simd_bitmask ICEs when result type is array

[matthiaskrgr]

from src/test/ui/simd/simd-bitmask.rs

//run-pass
//ignore-endian-big behavior of simd_select_bitmask is endian-specific
#![feature(repr_simd, platform_intrinsics)]

extern "platform-intrinsic" {
    fn simd_bitmask<T, U>(v: T) -> U;
}

#[derive(Copy, Clone)]
#[repr(simd)]
struct Simd<T, const N: usize>([T; N]);

fn main() {
    unsafe {
        let v = Simd::<i8, 4>([-1, 0, -1, 0]);
        let _a: [u8; 1] = simd_bitmask(v);
    }

}

thread 'rustc' panicked at 'assertion failed: dest.layout.ty.is_integral()', src/tools/miri/src/shims/intrinsics/simd.rs:550:17
stack backtrace:
   0:     0x7efe47365a7a - std::backtrace_rs::backtrace::libunwind::trace::h06c3ddb908575635
                               at /rustc/935dc07218b4bf6e20231e44eb9263b612fd649b/library/std/src/../../backtrace/src/backtrace/libunwind.rs:93:5
   1:     0x7efe47365a7a - std::backtrace_rs::backtrace::trace_unsynchronized::h8bfc3753e1cc3345
                               at /rustc/935dc07218b4bf6e20231e44eb9263b612fd649b/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x7efe47365a7a - std::sys_common::backtrace::_print_fmt::h3c304feda98d6679
                               at /rustc/935dc07218b4bf6e20231e44eb9263b612fd649b/library/std/src/sys_common/backtrace.rs:65:5
   3:     0x7efe47365a7a - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h98af9289a7a29dc2
                               at /rustc/935dc07218b4bf6e20231e44eb9263b612fd649b/library/std/src/sys_common/backtrace.rs:44:22
   4:     0x7efe473c87ae - core::fmt::write::h02538848739b7e2a
                               at /rustc/935dc07218b4bf6e20231e44eb9263b612fd649b/library/core/src/fmt/mod.rs:1208:17
   5:     0x7efe47355f45 - std::io::Write::write_fmt::h4aac2f2c3f0128af
                               at /rustc/935dc07218b4bf6e20231e44eb9263b612fd649b/library/std/src/io/mod.rs:1682:15
   6:     0x7efe47365845 - std::sys_common::backtrace::_print::h2e2acfe71f2fdb67
                               at /rustc/935dc07218b4bf6e20231e44eb9263b612fd649b/library/std/src/sys_common/backtrace.rs:47:5
   7:     0x7efe47365845 - std::sys_common::backtrace::print::h5bf077c253fab052
                               at /rustc/935dc07218b4bf6e20231e44eb9263b612fd649b/library/std/src/sys_common/backtrace.rs:34:9
   8:     0x7efe4736858f - std::panicking::default_hook::{{closure}}::h0ceeeedebdf177d0
                               at /rustc/935dc07218b4bf6e20231e44eb9263b612fd649b/library/std/src/panicking.rs:267:22
   9:     0x7efe473682cb - std::panicking::default_hook::h7d6f04e84940bd1d
                               at /rustc/935dc07218b4bf6e20231e44eb9263b612fd649b/library/std/src/panicking.rs:286:9
  10:     0x7efe4a640511 - rustc_driver[3622bb61b1ebe64a]::DEFAULT_HOOK::{closure#0}::{closure#0}
  11:     0x7efe47368dcd - <alloc::boxed::Box<F,A> as core::ops::function::Fn<Args>>::call::hc60f5ae29562b326
                               at /rustc/935dc07218b4bf6e20231e44eb9263b612fd649b/library/alloc/src/boxed.rs:2032:9
  12:     0x7efe47368dcd - std::panicking::rust_panic_with_hook::hce0f8648dbfb386b
                               at /rustc/935dc07218b4bf6e20231e44eb9263b612fd649b/library/std/src/panicking.rs:692:13
  13:     0x7efe47368b02 - std::panicking::begin_panic_handler::{{closure}}::h6ffcdb3a218d0ebe
                               at /rustc/935dc07218b4bf6e20231e44eb9263b612fd649b/library/std/src/panicking.rs:577:13
  14:     0x7efe47365f2c - std::sys_common::backtrace::__rust_end_short_backtrace::hc576a427f61fdaa5
                               at /rustc/935dc07218b4bf6e20231e44eb9263b612fd649b/library/std/src/sys_common/backtrace.rs:137:18
  15:     0x7efe47368852 - rust_begin_unwind
                               at /rustc/935dc07218b4bf6e20231e44eb9263b612fd649b/library/std/src/panicking.rs:575:5
  16:     0x7efe473c5193 - core::panicking::panic_fmt::habc1ee4a9702013e
                               at /rustc/935dc07218b4bf6e20231e44eb9263b612fd649b/library/core/src/panicking.rs:64:14
  17:     0x7efe473c526d - core::panicking::panic::h73f802489c27713b
                               at /rustc/935dc07218b4bf6e20231e44eb9263b612fd649b/library/core/src/panicking.rs:111:5
