clang finds the compiler-rt builtins for bare-metal Arm now, but there
is no C library for these targets:

    $ echo '#include <stdio.h>' | clang --target=arm-none-eabi -mcpu=cortex-m4 -x c -c - -o /dev/null
    <stdin>:1:10: fatal error: 'stdio.h' file not found

picolibc is the C library for embedded systems that Arm’s LLVM Embedded
Toolchain uses. Build it for the six Cortex-M variants of
compiler-rt.be0, into the same directories below
/usr/lib/clang-runtimes/arm-none-eabi, so that clang’s multilib.yaml
selects both together.

Each variant gets its own meson cross file, as picolibc’s own
cross-clang-*.txt files call arm-none-eabi-ar and link against libgcc
from an arm-none-eabi-gcc installation. Deviations from them:

- -nostdlibinc, so that the headers of an installed picolibc do not
  shadow the ones being built.
- c_ld = 'lld', as clang’s bare-metal driver defaults to GNU ld, which
  on MarIuX only supports x86:

      ERROR: Unable to detect linker for compiler `clang --target=thumbv6m-none-eabi -mfloat-abi=soft -nostdlibinc -nostdlib -Wl,--version`
      stderr: /usr/bin/ld: unrecognised emulation mode: armelf

- No -fshort-enums. picolibc passes it to match arm-none-eabi-gcc, but
  clang defaults to 32-bit enums, and the library has to match the code
  compiled with clang that links against it.

-Dspecsdir=none, as picolibc would otherwise install picolibc.specs into
clang’s install directory, /usr/bin.

Linked with the defaults (crt0-semihost.o, picolibc.ld, -lsemihost,
-fuse-ld=lld), a hello world doing 64-bit division and float/double
printf runs in QEMU 11.1.0 for every variant:

    -mcpu=cortex-m0plus                microbit      armv6m_soft_nofp
    -mcpu=cortex-m3                    mps2-an385    armv7m_soft_nofp
    -mcpu=cortex-m4 -mfloat-abi=soft   mps2-an386    armv7m_soft_nofp
    -mcpu=cortex-m4                    mps2-an386    armv7em_soft_fpv4_sp_d16
    -mcpu=cortex-m4 -mfloat-abi=hard   mps2-an386    armv7em_hard_fpv4_sp_d16
    --target=thumbv7em-none-eabihf     mps2-an386    armv7em_hard_fpv4_sp_d16
    -mcpu=cortex-m7                    mps2-an500    armv7em_soft_fpv4_sp_d16
    -mcpu=cortex-m7 -mfloat-abi=hard   mps2-an500    armv7em_hard_fpv4_sp_d16
    -mcpu=cortex-m33                   mps2-an505    armv8m.main_soft_fp
    -mcpu=cortex-m33 -mfloat-abi=hard  mps2-an505    armv8m.main_hard_fp
    -mcpu=cortex-m33 -mfloat-abi=soft  mps2-an505    armv7m_soft_nofp

    hello: 176366841446 6.250 0.333333

Assisted-by: claude-opus-5.5 (Claude Code)

compiler-rt.be0 and picolibc.be0 build the same six Cortex-M variants
into /usr/lib/clang-runtimes, and multilib.yaml lets clang pick one. A
wrong pick still links and only fails on the hardware, so check the
selection for 17 combinations of core, float ABI and target spelling,
the files of every variant, and run a program doing 64-bit division and
float and double printf for eleven of them in QEMU with semihosting.
Like llvm-smoke-test, --prefix mounts packages over /usr in a user
namespace, so they are tested before they are installed.

With the packages from this branch:

    $ TMPDIR=/dev/shm scripts/smoke-tests/baremetal-arm-smoke-test \
          --qemu /pkg/qemu-11.1.0-0/bin/qemu-system-arm \
          --prefix compiler-rt-23.1.3-1.x86_64.bee.tar.bz2 \
          --prefix picolibc-1.8.12-0.x86_64.bee.tar.bz2
    […]
    OK    Cortex-M33 softfp runs on mps2-an505 with arm-none-eabi/armv8m.main_soft_fp
    OK    Cortex-M33 hard runs on mps2-an505 with arm-none-eabi/armv8m.main_hard_fp
    OK    default crt0, libc and builtins from arm-none-eabi/armv7em_hard_fpv4_sp_d16
    PASS (0 failed, 0 warned)

The QEMU 5.1.0 in /usr/bin has no Cortex-M4 or Cortex-M7 board, so with
it those six programs are only linked, and reported as WARN.

