# XO1505: the LLVM backend's clang is older than 16

The LLVM backend (`--backend=llvm`) compiles its IR with clang: `$XO_CLANG`, else `clang` on PATH, or `--cc`. The generated code reads thread locals through llvm.threadlocal.address, which LLVM 16 added: an older clang leaves it an undefined function and the link fails (`undefined reference to llvm.threadlocal.address.p0`). The build reads `clang --version` and stops before generating code. Apple's clang numbers its versions apart from LLVM's and is not checked. The native backends, the default, do not use clang.

## Example

```xo
$ clang --version
clang version 15.0.7
$ xo build --backend=llvm .
XO1505: the LLVM backend needs clang 16 or later, but /usr/bin/clang is clang 15
```

## Fix

```xo
# install clang 16 or newer and point Xo at it:
XO_CLANG=/usr/lib/llvm-19/bin/clang xo build --backend=llvm .
# or use the native backend, which does not run clang:
xo build .
```

Run `xo explain XO1505` for this text in a terminal, or see
[section 10.6 of the specification](../../spec/core/#106-diagnostic-codes).