picolibc 与 compiler-rt-builtins —— C 库以源码包形态进入索引

[Sunrisepeak]

picolibc 1.8.12 and compiler-rt-builtins 22.1.8 — the C library, as source

⭐⭐ SOURCE PACKAGES, NOT A PREBUILT SYSROOT, AND THE MEASUREMENTS SAY WHY.

A prebuilt C library ships one build per ABI a target table can name — seven for
Cortex-M alone — and every consumer then finds the right one through a libdir
convention that must match the payload byte for byte. That convention is what
#481 was filed about, and on ARM it cannot work at all: the key
<march>/<mabi> does not separate the float ABI, because mabi there names the
procedure call standard and is aapcs either way.

Measured while building the prebuilt first: the seven profiles collapsed into
FIVE directories and the soft-float row received a library carrying
Tag_ABI_HardFP_use. Nothing failed at build time.

⚠️ And the headers were never per-profile. Across those seven builds the whole
include tree — picolibc.h and newlib.h, which meson GENERATES, included — is
byte-identical. The prebuilt shipped seven copies of one directory.

Compiled with the same compile_flags as the program consuming it, ABI
agreement holds by construction, and the version lands in mcpp.lock rather
than being pinned inside a target table.

What the source route cost instead

Four upstream partitions a flat library has to be told about, each found by a
link error rather than by reading:

• libos/semihost and libos/dummyhost define the same syscalls and upstream
  keeps them in separate ARCHIVES, where link order decides. A feature here.
• semihost/fake is the COMPLEMENT of semihost/common — 13 of 26 files — and
  the complement cannot be computed from filenames: fake_io.c collides with
  common/iob.c on stdin/stdout/stderr.
• the machine directory carries variants for every ARM profile upstream serves;
  memcpy-armv7a.S needs ARM mode and Thumb-2, which no M-profile core has.
• a negative glob in a base source set applies to the MERGED set, so
  !…/machine/** cancels the per-target block that adds the right one back.

And for the builtins, one more: upstream picks per CPU and the sets are nested —
25 files on armv6-m, 90 on armv8-m.main — so the list is the INTERSECTION, and
the routines it omits come from the generic C. An exclusion pattern was not
enough either: !builtins/arm/*vfp.S misses save_vfp_d8_d15_regs.S.

Measured

Both build for all seven M-profile rows. A printf("%.2f") program built
against them plus cortex-m-rt boots on QEMU's mps2-an385, prints 13.00 and
exits 0 — and the same source without the feature fails at 'stdio.h' file not found, which is the zero-libc tier behaving as documented.

Both tarballs verified against their bytes and re-fetched from the CN mirror,
byte-identical. Index lints pass.