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swung0x48andClaude Opus 5
f4cbaa8945
The shared control page both roles map had a mixed-writer cache line:
{appliedSeq, submittedSeq, retiredSeq, completedFrameSerial, presentAckSerial}
sat together, but submittedSeq is producer-written (Ring.h's "THE FIVE
WATERMARKS") while the other four are consumer-written. That is why no
single-writer progress block could be aliased over it, and it is what this
package fixes: submittedSeq moves up beside cmdHead, the other thing the
producer writes, and the four that remain become a named LinkProgress member -
the struct c6 declared in ILink.h - so a data plane with no shared page can own
the same four values and every wait predicate in SessionRings.h keeps ONE
implementation at source level.
The two event flags deliberately stay in the doorbell group. Promoting them
would put the client's per-drain exchange(0) on eventRingFull (EventRing.h:200)
onto the line the client's own spin predicate reads 1e5-1e6 times a frame.
73 call sites rewritten, 15 in the library and 58 in tests. The rewrite is
receiver-scoped rather than field-scoped, because SplitRuntimePeek.h:77 has its
own struct with a field of the same name and SplitRuntimePeek.cpp:146 has both
on one line - a field-scoped rename would have silently broken the peek struct
that MagmaRunAheadScenario reads nine times.
THE ASSERTS ARE THE PART THAT NEEDED A SECOND PASS. The first revision shipped
four aggregate asserts and an adversarial review broke them by counterexample:
swapping retiredSeq and completedFrameSerial inside LinkProgress compiled
clean, passed all four, and passed the runtime layout test too - because
appliedSeq stays at offset 0. Three wire-visible offsets on a shared page were
pinned by nothing. Nothing else in the tree covers it either: the ABI
fingerprint takes no input from this page, and sizeof(RingControl) is 4096
whatever the members do, because alignas(4096) makes it so.
So the block is now per-field as CONTRACT-P6 §8.3 actually asked, plus a
standard-layout precondition, a tail bound that alignas cannot satisfy for us,
and the LinkEventFlags mirror. Each was run red: the field swap reds the
per-field asserts by name, a fifth member inside LinkProgress reds sizeof.
One of them cannot go red, and Ring.h now says so rather than implying
otherwise. A SMALL member inserted between Progress and serverEpoch lands at
offset 288, inside Progress's own line, and serverEpoch's alignas(64) absorbs
up to 32 bytes of it - so both line asserts still pass. The review's proposed
fix does not catch it either; I tried it. A gate nobody can falsify is worse
than no gate, so the limit is written down beside the asserts.
Also: RingTest.ControlPageLayoutIsTheSharedContract said nothing about the one
field this package moved, and now asserts the producer/consumer separation and
the event flags' exclusion itself rather than borrowing the claim from Ring.h.
Ring.h grew 30 lines, so the Ring.h:NN citations this package and c6 authored
are re-anchored to named blocks instead of line numbers. ARCHITECTURE.md §11.2
is updated for the new layout, and a factual error introduced in the previous
revision of that paragraph - submittedSeq annotated as releasing staging, which
retiredSeq does - is corrected.
Gates: pull 0 added / 0 removed, .text size 10822147 unchanged; split 0/0,
.text size 11681395 unchanged; unit suite 2326/2328 with the same two
pre-existing environmental failures as the baseline. The five edits behind
MGITEST_SPLIT_RUNTIME_PEEK were proven compiled by a red-once, not assumed.
Note on the .text claim: that is a SIZE, not the bytes - field offsets moved,
so immediates moved with them. And this page's layout IS observable across
binaries; what is unobservable is behaviour within one binary. The contract
already books the layout change deliberately, before spawn exists to care.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>