# LLVM Cannot select: v16bf16 = insert\_subvector on Float32→BFloat16 vector store (Julia 1.12.6, znver5)

**URL:** <https://discourse.julialang.org/t/llvm-cannot-select-v16bf16-insert-subvector-on-float32-bfloat16-vector-store-julia-1-12-6-znver5/136766>\
**Category:** Internals & Design\
**Tags:** question\
**Created:** [April 18, 2026, 2:15pm UTC](https://discourse.julialang.org/t/llvm-cannot-select-v16bf16-insert-subvector-on-float32-bfloat16-vector-store-julia-1-12-6-znver5/136766 "2026-04-18T14:15:56Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![cscherrer](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/cscherrer/32/7631_2.png) [@cscherrer](https://discourse.julialang.org/u/cscherrer)\
**Post date:** [April 18, 2026, 2:15pm UTC](https://discourse.julialang.org/t/llvm-cannot-select-v16bf16-insert-subvector-on-float32-bfloat16-vector-store-julia-1-12-6-znver5/136766/1 "2026-04-18T14:15:57Z")

</div>

Hello all — is this a known LLVM codegen bug?

**Minimal repro**

```julia
using BFloat16s: BFloat16

function f!(dst, src)
    for j in eachindex(dst)
        dst[j] = src[j]
    end
end

f!(Vector{BFloat16}(undef, 16), rand(Float32, 16))

