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2843-llvm-asm.md

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Summary

Deprecate the existing asm! macro and provide an identical one called llvm_asm!. The feature gate is also renamed from asm to llvm_asm.

Unlike asm!, llvm_asm! is not intended to ever become stable.

Motivation

This change frees up the asm! macro so that it can be used for the new asm! macro designed by the inline asm project group while giving existing users of asm! an easy way to keep their code working.

It may also be useful to have an inline asm implementation available (on nightly) for architectures that the new asm! macro does not support yet.

Guide-level explanation

The Rust team is currently in the process of redesigning the asm! macro. You should replace all uses of asm! with llvm_asm! in your code to avoid breakage when the new asm! macro is implemented.

Reference-level explanation

All references to asm! inside the compiler will be changed to refer to llvm_asm! instead. asm! will become a simple (deprecated) macro_rules! which redirects to llvm_asm!. The deprecation warning will advise users that the semantics of asm! will change in the future and invite them to use llvm_asm! instead. The llvm_asm! macro will be guarded by the llvm_asm feature gate.

Drawbacks

This change may require people to change their code twice: first to llvm_asm!, and then to the new asm! macro once it is implemented.

Rationale and alternatives

We could skip the deprecation period and perform the renaming at the same time the new asm! macro is implemented. However this is guaranteed to break a lot of code using nightly Rust at once without any transition period.

Prior art

The D programming language also support 2 forms of inline assembly. The first one provides an embedded DSL for inline assembly, which allows direct access to variables in scope and does not require the use of clobbers, but is only available on x86 and x86_64. The second one is a raw interface to LLVM's internal inline assembly syntax, which is available on all architectures but only on the LDC backend.

Unresolved questions

None

Future possibilities

When the new asm! macro is implemented it will replace the current one. This will break anyone who has not yet transitioned their code to llvm_asm!. No silent miscompilations are expected since the operand separator will be changed from : to ,, which will guarantee that any existing asm! invocations will fail with a syntax error with the new asm! macro.