| Issue |
146866
|
| Summary |
aarch64: `dit` target feature cannot be reliably en/disabled from assembly
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
folkertdev
|
The rust [graviola](https://github.com/ctz/graviola/blob/3fe81f602eaf454e5e7fde18f6978913fabd18d5/graviola/src/low/aarch64/cpu.rs#L220-L229) crate defines this function
```rust
#[target_feature(enable = "dit")]
unsafe fn write(on: u32) {
if on > 0 {
// SAFETY: `msr DIT, _` is defined only if `dit` cpu feature is supported
unsafe { core::arch::asm!("msr DIT, #1") }
} else {
// SAFETY: `msr DIT, _` is defined only if `dit` cpu feature is supported
unsafe { core::arch::asm!("msr DIT, #0") }
}
}
```
The `target_feature` annotation makes the `msr DIT` instructions work inside the body. In a function without that annotation, LLVM would error.
But, it currently appears to be impossible to write this function reliably in inline assembly. The standard mechanism doesn't work:
```rust
std::arch::global_asm!(".arch_extension dit");
```
errors with
```
error: unsupported architectural extension: dit
|
note: instantiated into assembly here
--> <inline asm>:1:17
|
1 | .arch_extension dit
| ^
```
One way we've found to make the code compile is to use `.arch`
```rust
#[unsafe(naked)]
unsafe extern "C" fn write() {
core::arch::naked_asm!(
".arch armv8.4-a",
"msr DIT, #0",
"ret"
)
}
```
However `.arch` has no stack-like mechanism to "pop" a state and return to a previous one. Hence, `.arch` cannot be used reliably. Actually even `.arch_extension` doesn't have such a mechanism. Many other targets do.
So, can LLVM provide a reliable way to toggle this target feature from inline assembly, preferably something like what riscv and powerpc do with a stack of features. s390x recently added support for a similar mechanism https://github.com/llvm/llvm-project/issues/129053.
see also https://github.com/rust-lang/rust/pull/137720 for examples of how other targets deal with toggling target features in assembly. This concrete problem came up in https://github.com/rust-lang/rustc_codegen_cranelift/issues/1586
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