Inline Assembly

Rust-GPU has support for inline SPIR-V assembly. In addition the backend provides several conveniences for writing inline assembly that are documented below. For more information on specific instruction behaviour and syntax, please refer to the SPIR-V specification.

Basic syntax & usage.

You can write inline assembly using the new asm! macro available with the asm feature on nightly. Refer to the Rust unstable book for more information on how to use the macro.

Non-ID arguments are written as-is, e.g.

#![allow(unused)]
fn main() {
asm! {
    "OpCapability DerivativeControl"
}
}

ID based arguments are prefixed with % and their name. Result<id>s accessed with a = and a ID on the left hand side of the expression. E.g.

#![allow(unused)]
fn main() {
let vector = spirv_std::glam::Vec2::new(1.0, 0.0);
let mut result = f32::default();

asm! {
    "%vector = OpLoad _ {vector}",
    "%element = OpVectorExtractDynamic _ %vector {index}",
    "OpStore {element} %element",
    vector = in(reg) &vector,
    index = in(reg) index,
    element = in(reg) &mut result
}
}

asm! only accepts integers, floats, SIMD vectors, pointers and function pointers as input variables. However you can have the pointer point to a generic variable, so you can write generic assembly code like so.

#![allow(unused)]
fn main() {
use spirv_std::{scalar::Scalar, vector::Vector};

// This fn is available as `spirv_std::arch::vector_extract_dynamic`
pub unsafe fn vector_extract_dynamic<T: Scalar, V: Vector<T>>(vector: V, index: usize) -> T {
    let mut result = T::default();

    asm! {
        "%vector = OpLoad _ {vector}",
        "%element = OpVectorExtractDynamic _ %vector {index}",
        "OpStore {element} %element",
        vector = in(reg) &vector,
        index = in(reg) index,
        element = in(reg) &mut result
    }

    result
}
}

Additional syntax