Jahni: "I do not need your work to be identical or near identical to what I had
before, only having it 99.99% at worst reproducible if two people share the same
seed, since everyone rebuilds it on multiplayer."
Recorded as OPSTACK-PLAN 2.6.1, superseding 2.6's "recognisably the same place".
Consequences, each recorded where it will be found:
- C10 closed permanently rather than parked: it measures old-path vs new-path
agreement and the two never coexist in a shipped world.
- The equivalence tests keep their value as PORT-CORRECTNESS checks, not fidelity
checks. Isosurface hard-fail stays; ULP grading is diagnostic only.
- C9 promoted to top open risk. "Two people share a seed" is exactly what /fp:fast
weakens across toolchains, and a Linux dedicated server against Windows clients
compiles the density path under opposite float models. FPSemantics = Precise is
the fix and the IWYU debt now blocks something that matters.
- C1 unblocked: it was deferred only because it re-rolls the world's noise.
Then, doing C1's arithmetic before applying its documented one-liner: THE FIX IS
WRONG. It bounds SeedF but keeps the * 97.7 multiplier, so the coordinate term
still reaches 1.6e6 where the ULP is 0.19 — 9.5x the ~0.02/voxel step. It would
have left the bug live for mid-range seeds while closing the ticket.
The real fix deletes the multipliers: they only decorrelate the ~40 noise sites,
which is a hashing job. VoxelHash::SeedOffset(Seed, Site) gives a site-salted,
bounded, final-units offset. Bounding SeedF alone would also funnel every seed
through 16384 offsets shared by all sites; per-site salting requires a collision at
all ~40 sites instead.
The op stack has already inherited the bug via FSlabVoidSource, so it must land in
both paths at once — and every further port copies it again.
Docs only; the C1 fix is not written.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Jahni's call to pin it and move on, and the right one -- six builds spent and the
information stopped being worth the cost.
The final run closed it as far as it can be: SDFs identical everywhere (counted
unconditionally, 0 differ), yet two character-identical carve implementations in
the SAME translation unit fed a provably identical input differ by 1 ULP on
126/5000. That is only possible if they compile to different instruction
sequences, which /fp:fast permits based on surrounding context with no single
isolable axis. Hypothesis 3 was right about the mechanism and wrong about every
clean variable proposed for it, which is why four well-designed isolation tests
came back negative.
AUDIT C10 records the observation, what is proven (SDF bit-exact 126/126, zero
isosurface crossings), the five refuted hypotheses in a table so nobody repeats
them at a build each, why the settling experiment is blocked (shared-PCH / IWYU
debt), and the rule that actually matters: never run both density paths in one
world and never compare them for equality. That is NOT a client-desync risk --
within a binary the field is proven bit-pure and every peer runs the same path --
the cross-platform concern is C9, which stands on its own.
Corrected OPSTACK-PLAN 2.6 and C9: my earlier "/fp:fast across translation units"
explanation was measurably wrong and is removed rather than softened.
MazeEquivalence keeps the permanent value (equivalence with ULP grading,
window-invariance, box-verdict brute force) and drops the verbatim copy,
three-way, bisect, inlining and constness experiments.
Phase 1 closed: Maze decomposes into 7 ops, SDF bit-exact, 0 isosurface
crossings, window-invariant, and 23 of 60 tiles proved uniform where ClassifyTile
proves zero.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Jahni asked the right question: does the same seed produce identical results
across OS builds today? Checking the other toolchain made the answer sharper and
worse than what C9 originally said.
ClangToolChain.cs (Linux, Mac, Windows-with-Clang):
case FPSemanticsMode.Default: // Default to precise FP semantics.
case FPSemanticsMode.Precise: Arguments.Add("-ffp-contract=off");
and VCToolChain forces Precise when Windows uses Clang. So the same
FPSemanticsMode.Default resolves to OPPOSITE float models per toolchain, and
Windows/MSVC is the only imprecise configuration in the engine's defaults. Two
builds of identical source are not merely permitted to diverge -- they are
compiled under different rules.
Also added, so the entry does not over-fear itself:
- Calibration: 0 of 20000 samples crossed the isosurface under a 1-ULP
perturbation, so divergence means occasional single-voxel surface differences,
not different terrain. The case that bites is topological (a cave pinch-point
connecting on one build and not the other), which is rare and unreproducible --
the expensive kind.
- Precise everywhere still would not guarantee cross-platform bit-identity:
FMath::Sin/Cos route to platform libm, which is not bit-standardised. It closes
the large gap, not every gap.
- The knob would ALIGN Windows with every other platform rather than being a
one-sided cost -- but still must not be turned speculatively.
- The claim is inferred, not measured. The cheap decisive test is one Windows
build with FPSemantics = Precise: if MazeEquivalence's 454-sample residue
vanishes, the FP model is confirmed as the sole cause.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
The bisect settled it. The difference survives every stage removal down to
"corridors + carve ONLY", which is character-for-character transcribed code, so
it is not in anything the decomposition added.
Cause, read out of the engine rather than assumed (VCToolChain.cs):
case FPSemanticsMode.Default: // Default is imprecise FP semantics.
case FPSemanticsMode.Imprecise: Arguments.Add("/fp:fast"); break;
with UBT's own doc: "the compiler is allowed to transform math expressions in
ways that might result in differently rounded results". Identical source in two
translation units may reassociate differently, worth ~1 ULP. It shows up on
exactly the ~2% of samples inside the SDF blend shell, where Blend - Sdf
catastrophically cancels; outside it Carve is exactly 0 or 1 and both agree.
So MazeEquivalence now grades what it can actually assert:
- hard fail : any isosurface crossing (geometry moves)
- info : differences at ULP scale (the unavoidable floor)
- warn : anything larger, which IS port drift, and runs the bisect
A test that warns on every port would get ignored by the port that matters.
Recorded in OPSTACK-PLAN 2.6, and as AUDIT C9 for the part that outlives this
refactor: ARCHITECTURE 9.1's "every peer regenerates identically" holds only
between bit-identical binaries under /fp:fast. Fine for one build on one
platform; a real desync source for a Linux server plus Windows clients both
regenerating authoritative geometry. The FPSemantics::Precise knob exists but
must not be turned speculatively -- it blocks the vectorisation T2.a was chasing,
on the hot loop, for an unmeasured cost.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
AUDIT P1: every markdown design doc except CODEMAP.md was untracked, so
ARCHITECTURE / AUDIT / OPSTACK-PLAN / fable-idea / REVIEW_FINDINGS lived
only on disk. Replaces the single !CODEMAP.md exception with !*.md.
Also makes OPSTACK-PROGRESS.md commits actually record something, which
the unattended crash-safety discipline depends on.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Fr0zka