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rustinline-assemblyarm64

How do I use a constant in Rust's inline assembly macro?

I'm porting some legacy assembly code to Rust and I need to call it through the asm! macro. However, the assembly code depends on some constants stored in a C header file. I'd like to keep it similar, define the constants in Rust and have the names of the constants in the asm macro.

Legacy C header:

#define HCR_VALUE 0xffff0000

Legacy ASM file:

.func
...
ldr    x0, =HCR_VALUE
...

Rust code:

pub const HCR_VALUE: u32 = 0xffff0000;

unsafe { asm!("ldr x0, HCR_VALUE":::"x0"); }

Building the application ends up with a linker error:

lld-link: error: undefined symbol: HCR_VALUE
Solution

  • You need to pass the constant with a suitable constraint, like this:

    unsafe { asm!("ldr x0, =${0:c}" : : "i" (HCR_VALUE) : "x0"); }

    The right constraint depends on the architecture; on RISC CPUs, not all constants can be represented as immediate values. So you may have to use a register constraint instead and have LLVM materialize the constant there.