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fix(core): order RingBuffer::buf_ before the atomics to unblock the CI compiler

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First real signal from the restored pipeline: with checkout fixed, the cpp-tests
job got far enough to fail at `-Werror=stringop-overflow` in ring_buffer.hpp on
GitHub's GCC 13, a failure that had been invisible behind the broken submodule
checkout for a month. Reproduced locally against gcc 13.4.0 (local default is
gcc 14, which does not fire).

The warning is a false positive: GCC anchors the destination object to the
member at offset 0 and reports `buf_[head & kMask]` as writing past
`head_._M_i` (size 8) at offset [16, 268] — offsets that are precisely
buf_[0..63] of a 64-entry, 4-byte ControlEvent array. Adding an explicit
`__builtin_unreachable()` bound hint does not help, because the index range was
never what GCC got wrong. Declaring buf_ first anchors the analysis correctly.

Not a suppression, and behaviour-preserving: RingBuffer has exactly one
production user (ModeBase::events_) and is never serialized, copied, or sent
over a wire, so member order is unobservable. Reasoning recorded at the
declaration so nobody "tidies" the order back.

Verified: full ctest suite green under gcc 13.4.0 (the CI compiler) as well as
gcc 14.2, and scripts/run-all-tests.sh ALL GREEN (parity 1273 floats within
1e-5, lint, 33 Playwright specs).
This commit is contained in:
monkey-w1n5t0n 2026-07-21 12:08:52 +02:00
parent 8c249ea8af
commit 3552ee46f6
1 changed files with 11 additions and 1 deletions
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12
nisps/core/ring_buffer.hpp
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@ -71,10 +71,20 @@ class RingBuffer {
private:
static constexpr std::size_t kMask = N - 1u;
// `buf_` MUST stay ahead of the atomics. `buf_[head & kMask]` is provably
// in bounds, but GCC 12/13 -Wstringop-overflow anchors the destination
// object to whichever member sits at offset 0; with the atomics first it
// reports the write as "1 byte into a region of size 0" against
// `head_._M_i` (size 8) at offset [16, 268] — offsets that are in fact
// exactly buf_[0..63]. A false positive, but -Werror makes it fatal, and
// it only fires on the CI toolchain (GCC 14 locally does not). Ordering
// buf_ first anchors the analysis correctly. Nothing depends on this
// layout: RingBuffer is never serialized, copied, or sent over a wire.
//
// Head/tail use std::size_t and rely on natural unsigned wrap.
T buf_[N]{};
std::atomic<std::size_t> head_;
std::atomic<std::size_t> tail_;
T buf_[N]{};
};
} // namespace nisps