Use of Custom datapath extension

[Jsbjr]

Arm has introduced Custom Datapath Extensions in the armv8 architecture.
The support for CDE was introduce in LLVM 11 [ARM] Release notes for the Custom Datapath Extension (CDE).

These are assembly instructions the founder of a device can use to implement some special operations like using a coprocessor.

In order to use then in C one would include the Arm header file "arm_cde.h" which defines some CDE intrinsics like this one:

uint32_t __arm_cx3(int coproc, uint32_t n, uint32_t m, uint32_t imm);

Using this will generate assembly code like this:

CX3 p1, r0, r0, r3, #8

I searched in the experimental core::arch::arm but the CDE instructions don't seem to be supported there.
I tried to use use inline assembly like this:

let result: u32;
unsafe {
     core::arch::asm!("cx3 p1, {}, {}, {}, #8", out(reg) result, in(reg) a, in(reg) b);
}

As one could expect the compiler doesn't like it. Here is the error:

error: invalid instruction
  |
note: instantiated into assembly here
 --> <inline asm>:1:2
  |
1 |     cx3 p1, r0, r1, r2, #8
  |     ^

Do you know if it is possible to use CDE in Rust? if Yes how do I use it?
If it is currently not possible, what would be the steps to add support for CDE in the Rust compiler?
Would core::arch::arm be the right place to add these?

[alice]

@Jsbjr

Do you know if it is possible to use CDE in Rust? if Yes how do I use it?

If nothing else works, you can add a C or assembly file to your project and call it from Rust via FFI.

[jschievink]

#![no_std]
#![no_main]

#[no_mangle]
extern fn main() {
    unsafe {
        core::arch::asm!(
            "CX3 p1, r0, r0, r3, #8"
        );
    }
}

Build with rustc --crate-type lib main.rs --target thumbv8m.main-none-eabi -Ctarget-feature=+cdecp1

Note that the cdecp1 feature is an LLVM target feature with no guarantees from Rust, so it may be changed or removed.

[Jsbjr]

Thanks Alice for your answer.
That would be work but these custom instruct are often there for performance reason. therefore adding some jumps to C code maybe not so good.
But still I will try it maybe with "lto" the compiler can inline these functions.

[Jsbjr]

Thanks jschievink,
I have tested but the assembly in the library is not what I expected.

CDP p1, #9, c3, c0, c0, #0

I will have to find out why