Read value of ebx register in Rust with inline assembly ("rbx is used internally by LLVM and cannot be used as an operand for inline asm")

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I'm writing Rust code and need to get the value that is currently stored inside "ebx" register (x86).

My code [0] looks like this:

#![feature(asm)]

fn main() {
    let ebx: u32;
    unsafe { asm!("", out("ebx") ebx) };
    println!("current value of register ebx is: {}", ebx);
}

The error code from rustc 1.54.0-nightly is:

invalid register `ebx`: rbx is used internally by LLVM and cannot be used as an operand for inline asm

The only working workaround I found is the legacy way:

unsafe {
    llvm_asm!("" : "={ebx}"(ebx) : :);
}

Is there a way to achieve the desired functionality with the new asm! macro?

PS: Due to my research I came across this: https://github.com/rust-lang/rust/pull/84658. But it still doesn't offer a nice and simple solution.

[0] https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=8d0a322236cce1efbebedccc40ddbf71

rust
1 Answers

You can move it to another register.

#![feature(asm)]

fn main() {
    let ebx: u32;
    unsafe { asm!("mov {:e}, ebx", out(reg) ebx) };
    println!("current value of register ebx is: {}", ebx);
}
user_27023910
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