repr(simd) is unsound #44367
Description
The following should be discussed as part of an RFC for supporting portable vector types (repr(simd)) but the current behavior is unsound (playground):
#![feature(repr_simd)] #![feature(target_feature)] #![allow(non_camel_case_types)] // Given a SIMD vector type: #[derive(Debug)] #[repr(simd)] struct f32x8(f32, f32, f32, f32, f32, f32, f32, f32); // and the following two functions: #[target_feature = "+avx"] fn foo() -> f32x8 { f32x8(0.,1.,2.,3.,4.,5.,6.,7.) } // f32x8 will be a 256bit vector #[target_feature = "+sse3"] fn bar(arg: f32x8) { // f32x8 will be 2x128bit vectors println!("{:?} != f32x8(0, 1, 2, 3, 4, 5, 6, 7)", arg); // prints: f32x8(0, 1, 2, 3, 6, 0, 0, 0) != f32x8(0, 1, 2, 3, 4, 5, 6, 7) } // what are the semantics of the following when // executing on a machine that supports AVX? fn main() { bar(foo()); }Basically, those two objects of type f32x8 have a different layout, so foo and bar have a different ABI / calling convention. This can be introduced without target_feature, by compiling two crates with different --target-cpus and linking them, but target_feature was used here for simplicity.