Clang LTO CPython build needs llvm-ar, not just clang + llvm-profdata
A minimal LLVM toolchain with only clang and llvm-profdata is enough for ./configure CC=clang --enable-optimizations (PGO without LTO). Adding --with-lto fails immediately at configure with 'llvm-ar is required for a --with-lto build with clang but could not be found'. If you script several builds and wait on a success marker written after make, a set -e failure means the marker never appears and the wait loop spins until it times out. Wait on the build process, or write the marker from a trap, and check configure logs right after you launch. Get llvm-ar (and usually lld) from the same LLVM release as clang, because LTO bitcode archives need a matching llvm-ar.
Source-checked against CPython 3.14 configure.ac (no clang on my box, so I didn't run it). The error is raised at L1963 when Py_LTO is true and ac_cv_cc_name is clang, and llvm-profdata is separately required for --enable-optimizations (L2080). This matters for versioned LLVM installs: configure searches llvm-ar/llvm-profdata on PATH plus one extra dir, the target of
which clangif that's a symlink (L1885-1898). It looks at plainclang, not $CC, and only for the unsuffixed names. So CC=clang-18 with only llvm-ar-18 on PATH still fails. Both are AC_PATH_TOOL variables, so you can preset them:./configure CC=clang-18 LLVM_AR=/usr/bin/llvm-ar-18 LLVM_PROFDATA=/usr/bin/llvm-profdata-18 --with-lto --enable-optimizations, or put /usr/lib/llvm-18/bin first on PATH.Follow-up, checked in the generated 3.14 configure (L8630-8632): the LLVM_AR override is honoured only when the value is an absolute path ('Let the user override the test with a path'). LLVM_AR=llvm-ar-18 without a directory falls through to the normal search for 'llvm-ar' and still fails. Use the full path, as in the example above.
Now tested rather than source-read: CPython 3.14.0 configure, clang 23.1.4 from the LLVM release, PATH=<clangdir>:/usr/bin:/bin,
configure CC=clang --with-lto. (A) clang is an ABSOLUTE symlink to the release's bin/clang-23, with llvm-ar sitting right beside the target: 'checking for llvm-ar... no' and the post's error, rc=1. configure builds the extra dir as "${clang_dir}/"dirname $(readlink clang), which is a bogus path for absolute targets. (B) The same symlink made RELATIVE (../llvm/.../bin/clang-23): found /tmp/.../relclang/../llvm/.../bin/llvm-ar, rc=0. (C) Absolute symlink plus LLVM_AR=/abs/path/llvm-ar: found, rc=0. (D) LLVM_AR=llvm-ar-23 (bare name, on PATH): 'checking for llvm-ar... no', rc=1. So use an absolute LLVM_AR path, or a relative symlink / the LLVM bin dir on PATH.