  18:     0x565111e3a7cf - <rustc_const_eval[60209f501fc07249]::interpret::eval_context::InterpCx<miri[df45d2962384a2a4]::machine::MiriMachine> as miri[df45d2962384a2a4]::shims::intrinsics::simd::EvalContextExt>::emulate_simd_intrinsic
  19:     0x565111de0bf9 - <rustc_const_eval[60209f501fc07249]::interpret::eval_context::InterpCx<miri[df45d2962384a2a4]::machine::MiriMachine>>::eval_fn_call
  20:     0x565111e166a8 - <rustc_const_eval[60209f501fc07249]::interpret::eval_context::InterpCx<miri[df45d2962384a2a4]::machine::MiriMachine> as miri[df45d2962384a2a4]::concurrency::thread::EvalContextExt>::run_threads
  21:     0x565111ec1e1d - miri[df45d2962384a2a4]::eval::eval_entry
  22:     0x565111d1d0e4 - <rustc_interface[299971c75fff0058]::passes::QueryContext>::enter::<<miri[2cc0a1151e659598]::MiriCompilerCalls as rustc_driver[3622bb61b1ebe64a]::Callbacks>::after_analysis::{closure#0}, ()>
  23:     0x565111d22263 - <miri[2cc0a1151e659598]::MiriCompilerCalls as rustc_driver[3622bb61b1ebe64a]::Callbacks>::after_analysis
  24:     0x7efe498d1661 - <rustc_interface[299971c75fff0058]::interface::Compiler>::enter::<rustc_driver[3622bb61b1ebe64a]::run_compiler::{closure#1}::{closure#2}, core[62c015fcd585be4a]::result::Result<core[62c015fcd585be4a]::option::Option<rustc_interface[299971c75fff0058]::queries::Linker>, rustc_errors[19d12f94925af316]::ErrorGuaranteed>>
  25:     0x7efe498cc658 - rustc_span[ea560f4838c45ec0]::with_source_map::<core[62c015fcd585be4a]::result::Result<(), rustc_errors[19d12f94925af316]::ErrorGuaranteed>, rustc_interface[299971c75fff0058]::interface::run_compiler<core[62c015fcd585be4a]::result::Result<(), rustc_errors[19d12f94925af316]::ErrorGuaranteed>, rustc_driver[3622bb61b1ebe64a]::run_compiler::{closure#1}>::{closure#0}::{closure#0}>
  26:     0x7efe498cc145 - <scoped_tls[6f6de2f0677d8068]::ScopedKey<rustc_span[ea560f4838c45ec0]::SessionGlobals>>::set::<rustc_interface[299971c75fff0058]::interface::run_compiler<core[62c015fcd585be4a]::result::Result<(), rustc_errors[19d12f94925af316]::ErrorGuaranteed>, rustc_driver[3622bb61b1ebe64a]::run_compiler::{closure#1}>::{closure#0}, core[62c015fcd585be4a]::result::Result<(), rustc_errors[19d12f94925af316]::ErrorGuaranteed>>
  27:     0x7efe498cb732 - std[688abfdbcfe52504]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[299971c75fff0058]::util::run_in_thread_pool_with_globals<rustc_interface[299971c75fff0058]::interface::run_compiler<core[62c015fcd585be4a]::result::Result<(), rustc_errors[19d12f94925af316]::ErrorGuaranteed>, rustc_driver[3622bb61b1ebe64a]::run_compiler::{closure#1}>::{closure#0}, core[62c015fcd585be4a]::result::Result<(), rustc_errors[19d12f94925af316]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[62c015fcd585be4a]::result::Result<(), rustc_errors[19d12f94925af316]::ErrorGuaranteed>>
  28:     0x7efe49f03aba - <<std[688abfdbcfe52504]::thread::Builder>::spawn_unchecked_<rustc_interface[299971c75fff0058]::util::run_in_thread_pool_with_globals<rustc_interface[299971c75fff0058]::interface::run_compiler<core[62c015fcd585be4a]::result::Result<(), rustc_errors[19d12f94925af316]::ErrorGuaranteed>, rustc_driver[3622bb61b1ebe64a]::run_compiler::{closure#1}>::{closure#0}, core[62c015fcd585be4a]::result::Result<(), rustc_errors[19d12f94925af316]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[62c015fcd585be4a]::result::Result<(), rustc_errors[19d12f94925af316]::ErrorGuaranteed>>::{closure#1} as core[62c015fcd585be4a]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  29:     0x7efe47372dc3 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h16c8b1a5112fc07f
                               at /rustc/935dc07218b4bf6e20231e44eb9263b612fd649b/library/alloc/src/boxed.rs:2000:9
  30:     0x7efe47372dc3 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h0cc5ff4fc4b4dcff
                               at /rustc/935dc07218b4bf6e20231e44eb9263b612fd649b/library/alloc/src/boxed.rs:2000:9
  31:     0x7efe47372dc3 - std::sys::unix::thread::Thread::new::thread_start::ha7c6dff74603472f
                               at /rustc/935dc07218b4bf6e20231e44eb9263b612fd649b/library/std/src/sys/unix/thread.rs:108:17
  32:     0x7efe471058fd - <unknown>
  33:     0x7efe47187a60 - <unknown>
  34:                0x0 - <unknown>

