Fr0zka
0379d59c1c
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>