in asm `const` operand #138618

Support using const pointers in asm `const` operand #138618

nbdd0121 commented Mar 17, 2025 •

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