error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.68.0-nightly (935dc0721 2022-12-19) running on x86_64-unknown-linux-gnu

note: compiler flags: --crate-type bin -C embed-bitcode=no -C debuginfo=2 -C incremental=[REDACTED] -C target-cpu=native

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
end of query stack

Miri caused an ICE during evaluation. Here's the interpreter backtrace at the time of the panic:
note: the place in the program where the ICE was triggered
  --> src/main.rs:16:27
   |
16 |         let _a: [u8; 1] = simd_bitmask(v);
   |                           ^^^^^^^^^^^^^^^
   |
   = note: inside `main` at src/main.rs:16:27: 16:42
   = note: inside `<fn() as std::ops::FnOnce<()>>::call_once - shim(fn())` at /home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:507:5: 507:71
   = note: inside `std::sys_common::backtrace::__rust_begin_short_backtrace::<fn(), ()>` at /home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/sys_common/backtrace.rs:121:18: 121:21
   = note: inside closure at /home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/rt.rs:166:18: 166:82
   = note: inside `std::ops::function::impls::<impl std::ops::FnOnce<()> for &dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe>::call_once` at /home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:606:13: 606:31
   = note: inside `std::panicking::r#try::do_call::<&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe, i32>` at /home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panicking.rs:483:40: 483:43
   = note: inside `std::panicking::r#try::<i32, &dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe>` at /home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panicking.rs:447:19: 447:81
   = note: inside `std::panic::catch_unwind::<&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe, i32>` at /home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panic.rs:137:14: 137:33
   = note: inside closure at /home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/rt.rs:148:48: 148:73
   = note: inside `std::panicking::r#try::do_call::<[closure@std::rt::lang_start_internal::{closure#2}], isize>` at /home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panicking.rs:483:40: 483:43
   = note: inside `std::panicking::r#try::<isize, [closure@std::rt::lang_start_internal::{closure#2}]>` at /home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panicking.rs:447:19: 447:81
   = note: inside `std::panic::catch_unwind::<[closure@std::rt::lang_start_internal::{closure#2}], isize>` at /home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panic.rs:137:14: 137:33
   = note: inside `std::rt::lang_start_internal` at /home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/rt.rs:148:20: 148:98
   = note: inside `std::rt::lang_start::<()>` at /home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/rt.rs:165:17: 170:6

warning: 1 warning emitted

[RalfJung]

This looks okay to me, simd_bitmask returns an integer, not an array. If you use intrinsics at the wrong type you get to keep the pieces.

Cc @workingjubilee is simd_bitmask intended to allow array return types? I think not (in portable-simd it's always an integer return type), so I am inclined to close this issue.

[RalfJung]

Also Cc @calebzulawski

[oli-obk]

I'm surprised this passes in the rustc test suite...

[bjorn3]

cg_llvm explicitly allows it. I don't think it is used though. https://github.com/rust-lang/rust/blob/1d12c3cea30b8ba4a09650a9e9c46fe9fbe25f0b/compiler/rustc_codegen_llvm/src/intrinsic.rs#L1096

[oli-obk]

I opened rust-lang/rust#105990 to remove it. This is needless complexity when there are dedicated stable functions for it in libstd

[oli-obk]

Looks like this is a real thing, but I guess we can just make it an unsupported error

[RalfJung]

Sounds like portable-simd will start using this as well (just hasn't been upstreamed yet). So we should just implement this. But we'll need tests that actually check that the bytes are all in the right order etc.

Is this really the same as using to_be_bytes or is it host byte order?

[programmerjake]

it uses llvm bit vector order, which currently matches target byte-endian.

[programmerjake]

it uses llvm bit vector order, which currently matches target byte-endian.

see https://rust-lang.zulipchat.com/#narrow/stream/257879-project-portable-simd/topic/.E2.9C.94.20PR.20open.20to.20rust-lang.2Frust/near/260888464
https://github.com/llvm/llvm-project/blob/18334a1c1afe1b8a83c2a6108413bc48d80107df/llvm/lib/Analysis/ConstantFolding.cpp#L218
iirc it also obliquely states that in the llvm lang ref under vector in-memory layout.