Inline Assembly in Rust

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When diving into the depths of Rust, you might come across situations where you'll want to squeeze every bit of performance out of your code. One way to do this is by using inline assembly. Inline assembly allows you to write assembly code directly within your Rust source code, blending the power of low-level assembly with the safety and convenience of Rust.

Rust Inline Assembly Syntax

Rust's inline assembly syntax is borrowed from LLVM's integrated assembler. It is embedded in the Rust code using the asm! macro. The syntax looks like this:

asm!(assembly_template, output_operands, input_operands, clobbers, options);

Let's break that down:

  • assembly_template: A string literal containing the assembly code to be executed.
  • output_operands: A list of output operands, separated by commas.
  • input_operands: A list of input operands, separated by commas.
  • clobbers: A list of clobbered registers, separated by commas.
  • options: A list of options that affect the behavior of the asm! macro, separated by commas.

Here's a simple example that adds two numbers using inline assembly:

fn main() { let a: u32 = 42; let b: u32 = 23; let mut result: u32; unsafe { asm!("add {}, {}, {}", out(reg) result, in(reg) a, in(reg) b); } println!("Result: {}", result); // Output: Result: 65 }

Note that the asm! macro is enclosed within an unsafe block. This is because the Rust compiler cannot guarantee safety when working with inline assembly. Always make sure you carefully review the assembly code and understand the implications of using it.


In the example above, we used out(reg), in(reg), and placeholders {} in the assembly template. These are called operands. They are used to specify the inputs and outputs of the inline assembly block.

  • out(reg) var: This operand marks var as an output variable, and it must be mutable. The value will be stored in a register.
  • in(reg) var: This operand marks var as an input variable. The value will be passed as an argument into a register.
  • {}: Placeholders represent the variables in the assembly code. They are filled in by the order they appear in the output and input operands.

Clobbers and Options

Clobbers are used to inform the compiler about registers that will be modified by the inline assembly code. This allows the compiler to avoid using those registers for other purposes during the assembly block. Clobbers are specified using the syntax lateout(reg) or inlateout(reg).

Options allow you to fine-tune the behavior of the asm! macro. Some common options include:

  • pure: Indicates that the assembly code has no side effects and depends only on its inputs.
  • nomem: Specifies that the assembly code does not read or write memory.
  • readonly: Indicates that the assembly code only reads memory, but does not modify it.
  • preserves_flags: Tells the compiler that the assembly code does not modify the CPU flags.

For a more comprehensive list of options and their syntax, refer to the Rust inline assembly documentation.


Inline assembly can provide powerful optimization opportunities in Rust when used with care. It's essential to understand the risks and responsibilities that come with using low-level assembly code. Always use unsafe blocks to enclose inline assembly and thoroughly review your code to ensure it meets Rust's safety guarantees. With this knowledge, you'll be able to harness the full potential of Rust and inline assembly for your high-performance projects.


What is inline assembly in Rust?

Inline assembly in Rust allows you to embed assembly instructions directly into your Rust code to improve performance and optimize low-level operations. It enables the programmer to have direct control over the hardware, allowing for fine-tuned optimizations and greater flexibility.

How can I use inline assembly in my Rust program?

To use inline assembly in Rust, you'll need to utilize the asm! macro. Here's a simple example:

#![feature(asm)] fn main() { let x: u64; unsafe { asm!("mov {}, 42", out(reg) x); } println!("The value of x is: {}", x); }

In this example, we use the asm! macro to move the constant value "42" into the variable "x". Remember that using inline assembly is unsafe, so it must be wrapped in an unsafe block.

What are the common use cases for inline assembly in Rust?

Inline assembly is particularly useful in situations where you need precise control over hardware or want to optimize performance-critical sections of your code. Some common use cases include:

  • Low-level hardware control
  • Implementing specific processor instructions
  • Performance-critical algorithms
  • Cryptographic operations
  • Embedded systems programming

Are there any risks or drawbacks when using inline assembly in Rust?

While inline assembly offers many benefits, it also comes with some risks and drawbacks:

  • Using inline assembly can make your code less portable, as it may rely on specific hardware or processor features.
  • It can make your code harder to read, understand, and maintain.
  • Inline assembly is inherently unsafe, meaning you'll need to take extra care to avoid introducing security vulnerabilities or other issues.
  • It may bypass some of the safety features and optimizations provided by the Rust compiler. Always weigh the pros and cons before deciding to use inline assembly in your Rust project.

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