BUG: Clang linker fails on macOS 27 Beta ARM64 — ld: library 'd64' not found #2268

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1. Bllq21

   added

   bugSomething isn't workingSomething isn't working

   on Jun 30, 2026
2. SneachChea commented on Sep 14, 2026

   

   SneachChea

   on Sep 14, 2026

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   Confirming this on macOS 27.0 (26A428) + Command Line Tools 27.0.0.0 + Apple clang 21.0.0 (clang-2100.3.34.2), PyTensor 3.2.3 / PyMC 6.2.0.

   The flag is still on main (pytensor/link/c/cmodule.py, gated on int(platform.mac_ver()[0].split(".")[0]) >= 15), so no released version is unaffected.

   Two things that may narrow the fix:

   1. On this toolchain the classic-linker selection is not renamed, it is gone:

      • clang++ -ld64 → ld: library 'd64' not found
      • clang++ -Wl,-ld64 → same error, so ld itself rejects it too
      • clang++ -Wl,-ld_classic → ld: warning: -ld_classic is no longer supported and will be ignored

      The mac_ver() >= 15 gate therefore has no target left: on macOS ≥ 27 the flag should just be dropped.

   2. cxx="" works but disables C compilation entirely — much slower, and it rules out samplers that need the compiled logp. A wrapper that strips the flag keeps C compilation:

      cat > /usr/local/bin/clang-no-ld64 <<'EOF'
      #!/bin/bash
      args=()
      for a in "$@"; do [[ "$a" == "-ld64" ]] || args+=("$a"); done
      exec /usr/bin/clang++ "${args[@]}"
      EOF
      chmod +x /usr/local/bin/clang-no-ld64
      export PYTENSOR_FLAGS="cxx=/usr/local/bin/clang-no-ld64"   # or cxx = ... in .pytensorrc

      Verified: models that failed with the exact CompileError above compile and sample normally.

   3. Why it looks like two errors: .so files compiled before the OS upgrade keep loading, so the failure only appears on the next recompile. Any Python patch bump is enough, since the compiledir tag changes (compiledir_macOS-27.0-arm64-arm-64bit-arm-3.12.14-64) and the lazylinker must be rebuilt — hence the follow-up "Version check of the existing lazylinker compiled file. Looking for version 0.32, but found None".
3. drbenvincent commented on Sep 19, 2026

   

   drbenvincent

   on Sep 19, 2026

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   Can I request your attention on this one, maybe @ricardoV94? This is a major issue for anyone who has updated to MacOS 27
4. ricardoV94 commented on Sep 20, 2026

   

   ricardoV94

   on Sep 20, 2026

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   Can I request your attention on this one, maybe @ricardoV94? This is a major issue for anyone who has updated to MacOS 27

   Are you installing via conda?
5. SneachChea commented on Sep 20, 2026

   

   SneachChea

   on Sep 20, 2026

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   In my case, no conda involved. I’m installing from PyPI using uv in a virtual environment, with Homebrew Python 3.12.14 (macOS ARM64), PyTensor 3.2.3 and PyMC 6.2.0.
6. ricardoV94 commented on Sep 20, 2026

   

   ricardoV94

   on Sep 20, 2026

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   Can you try with conda. The suggested way of working with C backend and respective compilers is to use the conda release/env. If that fails equally we need to take a look
7. drbenvincent commented on Sep 21, 2026

   

   drbenvincent

   on Sep 21, 2026

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   I have applied and validated a host-local workaround on macOS 27.0 / Apple Clang 21. It unblocks current development across six separate PyTensor worktrees while retaining C compilation: a forced PyTensor C-link and a small PyMC NUTS sample both pass.

   Workaround applied on this machine

   ~/.local/bin/pytensor-clang++-macos27 (made executable):

   #!/bin/bash
   set -euo pipefail
   
   if [[ "$(/usr/bin/uname -s)" == "Darwin" ]]; then
       macos_major="$(/usr/bin/sw_vers -productVersion | /usr/bin/cut -d. -f1)"
   else
       macos_major=0
   fi
   
   if [[ "$macos_major" =~ ^[0-9]+$ ]] && ((macos_major >= 27)); then
       args=()
       for arg in "$@"; do
           [[ "$arg" == "-ld64" ]] || args+=("$arg")
       done
       exec /usr/bin/clang++ "${args[@]}"
   fi
   
   exec /usr/bin/clang++ "$@"

   ~/.pytensorrc:

   [global]
   cxx=/absolute/path/to/pytensor-clang++-macos27

   The shim removes only -ld64 on macOS >= 27 and passes every other compiler argument through. Because PyTensor reads this user configuration, it applies to all local PyTensor environments without changing repository files or individual virtualenvs. Existing Python kernels need a restart.

   This is a local mitigation, not a proposed substitute for the upstream fix. To answer the installation question cleanly, I will upgrade another machine to macOS 27 and test the same minimal reproduction in fresh pip and conda environments with this shim disabled, then report back.
8. alex-andrzejek-og commented on Oct 7, 2026

   

   alex-andrzejek-og

   on Oct 7, 2026

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   Adding a data point: this is triggered by the Command Line Tools / Xcode 27.0 toolchain, not by macOS 27. It reproduces on macOS 26.7 (25G229) once Software Update installs Command Line Tools for Xcode 27.0, which it offers to the previous macOS as a separate update.

   • macOS 26.7 (25G229), arm64, no Xcode.app; xcode-select -p → /Library/Developer/CommandLineTools
   • Command Line Tools for Xcode 27.0 (pkgutil --pkg-info=com.apple.pkg.CLTools_Executables → 27.0.0.0.1788430756), Apple clang 21.0.0, ld-27037.1
   • PyTensor 2.38.3 / PyMC 5.28.5 from PyPI in a uv venv (no conda), Python 3.12.13
   • The same machine compiled fine on macOS 26.6.x with Command Line Tools 26.6. The first fresh compile after the 27.0 tools landed fails with ld: library 'd64' not found.

   So a fix or workaround gated on macOS >= 27 would miss this case. Both shims earlier in this thread gate that way, so they're no-ops on macOS 26 with the 27 tools. The flag should also not be dropped for every macOS ≥ 15: older toolchains may still rely on -ld64 to avoid the Xcode 15 ld_prime problems the gate was written for.

   Suggested fix: decide from the linker, not the OS version. Probe once per process: link a trivial shared object with -ld64, and drop the flag only if that fails. For example, in compile_args:

   def _ld64_supported(cxx):
       # tiny C file -> cxx -shared -undefined dynamic_lookup -ld64 t.c -o t.so
       # return False if it fails with "library 'd64' not found"

   We're carrying this as a local patch (wrapping GCC_compiler.compile_args). It has been verified with the PyMC NUTS and nutpie samplers, and it's a no-op on toolchains that still accept -ld64.
9. ricardoV94

   closed this as completedin #2432on Oct 12, 2026