[hexagon] "Unhandled HVX operation" in portable_simd #182520
Description
LLVM crashes with "Unhandled HVX operation" when truncating a sub-HVX-width integer vector to a boolean vector in HVX 128-byte mode (e.g., <64 x i8> to <64 x i1>).
This issue was discovered when compiling Rust code using SIMD vector operations targeting Hexagon with HVX. The Rust code performed SIMD comparisons that produce boolean masks, which are then converted to bitmasks.
A minimal Rust reproducer:
#![feature(repr_simd, platform_intrinsics)]
#![no_std]
#![no_main]
use core::panic::PanicInfo;
#[panic_handler]
fn panic(_: &PanicInfo) -> ! { loop {} }
#[repr(simd)]
#[derive(Copy, Clone)]
pub struct u8x64([u8; 64]);
#[repr(simd)]
#[derive(Copy, Clone)]
pub struct i8x64([i8; 64]);
extern "platform-intrinsic" {
fn simd_lt<T, U>(a: T, b: T) -> U;
fn simd_bitmask<T, U>(a: T) -> U;
}
// This function triggers the crash
#[no_mangle]
pub fn compare_and_bitmask(a: u8x64, b: u8x64) -> u64 {
unsafe {
// Compare a < b, producing i8x64 of -1 or 0
let mask: i8x64 = simd_lt(a, b);
// Convert to bitmask - triggers trunc <64 x i8> to <64 x i1>
simd_bitmask(mask)
}
}Build with cargo (requires nightly for -Zbuild-std):
# Setup
mkdir hvx_repro && cd hvx_repro
cargo init --lib
cp hvx_trunc_rust_repro.rs src/lib.rs
echo '[lib]
crate-type = ["staticlib"]' >> Cargo.toml
# Create .cargo/config.toml
mkdir -p .cargo
echo '[target.hexagon-unknown-linux-musl]
rustflags = ["-C", "target-feature=+hvx,+hvx-length128b"]' > .cargo/config.toml
# Build - this triggers the crash
cargo +nightly build --target=hexagon-unknown-linux-musl \
-Zbuild-std=core --releaseWhen compiled for Hexagon with HVX enabled, simd_bitmask generates LLVM IR that truncates a <64 x i8> vector to <64 x i1>, which triggers the crash.
Symptom in Rust
error: could not compile `myproject` (lib)
Caused by:
process didn't exit successfully: `rustc ...` (signal: 6, SIGABRT: process abort signal)
LLVM ERROR: Unhandled HVX operation
UNREACHABLE executed at HexagonISelLoweringHVX.cpp
Minimal LLVM IR Reproducer
; trunc-v64i8-to-v64i1.ll
target datalayout = "e-m:e-p:32:32:32-a:0-n16:32-i64:64:64-i32:32:32-i16:16:16-i1:8:8-f32:32:32-f64:64:64-v32:32:32-v64:64:64-v512:512:512-v1024:1024:1024-v2048:2048:2048"
target triple = "hexagon-unknown-linux-musl"
define i64 @crash(<64 x i8> %v) #0 {
%1 = trunc <64 x i8> %v to <64 x i1>
%2 = bitcast <64 x i1> %1 to i64
ret i64 %2
}
attributes #0 = { "target-features"="-small-data,+hvx,+hvx-length128b" }How to Reproduce
llc -mtriple=hexagon-unknown-linux-musl -mcpu=hexagonv73 \
-mattr=+hvxv73,+hvx-length128b \
trunc-v64i8-to-v64i1.ll -o /dev/nullCrash Output
t5: v64i1 = truncate t4
t4: v64i8 = extract_subvector t2, Constant:i32<0>
t2: v128i8,ch = CopyFromReg t0, Register:v128i8 %0
t0: ch,glue = EntryToken
Unhandled HVX operation
UNREACHABLE executed at llvm/lib/Target/Hexagon/HexagonISelLoweringHVX.cpp:3554!
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace.
Stack dump:
0. Program arguments: llc -mtriple=hexagon-unknown-linux-musl -mcpu=hexagonv73 -mattr=+hvxv73,+hvx-length128b trunc-v64i8-to-v64i1.ll -o /dev/null
1. Running pass 'Function Pass Manager' on module 'trunc-v64i8-to-v64i1.ll'.
2. Running pass 'Hexagon DAG->DAG Pattern Instruction Selection' on function '@crash'
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