With the first draft of multilib.yaml, which had no softfp variants and
did not require -mfpu=none for the variants without FPU, it fails:

    FAIL  Cortex-M4 softfp (--target=arm-none-eabi -mcpu=cortex-m4): expected arm-none-eabi/armv7em_soft_fpv4_sp_d16, got 'arm-none-eabi/armv7m_soft_nofp'
    […]
    FAIL  Cortex-M4 softfp on mps2-an386: exit status 1: ARM fault: hardfault
    FAIL  Cortex-M7 softfp on mps2-an500: exit status 1: ARM fault: hardfault
    FAIL  Cortex-M33 softfp on mps2-an505: exit status 1: ARM fault: hardfault
    FAIL (7 failed, 0 warned)

Every link passes -fuse-ld=lld. For bare-metal targets clang defaults
to the GNU ld in /usr/bin, which on MarIuX only knows x86:

    $ clang --target=arm-none-eabi -mcpu=cortex-m3 -T picolibc.ld \
          -o ld.elf hello.c -lsemihost
    /usr/bin/ld: unrecognised emulation mode: armelf

That is clang’s configuration, not a fault of the runtimes, so it is
not checked. CLANG_DEFAULT_LINKER would change the host’s linker too;
clang configuration files per target triple are the better candidate.

Two things found on the way: an automount such as /pkg/qemu-11.1.0-0
cannot be triggered from within the user namespace ("Too many levels of
symbolic links"), so the script runs QEMU once before entering it; and
QEMU 11 writes the semihosting console to stderr unless that is a tty.

Assisted-by: claude-opus-5.5 (Claude Code)

For --prefix, the smoke tests re-run themselves in a mount namespace
made with `unshare --user --map-root-user --mount`, and unshare(1)
defaults to private mount propagation. An automount triggered from
inside, such as a /pkg package not yet mounted, is then mounted by the
daemon in the host’s namespace and never shows up in the private copy:

    $ unshare --user --map-root-user --mount ls /pkg/boost-1.86.0-0
    ls: cannot open directory '/pkg/boost-1.86.0-0': Too many levels of symbolic links

So pass `--propagation slave`: host mounts, the automounts included,
then reach the namespace, while the overlay over /usr and every other
mount made inside still do not leak out to the host.

    $ unshare --user --map-root-user --mount --propagation slave \
          ls /pkg/perl-5.34.0-0/profile
    /pkg/perl-5.34.0-0/profile

This makes running QEMU once before re-executing, which
baremetal-arm-smoke-test did to mount /pkg/qemu-11.1.0-0, otiose, so
drop it. Now every /pkg path works, not only QEMU’s.

Host mount changes now also reach the namespace, among them the autofs
expiry after timeout=300, but the next access simply mounts it again.

With /pkg/qemu-11.1.0-0 not mounted beforehand:

    $ TMPDIR=/dev/shm scripts/smoke-tests/baremetal-arm-smoke-test \
          --qemu /pkg/qemu-11.1.0-0/bin/qemu-system-arm \
          --prefix compiler-rt-23.1.3-1.x86_64.bee.tar.bz2 \
          --prefix picolibc-1.8.12-0.x86_64.bee.tar.bz2
          testing, over /usr: /dev/shm/baremetal-arm-smoke-d9n6vdgy/0 /dev/shm/baremetal-arm-smoke-d9n6vdgy/1
          running with /pkg/qemu-11.1.0-0/bin/qemu-system-arm (QEMU emulator version 11.1.0)
    […]
    OK    Cortex-M33 hard runs on mps2-an505 with arm-none-eabi/armv8m.main_hard_fp
    OK    default crt0, libc and builtins from arm-none-eabi/armv7em_hard_fpv4_sp_d16
    PASS (0 failed, 0 warned)

Assisted-by: claude-opus-5.5 (Claude Code)