[gburd]

Purpose (gburd fork): exercise CI + OCR AI review on the stdatomic
series before pgsql-hackers. Targets the fork's master; opened draft.
cf-7005: https://commitfest.postgresql.org/patch/7005/

---

This series reimplements PostgreSQL's atomics (port/atomics.h) on top of
C11 <stdatomic.h>, picking up Thomas Munro's WIP and finishing it into a
reviewable, benchmarked form.

It is deliberately structured as an alternative, selectable
implementation -- not a replacement:

• The build option (-Duse_stdatomic / --with-stdatomic) is a tri-state
  auto|yes|no and DEFAULTS TO "no". With the default, the traditional
  platform-specific atomics remain the implementation and are
  behavior-preserving; stdatomic is strictly opt-in.
• Patches 1-5 are the proposal: they ADD the C11 path alongside the
  existing arch-/generic- code and wire it up behind the option.
  Nothing is removed; both build systems keep building both ways.
• Patch 6 is marked "[NOT FOR MERGE... yet.]". It shows the eventual
  endgame -- deleting the traditional code once stdatomic has proven
  itself in the field -- purely so reviewers and the buildfarm can
  exercise the stdatomic-only end state. It is NOT being proposed for
  commit now, and the aggregate diffstat below includes its deletions.

So the answer to "is this making <stdatomic.h> mandatory?" is no:
through patch 5 the traditional path is intact, default, and selectable.

The one substantive semantics decision worth review is the memory
ordering of the generic read/write. Short version: the WRITE is relaxed
(plain STR, matching the documented "no barrier" contract), and the READ
is seq_cst -- which weak-memory correctness actually required (a C11
relaxed load is NOT equivalent to stock's volatile load; a parallel hash
join lost tuples on RISC-V with a relaxed read). Uncontended this costs
nothing measurable, and the earlier ~2% aarch64 regression (which was
the seq_cst STORE, now relaxed) is gone. Relaxing the read too and
giving just the hash-join consumer its ordering is documented follow-up.
Full data, per-op mapping, and methodology are in the accompanying
pgsql-hackers mail.

CI is green on all platforms (Linux 32/64 meson + autoconf, macOS,
Windows MSVC + MinGW, CompilerWarnings), Test world included.