std::simd scatter miscompilation #1439
Description
The example for Simd::scatter
#![feature(portable_simd)]
use std::simd::Simd;
fn main() {
let mut vec: Vec<i32> = vec![10, 11, 12, 13, 14, 15, 16, 17, 18];
let idxs = Simd::from_array([9, 3, 0, 0]); // Note the duplicate index.
let vals = Simd::from_array([-27, 82, -41, 124]);
vals.scatter(&mut vec, idxs); // two logical writes means the last wins.
assert_eq!(vec, vec![124, 11, 12, 82, 14, 15, 16, 17, 18]);
}results in
thread 'main' panicked at src/main.rs:11:5:
assertion `left == right` failed
left: [-1, -1, 12, -1, -1, 15, 16, 17, 18]
right: [124, 11, 12, 82, 14, 15, 16, 17, 18]
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
free(): invalid pointer
[1] 483914 IOT instruction (core dumped) cargo run
Sort of a minimal reproducer is the following code:
#![feature(portable_simd)]
use std::simd::Simd;
fn main() {
let mut vec: Vec<u8> = vec![0];
let idxs = Simd::from_array([0]);
let vals = Simd::from_array([1]);
vals.scatter(&mut vec, idxs);
assert_eq!(vec, vec![1]);
}which results in
thread 'main' panicked at src/main.rs:11:5:
assertion `left == right` failed
left: [255]
right: [1]
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
(Interestingly, the second example does not cause free(): invalid pointer error.)
Running the examples with standard LLVM backend works as expected.
$ rustc --version
rustc 1.77.0-nightly (d6d7a9386 2023-12-22)
If I can provide more information I will happily do so. I tried to get generated assembly using cargo-show-asm, but I got
error: Unknown option `-x86-asm-syntax`
Looking at the actual outputs, it seems that the problem manifests itself by scatter putting value -1 at index i and i + 1 for every (valid) i from the idxs vector.
// Only 0 and 3 indices are valid in the example.
let idxs = Simd::from_array([9, 3, 0, 0]);
// index: 0, 1, 2, 3, 4, 5, 6, 7, 8
// left: [-1, -1, 12, -1, -1, 15, 16, 17, 18]I would be interested in trying to fix this bug if I get some pointers to where to start. I have some basic knowledge of compilers in general and cranelift in particular.