rust-lang/rust · error

unsupported LLVM intrinsic {}

Error message

unsupported LLVM intrinsic {}

What it means

An `unimplemented!("unsupported LLVM intrinsic {}")` ICE in rustc_codegen_gcc's `intrinsic` function under `#[cfg(not(feature = "master"))]`. The non-master codegen path can only lower two LLVM intrinsic names (`llvm.x86.sse2.pause`, `llvm.x86.xgetbv`); every other LLVM intrinsic name falls through to the panic. The `master` cfg (the file's other branch) has a large table mapping LLVM intrinsics to GCC builtins, but the non-master build does not.

Source

Thrown at compiler/rustc_codegen_gcc/src/intrinsic/llvm.rs:1037

                return true;
            }
        }
        _ => (),
    }

    false
}

#[cfg(not(feature = "master"))]
pub fn intrinsic<'gcc, 'tcx>(name: &str, cx: &CodegenCx<'gcc, 'tcx>) -> Function<'gcc> {
    let gcc_name = match name {
        "llvm.x86.sse2.pause" => {
            // NOTE: pause is only a hint, so we use a dummy built-in because target built-ins
            // are not supported in libgccjit 12.
            "__builtin_inff"
        }
        "llvm.x86.xgetbv" => "__builtin_trap",
        _ => unimplemented!("unsupported LLVM intrinsic {}", name),
    };
    let func = cx.context.get_builtin_function(gcc_name);
    cx.functions.borrow_mut().insert(gcc_name.to_string(), func);
    func
}

#[cfg(feature = "master")]
pub fn intrinsic<'gcc, 'tcx>(name: &str, cx: &CodegenCx<'gcc, 'tcx>) -> Function<'gcc> {
    let gcc_name = match name {
        "llvm.prefetch.p0" => {
            let gcc_name = "__builtin_prefetch";
            let func = cx.context.get_builtin_function(gcc_name);
            cx.functions.borrow_mut().insert(gcc_name.to_string(), func);
            return func;
        }

        "llvm.aarch64.isb" => {
            // FIXME: GCC doesn't support __builtin_arm_isb yet, check if this builtin is OK.

View on GitHub (pinned to 7088e4b63a)

Solutions

1. Build rustc_codegen_gcc WITH the `master` cargo feature (recent libgccjit) so the large intrinsic table at llvm.rs:1044+ is compiled in.
2. Switch to the LLVM backend, which natively understands all LLVM intrinsics.
3. Avoid `#[target_feature]` / `std::arch` intrinsics in the crate being compiled by the non-master GCC backend.

Example fix

# before (non-master cg_gcc + intrinsics -> ICE)
rustc -Zcodegen-backend=gcc simd_crate.rs

# after — enable master feature when building cg_gcc, or use LLVM
# in rustc_codegen_gcc/Cargo.toml:  features = ["master"]
# or:
rustc simd_crate.rs

# Detect intrinsics usage + non-master cg_gcc
if grep -rqE 'core::arch|core::intrinsics|target_feature' src/ && [[ "${RUSTFLAGS:-}" == *codegen-backend=gcc* ]]; then
  echo "avoid: non-master cg_gcc only lowers 2 LLVM intrinsics; enable 'master' feature or use LLVM"
fi

When it happens

Trigger: Using inline-asm-derived or `std::arch`-derived LLVM intrinsics (e.g. AVX/SSE/NEON builtins) in code compiled with a `rustc_codegen_gcc` built WITHOUT the `master` feature. Any intrinsic name not in the two-entry allowlist at llvm.rs:1030-1037 panics.

Common situations: Crates that use platform intrinsics (`core::arch::x86_64::*`, SIMD, `core::intrinsics`) compiled via a distro-built (non-master) cg_gcc backend. Common when a user enables `-Zcodegen-backend=gcc` globally and then builds a numeric/simd-heavy crate.

Related errors

• not implemented
• simd {}
• funnel shift on {:?}
• Kind: {:?}
• unsupported Apple pointer width {pointer_width:?}

AI-assisted analysis of rust-lang/rust@7088e4b63a (2026-08-10). Data as JSON: /api/errors/85451f96e2958900. Report an issue: GitHub.