LLVM regression in beta with `-Ctarget-feature=-simd128`

[DaniPopes]

Code

I tried this code:

use std::arch::wasm32::*;

#[target_feature(enable = "simd128")]
pub unsafe fn some_simd128_fn(chunk: v128) -> bool {
    u8x16_all_true(chunk)
}

Flags: -Ctarget-feature=-simd128 --target wasm32-wasip1

Godbolt

I expected to see this happen: compiles

Instead, this happened: fails to compile with "rustc-LLVM ERROR: Do not know how to split this operator's operand!"

Version it worked on

It most recently worked on: 1.81.0

Version with regression

rustc --version --verbose:

rustc 1.82.0-beta.4 (8c27a2ba6 2024-09-21)
binary: rustc
commit-hash: 8c27a2ba6b21f3406a51118643080f0591949827
commit-date: 2024-09-21
host: x86_64-unknown-linux-gnu
release: 1.82.0-beta.4
LLVM version: 19.1.0
Compiler returned: 0

rustc 1.83.0-nightly (2bd1e894e 2024-09-26)
binary: rustc
commit-hash: 2bd1e894efde3b6be857ad345914a3b1cea51def
commit-date: 2024-09-26
host: x86_64-unknown-linux-gnu
release: 1.83.0-nightly
LLVM version: 19.1.0
Compiler returned: 0

@rustbot modify labels: +regression-from-stable-to-beta -regression-untriaged +A-LLVM

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-medium

[hanna-kruppe]

Note that enabling simd128 only for some functions is not really useful because wasm doesn’t support dynamic feature detection: the entire module has to pass validation before any of it is executed, so either you have SIMD support or your module won’t load anyway. See https://doc.rust-lang.org/core/arch/wasm32/index.html#simd and BurntSushi/memchr#144 for discussion.

There’s still a bug here that should be fixed, but the workaround of building the entire program (except the sysroot) with -Ctarget-feature=+simd128 should not have any drawbacks in 99% of cases.

[nikic]

Looks like CoalesceFeaturesAndStripAtomics replaces "-simd128,+simd128" with "+multivalue,+mutable-globals,+reference-types,+sign-ext,+simd128,-simd128,"

[nikic]

Unfortunately, I can't seem to figure out how to reproduce this outside rustc. Just feeding the IR through llc gives be "+multivalue,+mutable-globals,+reference-types,+sign-ext,+simd128," without the incorrect -simd128 at the end.

[nikic]

@alexcrichton Do you happen to know whether there is some kind of special option one has to pass to llc to reproduce how rust emits wasm?

[alexcrichton]

This is a part of LLVM I've never personally fully understood but I believe that the target-feature attribute on functions is not equivalent to what's passed on the command line. I got the above to crash and with -Csave-temps it emitted foo.foo.93210e5e38e8209c-cgu.0.rcgu.bc as the crashing bitcode. Locally I then see:

$ llc foo.foo.93210e5e38e8209c-cgu.0.rcgu.bc -o wat.o -filetype=obj
$ llc foo.foo.93210e5e38e8209c-cgu.0.rcgu.bc -o wat.o -filetype=obj -mattr=-simd128
SplitVectorOperand Op #1: t13: i32 = llvm.wasm.alltrue TargetConstant:i32<12090>, t25

LLVM ERROR: Do not know how to split this operator's operand!

PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace.
...

which is a long way of saying "maybe -mattr=-simd128 perhaps?"