#[target_feature(..)] no longer in effect in naked functions

[tmiasko]

Since #128004, target features enabled with #[target_feature(..)] are no longer in effect in naked functions. For example:

// --crate-typelib --target aarch64-unknown-linux-gnu
#![feature(naked_functions)]
use std::arch::{asm, naked_asm};

#[target_feature(enable = "aes")]
pub unsafe extern "C" fn f() {
    // ok
    asm!("aese.16b v0, v1");
}

#[target_feature(enable = "aes")]
#[naked]
pub unsafe extern "C" fn g() {
    // Before #128004: ok
    // After  #128004: error: instruction requires: aes
    naked_asm!("aese.16b v0, v1");
}

I haven't seen this discussed in the pull request. Opening the issue to give a chance to review the behaviour before stabilization.

cc @folkertdev, @Amanieu

[folkertdev]

This behavior is unfortunately expected, because of a bug in LLVM

#80608
llvm/llvm-project#61991

So basically LLVM just does not attach the target features to global assembly.

There are some target-specific fixes that use directives to enable the target features within the global assembly (e.g. this one for arm). That may work, but should not be needed in the future when LLVM, hopefully, fixes this issue.

[Amanieu]

This is a different issue than those you listed, which are related to global target feature (-Ctarget-feature). In the case of local target features we need to explicitly pass those into the assembly code since the naked function is never seen by LLVM.

[folkertdev]

ah, right. Then yeah we can at least do better (e.g. with .arch_extension for arm/aarch64), and I'll have to figure out what to do for other ISAs

[taiki-e]

I'll have to figure out what to do for other ISAs

• arm/aarch64: .arch_extension/.arch
• riscv32/riscv64: .option arch
• powerpc/powerpc64/s390x: .machine (powerpc, s390x)
• mips*: .set (https://sourceware.org/binutils/docs/as/MIPS-ISA.html, https://sourceware.org/binutils/docs/as/MIPS-ASE-Instruction-Generation-Overrides.html)
• m68k: .arch/.cpu

AFAIK, in x86/x86_64 all instructions available without such a directive. Also, in powerpc/powerpc64 LLVM currently actually ignores .machine and allows all instructions (see this fixme comment).

(Of these directives, I have actually used only aarch64 .arch_extension & .arch, arm .arch, riscv .option arch, and powerpc64 .machine.)

[folkertdev]

That's extremely helpful, thanks!

I think for the tier 2 targets, the ones I'm still missing are

• wasm: nothing jumps out to me in the LLVM asm parser, so does wasm just not have such a directive yet?
• loongarch: again nothing obvious in LLMV

Wasm does emit an error when you use inline assembly that requires a target feature https://godbolt.org/z/KaPqqM8Ez, while for loongarch I've not yet found an example where the target feature seems to matter.

And then there is a bunch of tier 3 targets which I did not all check but I'd assume most are not (close) to stable inline assembly, so we can skip those on a first pass.