```

**Output**

> **LLVM ERROR (click to expand)**
>
> ```julia-auto
> LLVM ERROR: Cannot select: v16bf16 = insert_subvector <prev>, <v8bf16 from VFPROUND>, Constant:i64<8>
> v16bf16 = insert_subvector undef:v16bf16, <v8bf16 from VFPROUND>, Constant:i64<0>
> v8bf16 = X86ISD::VFPROUND <v8f32 from load>
> v8bf16 = X86ISD::VFPROUND <v8f32 from load>
> Constant:i64<8>
> In function: julia_f!_...
> 
> ```
> 
> Full DAG + stack trace:
> 
> ```julia-auto
> LLVM ERROR: Cannot select: 0x3ddc45f0: v16bf16 = insert_subvector 0x3de48da0, 0x3ddc4f20, Constant:i64<8>, array.jl:991 @[array.jl:986 @[REPL[2]:3 ] ]
> 0x3de48da0: v16bf16 = insert_subvector undef:v16bf16, 0x3ddc4430, Constant:i64<0>, array.jl:991 @[array.jl:986 @[REPL[2]:3 ] ]
> 0x3ddc4b30: v16bf16 = undef
> 0x3ddc4430: v8bf16 = X86ISD::VFPROUND 0x3de48a90, /home/chad/.julia/packages/BFloat16s/lYUbX/src/bfloat16.jl:166 @[REPL[2]:3 ]
> 0x3de48a90: v8f32 = insert_subvector undef:v8f32, 0x3de486a0, Constant:i64<0>, /home/chad/.julia/packages/BFloat16s/lYUbX/src/bfloat16.jl:166 @[REPL[2]:3 ]
> 0x3de481d0: v8f32 = undef
> 0x3de486a0: v4f32,ch = load<(load (s128) from %ir.51, align 4, !tbaa !53, !alias.scope !56, !noalias !59)> 0x3cc2f6b0, 0x3de48710, undef:i64, essentials.jl:920 @[REPL[2]:3 ]
> 0x3de48710: i64 = add 0x3de48b00, Constant:i64<32>, essentials.jl:920 @[REPL[2]:3 ]
> 0x3de48b00: i64 = add 0x3ddc4510, 0x3de48c50, essentials.jl:920 @[REPL[2]:3 ]
> 0x3ddc4510: i64,ch = CopyFromReg 0x3cc2f6b0, Register:i64 %2, essentials.jl:920 @[REPL[2]:3 ]
> 0x3ddc4350: i64 = Register %2
> 0x3de48c50: i64 = shl 0x3de48fd0, Constant:i8<2>, essentials.jl:920 @[REPL[2]:3 ]
> 0x3de48fd0: i64,ch = CopyFromReg 0x3cc2f6b0, Register:i64 %8, essentials.jl:920 @[REPL[2]:3 ]
> 0x3ddc42e0: i64 = Register %8
> 0x3de48160: i8 = Constant<2>
> 0x3ddc4900: i64 = Constant<32>
> 0x3de48320: i64 = undef
> 0x3ddc51c0: i64 = Constant<0>
> 0x3ddc51c0: i64 = Constant<0>
> 0x3ddc4f20: v8bf16 = X86ISD::VFPROUND 0x3ddc4ba0, /home/chad/.julia/packages/BFloat16s/lYUbX/src/bfloat16.jl:166 @[REPL[2]:3 ]
> 0x3ddc4ba0: v8f32 = insert_subvector undef:v8f32, 0x3ddc49e0, Constant:i64<0>, /home/chad/.julia/packages/BFloat16s/lYUbX/src/bfloat16.jl:166 @[REPL[2]:3 ]
> 0x3de481d0: v8f32 = undef
> 0x3ddc49e0: v4f32,ch = load<(load (s128) from %ir.55, align 4, !tbaa !53, !alias.scope !56, !noalias !59)> 0x3cc2f6b0, 0x3ddc43c0, undef:i64, essentials.jl:920 @[REPL[2]:3 ]
> 0x3ddc43c0: i64 = add 0x3de48b00, Constant:i64<48>, essentials.jl:920 @[REPL[2]:3 ]
> 0x3de48b00: i64 = add 0x3ddc4510, 0x3de48c50, essentials.jl:920 @[REPL[2]:3 ]
> 0x3ddc4510: i64,ch = CopyFromReg 0x3cc2f6b0, Register:i64 %2, essentials.jl:920 @[REPL[2]:3 ]
> 0x3ddc4350: i64 = Register %2
> 0x3de48c50: i64 = shl 0x3de48fd0, Constant:i8<2>, essentials.jl:920 @[REPL[2]:3 ]
> 0x3de48fd0: i64,ch = CopyFromReg 0x3cc2f6b0, Register:i64 %8, essentials.jl:920 @[REPL[2]:3 ]
> 0x3ddc42e0: i64 = Register %8
> 0x3de48160: i8 = Constant<2>
> 0x3de48400: i64 = Constant<48>
> 0x3de48320: i64 = undef
> 0x3ddc51c0: i64 = Constant<0>
> 0x3ddc50e0: i64 = Constant<8>
> In function: julia_f!_595
> 
> [116013] signal 6 (-6): Aborted
> in expression starting at REPL[3]:1
> unknown function (ip: 0x7f787a846a2c) at /usr/lib/libc.so.6
> gsignal at /usr/lib/libc.so.6 (unknown line)
> abort at /usr/lib/libc.so.6 (unknown line)
> _ZN4llvm18report_fatal_errorERKNS_5TwineEb.cold at /home/chad/.julia/juliaup/julia-1.12.6+0.x64.linux.gnu/bin/../lib/julia/libLLVM.so.18.1jl (unknown line)
> _ZN4llvm16SelectionDAGISel15CannotYetSelectEPNS_6SDNodeE at /home/chad/.julia/juliaup/julia-1.12.6+0.x64.linux.gnu/bin/../lib/julia/libLLVM.so.18.1jl (unknown line)
> _ZN4llvm16SelectionDAGISel16SelectCodeCommonEPNS_6SDNodeEPKhj at /home/chad/.julia/juliaup/julia-1.12.6+0.x64.linux.gnu/bin/../lib/julia/libLLVM.so.18.1jl (unknown line)
> _ZN12_GLOBAL__N_115X86DAGToDAGISel6SelectEPN4llvm6SDNodeE at /home/chad/.julia/juliaup/julia-1.12.6+0.x64.linux.gnu/bin/../lib/julia/libLLVM.so.18.1jl (unknown line)
> _ZN4llvm16SelectionDAGISel22DoInstructionSelectionEv at /home/chad/.julia/juliaup/julia-1.12.6+0.x64.linux.gnu/bin/../lib/julia/libLLVM.so.18.1jl (unknown line)
> _ZN4llvm16SelectionDAGISel17CodeGenAndEmitDAGEv at /home/chad/.julia/juliaup/julia-1.12.6+0.x64.linux.gnu/bin/../lib/julia/libLLVM.so.18.1jl (unknown line)
> _ZN4llvm16SelectionDAGISel20SelectAllBasicBlocksERKNS_8FunctionE at /home/chad/.julia/juliaup/julia-1.12.6+0.x64.linux.gnu/bin/../lib/julia/libLLVM.so.18.1jl (unknown line)
> _ZN4llvm16SelectionDAGISel20runOnMachineFunctionERNS_15MachineFunctionE.part.0 at /home/chad/.julia/juliaup/julia-1.12.6+0.x64.linux.gnu/bin/../lib/julia/libLLVM.so.18.1jl (unknown line)
> ...
> Allocations: 2782850 (Pool: 2779994; Big: 2856); GC: 5
> fish: Job 1, 'julia' terminated by signal SIGABRT (Abort)
> 
> ```

**What I’ve checked**

| Variant | Result |
| --- | --- |
| `--cpu-target=generic` | OK |
| `--cpu-target=znver3` | OK |
| `--cpu-target=znver4` | crash |
| `--cpu-target=znver5` | crash |
| `--cpu-target=sapphirerapids` | crash |
| Scalar `BFloat16(rand(Float32))` | OK |
| Same loop but `Float16` instead of `BFloat16` | OK |
| `dst[j] = BFloat16(src[j])` (explicit) | crash (same error) |

So: any AVX-512-BF16 native target + a vectorizable `Float32 → BFloat16` conversion loop. Scalar is fine; emulated BF16 (znver3, generic) is fine; `Float16` is fine.

