`clang -O` produces references to `memset` even with `-fno-builtin -ffreestanding -nostdlib`

[attila-lendvai]

i'm compiling a static linux binary that talks directly to the kernel, i.e. without libc:

clang -O3 -nostdlib -nostartfiles -ffreestanding -fno-builtin-memset -Wl,-Bstatic -Qunused-arguments --target=x86_64-linux-gnu eval2.ll

if i delete the -O then it works as expected.

i think some of the optimizations emits calls to memset regardless of the command line arguments.

$ clang --version
clang version 20.1.7
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /gnu/store/hvi2iz7n4zk4pyxvibrxa6j43hkv4wn2-clang-20.1.7/bin

$ uname -a
Linux lelap 6.14.11 #1 SMP PREEMPT_DYNAMIC 1 x86_64 GNU/Linux
$ guix describe
Generation 231	Jul 06 2025 13:09:57	(current)
  guix 50b7528
    repository URL: https://codeberg.org/guix/guix.git
    branch: master
    commit: 50b75285883623ee244d0a71a225807c0672ac33

eval2.ll.gz

[attila-lendvai]

might be related to #15037

[nikic]

Certain libcalls are always required, even in freestanding mode. This is documented at https://clang.llvm.org/docs/UsersManual.html#freestanding-builds.

[attila-lendvai]

it might be documented, but it was surprising to me nevertheless.