Runs the instrument section 5.5.2 asked for, on a V4c tree staged from
`git revert --no-commit 543bc79f` and never committed: GL_SAMPLES_PASSED around
the raw-GL terrain draw, the dispatcher's entity draws, and the retained-UI
flush, collected outside the frame that issued them, with the desktop witness as
the verdict. All probe code is stripped; what survives here is the two gate
scripts and section 5.5.3/5.5.4.
Building it found a fourth instrument fault. Reading a query result on the CPU
timeline - glGetQueryObject guarded by RESULT_AVAILABLE, one frame late -
deadlocks V4c at the first frame that draws the world: 4/4 runs, and five
dotnet-stack samples four seconds apart all show the render thread inside the
driver in that call. Not a probe defect - the same probe ran 4,420 clean frames
on the V4c parent, and instrumenting only the UI flush reproduces the wedge while
creating the query objects and never beginning one does not.
Routing the result into a persistently-mapped GL_QUERY_BUFFER instead - the
driver writes it on the GPU timeline, so no client wait is possible, and a
sentinel separates "reported zero" from "never reached" - does not wedge, and
gives the answer. On blank runs no query result is ever produced at any site for
the whole run, including the UI, in the same frames where the desktop grab plainly
shows the UI on screen. On the rendered run of the same binary, 1,068 frames, not
one missing result.
So the mission's fork resolves to "never completes", but not as a stall: frame
time holds at 5.5 ms for ~3,700 frames, the frame-flight fences keep retiring,
and present keeps working. Every channel that carries a result back from the GPU
is dead - pixel readback, CPU query read, GPU-timeline query write - and every
channel that carries none is fine. The transition is one sharp event at the first
world frame and never reverses, and that frame rasterizes correctly: 1,692,830
terrain and 317,561 entity samples, the same two numbers the parent reports for
its own first world frame.
Section 5.5.4 lays out the three options with their costs and recommends (C):
bring Vulkan up first and decide V4c afterwards, because running the identical
ported world path on the Vulkan backend on this GPU is both the cheapest test of
the driver-defect reading and work the campaign owes anyway. (B), accepting the
GL-side fork, is probably the right conclusion but should be adopted on a
measurement rather than an inference. No fix was attempted and V4c is not
re-landed.
Apparatus: run-repeat-connected-gate.ps1 and run-blank-world-ab-probe.ps1 now
assert on the desktop grab and record the client's own capture as a second
column, which is the re-arming section 5.5.2 required before re-land condition 2
can mean anything. Both verified end-to-end.
Gates: Release build clean; App tests 3,866 passed / 3 skipped; offline pixel
gate PASS at 3.37e-05 differing fraction (19 px of 563,200), inside the
documented 15-23 px band.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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