**Diagnosis** (verified against the LLVM 18.x sources)

The failing DAG is two `X86ISD::VFPROUND` halves concatenated into a `v16bf16` via two `insert_subvector`s — one at offset 0, one at **offset 8**. Only the offset-0 side has a lowering pattern:

- **Offset-0 inserts** are handled by `subvector_subreg_lowering` in [`X86InstrVecCompiler.td`](https://github.com/llvm/llvm-project/blob/release/18.x/llvm/lib/Target/X86/X86InstrVecCompiler.td#L65-L112). The multiclass is explicitly commented _“Patterns for insert\_subvector/extract\_subvector to/from index=0”_ and its pattern hard-codes `(iPTR 0)`. bf16 entries exist (lines 86, 99, 112), added by [PR #83720](https://github.com/llvm/llvm-project/pull/83720) for [#83358](https://github.com/llvm/llvm-project/issues/83358).
- **Non-zero-offset inserts** (i.e. the `vinsertf128`-style concat of two 128-bit halves into a 256/512-bit register) are handled by `vinsert_for_size_lowering` in [`X86InstrAVX512.td`](https://github.com/llvm/llvm-project/blob/release/18.x/llvm/lib/Target/X86/X86InstrAVX512.td#L495-L510). f16 has three entries there (`v8f16→v16f16`, `v8f16→v32f16`, `v16f16→v32f16`); **bf16 has zero**.

So PR #83720 covered bf16 at offset 0, but the matching `vinsert_for_size_lowering` calls for bf16 were never added. The fix would be three lines mirroring the f16 entries at 495/502/509. (`vextract_for_size_lowering` at 796/811 for f16 also has no bf16 counterparts, so the extract-from-upper-half path is presumably broken too — I haven’t tripped it.)

**Environment**

```julia-auto
Julia 1.12.6, LLVM 18.1.7
AMD Ryzen 9 9950X (znver5, AVX-512 + BF16)
BFloat16s v0.6.1

```

```julia
julia> versioninfo()
Julia Version 1.12.6
Commit 15346901f00 (2026-04-09 19:20 UTC)
Build Info:
  Official https://julialang.org release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 32 × AMD Ryzen 9 9950X 16-Core Processor
  WORD_SIZE: 64
  LLVM: libLLVM-18.1.7 (ORCJIT, znver5)
  GC: Built with stock GC
Threads: 16 default, 1 interactive, 16 GC (on 32 virtual cores)
Environment:
  LD_LIBRARY_PATH = /opt/cuda/lib64
  JULIA_NUM_THREADS = 16
  JULIA_PKG_USE_CLI_GIT = true

```

---

<div class="post-metadata">

**Author:** ![adienes](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/adienes/32/37459_2.png) [@adienes](https://discourse.julialang.org/u/adienes)\
**Post date:** [April 18, 2026, 2:22pm UTC](https://discourse.julialang.org/t/llvm-cannot-select-v16bf16-insert-subvector-on-float32-bfloat16-vector-store-julia-1-12-6-znver5/136766/2 "2026-04-18T14:22:29Z")

</div>

looks like ["LLVM ERROR: Cannot select" in LinearAlgebra/triangular2 tests on Apple Silicon M4 · Issue #61599 · JuliaLang/julia · GitHub](https://github.com/JuliaLang/julia/issues/61599) ?

---

<div class="post-metadata">

**Author:** ![cscherrer](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/cscherrer/32/7631_2.png) [@cscherrer](https://discourse.julialang.org/u/cscherrer)\
**Post date:** [April 18, 2026, 2:33pm UTC](https://discourse.julialang.org/t/llvm-cannot-select-v16bf16-insert-subvector-on-float32-bfloat16-vector-store-julia-1-12-6-znver5/136766/3 "2026-04-18T14:33:15Z")

</div>

Thanks @adienes . Same error, not clear whether it’s the same bug. Different architecture, and I’m only seeing it with `BFloat16`.

Also, works fine in 1.13:

```julia
julia> using BFloat16s: BFloat16

julia> function f!(dst, src)
           for j in eachindex(dst)
               dst[j] = src[j]
           end
       end
f! (generic function with 1 method)

julia> f!(Vector{BFloat16}(undef, 16), rand(Float32, 16))

julia> versioninfo()
Julia Version 1.13.0-beta3
Commit 393f698a6cc (2026-03-14 13:54 UTC)
Build Info:
  Official https://julialang.org release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 32 × AMD Ryzen 9 9950X 16-Core Processor
  WORD_SIZE: 64
  LLVM: libLLVM-20.1.8 (ORCJIT, znver5)
  GC: Built with stock GC
Threads: 16 default, 1 interactive, 16 GC (on 32 virtual cores)
Environment:
  LD_LIBRARY_PATH = /opt/cuda/lib64
  JULIA_NUM_THREADS = 16
  JULIA_PKG_USE_CLI_GIT = true

```

---

<div class="post-metadata">

**Author:** ![giordano](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/giordano/32/2166_2.png) [@giordano](https://discourse.julialang.org/u/giordano)\
**Post date:** [April 18, 2026, 5:20pm UTC](https://discourse.julialang.org/t/llvm-cannot-select-v16bf16-insert-subvector-on-float32-bfloat16-vector-store-julia-1-12-6-znver5/136766/5 "2026-04-18T17:20:50Z")

</div>

> [@cscherrer](#):
>
> Also, works fine in 1.13:

That’s usually a good indication it’s an LLVM issue.