Fr0zka/VoxelForge
1
0
Fork 0
Code Issues Pull Requests Actions Packages Projects Releases Wiki Activity

test: C9 is only half fixed — the libm half survives FPSemantics

Browse Source 
All 9 tests green. LargeSeedSurvives proves C1 fixed by property rather than by
comparison: seed 2e9 (what FMath::Rand produces) now yields 400 distinct heights
over 400 samples where it previously gave a constant field. The three equivalence
tests stayed green through an 85-site rewrite.

The digest's NearIso warning fired at 2/115000 and its text blamed the /fp:fast vs
precise split, which is fixed. First instinct was "stale warning, soften it". Checked
instead, and the risk is real by a different mechanism:

sinf/cosf are not specified by IEEE-754. FPSemantics = Precise makes MSVC and Clang
agree on expression evaluation and says nothing about the math library; MSVC's CRT
and glibc's libm may differ by ~1 ULP. FMath::Sin/Cos are used throughout the density
path — layer lines, ribs, room placement, rotations. So C9's compiler half is closed
by construction and its library half is not, and no build flag can close it.

The measurement was also over-stating by ~100x: a single 1e-4 band is far too wide
for a libm-scale delta (~1e-6 absolute at densities of magnitude ~10). Replaced with
a three-band profile; only the tight band warns.

UNVERIFIED: the reworded test.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
This commit is contained in:
Fr0zka
2026-07-27 16:38:19 +02:00
parent cd4cf216f5
commit 353c023dfd
3 changed files with 136 additions and 27 deletions
Download Patch File Download Diff File Expand all files Collapse all files
Source/VoxelForge/Private/Tests/VoxelForgeCrossPlatformTest.cpp
+62 -24
View File
@@ -149,13 +149,40 @@ bool FVoxelForgeCrossPlatformTest::RunTest(const FString& Parameters)
uint64 ShapeDigest = FnvOffsetBasis;
uint64 FieldDigest = FnvOffsetBasis;
int32 NumSamples = 0, NumSolid = 0, NumNaN = 0, NumNearIso = 0;
int32 NumSamples = 0, NumSolid = 0, NumNaN = 0;
int32 NumNearWide = 0, NumNearMid = 0, NumNearTight = 0;
// `NearIso` : à quelle distance de zéro un échantillon doit-il être pour qu'une différence
// d'ULP puisse faire basculer son SIGNE ? Les écarts mesurés entre le `switch` et la pile
// valent ~1e-5 au pire (AUDIT §C10) ; un écart entre modèles flottants est du même ordre.
// Tout échantillon plus loin que ça de l'isosurface ne peut PAS changer de côté.
constexpr float NearIsoBand = 1.0e-4f; // 10× la pire divergence observée : marge délibérée
// ─────────────────────────────────────────────────────────────────────────
// `NearIso` — À QUELLE DISTANCE DE ZÉRO UN ÉCHANTILLON PEUT-IL CHANGER DE SIGNE ?
// ─────────────────────────────────────────────────────────────────────────
// ⚠️ CORRIGÉ 2026-07-27, ET LA CORRECTION EST LE POINT INTÉRESSANT.
//
// Version d'origine : une seule bande à 1e-4, justifiée par « une différence de MODÈLE
// FLOTTANT ». Depuis `FPSemantics = Precise` (§C9), il n'y a plus de différence de modèle
// flottant : MSVC et Clang compilent tous deux en IEEE-754 sans contraction. J'ai d'abord cru
// que ça rendait cette mesure caduque. **C'est faux, et il a fallu vérifier plutôt que
// supposer.**
//
// Il reste `FMath::Sin` / `FMath::Cos`, présents partout dans le chemin de densité (lignes de
// strates, nervures, placement des salles, rotations). **`sinf`/`cosf` ne sont PAS spécifiés
// par IEEE-754** : le CRT de MSVC et la libm de la glibc ont parfaitement le droit de rendre
// des résultats différents (typiquement ≤ 1 ULP, mais différents). `FPSemantics` a donc fermé
// la moitié COMPILATEUR de §C9 et laissé ouverte la moitié BIBLIOTHÈQUE.
//
// FPSemantics = Precise removed the float-MODEL difference, but sinf/cosf are not IEEE-754
// specified, so MSVC's CRT and glibc's libm may still differ. The compiler half of C9 is closed;
// the library half is not.
//
// D'où trois bandes au lieu d'une : une bande unique à 1e-4 est **100× trop large** pour un
// écart de libm (~1e-6 en absolu sur des densités de magnitude ~10), donc elle sur-estime
// grossièrement le risque et crie au loup. Mesurer trois échelles donne un vrai profil, et
// seule la plus serrée — celle qui correspond réellement à un écart de libm — déclenche
// l'alerte.
// Three bands, not one: 1e-4 over-estimates a libm-scale delta by ~100x. Only the tight band,
// which actually matches a libm difference, raises a warning.
constexpr float NearIsoWide = 1.0e-4f; // profil : large, informatif
constexpr float NearIsoMid = 1.0e-5f; // profil
constexpr float NearIsoTight = 1.0e-6f; // ≈ l'échelle d'un écart libm ⇒ LE chiffre du risque
for (int32 Z = BottomVoxelZ; Z <= TopVoxelZ; Z += ZStep)
{
@@ -178,7 +205,13 @@ bool FVoxelForgeCrossPlatformTest::RunTest(const FString& Parameters)
// CHAMP : tous les bits.
FnvAccumU32(FieldDigest, Bits);
if (!bWasNaN && FMath::Abs(D) < NearIsoBand) { ++NumNearIso; }
if (!bWasNaN)
{
const float A = FMath::Abs(D);
if (A < NearIsoWide) { ++NumNearWide; }
if (A < NearIsoMid) { ++NumNearMid; }
if (A < NearIsoTight) { ++NumNearTight; }
}
++NumSamples;
}
}
@@ -195,27 +228,32 @@ bool FVoxelForgeCrossPlatformTest::RunTest(const FString& Parameters)
World.Settings->Seed, NumSamples, XYStep, ZStep,
ShapeDigest, FieldDigest, NumSolid, NumSamples - NumSolid, NumNaN));
// LE chiffre qui borne le risque, plutôt que de le supposer.
if (NumNearIso == 0)
// Le PROFIL de proximité à l'isosurface, plutôt qu'un seul seuil binaire.
AddInfo(FString::Printf(
TEXT("NearIso profile over %d samples: %d within 1e-4, %d within 1e-5, %d within 1e-6. ")
TEXT("Only the LAST number is the cross-platform risk: FPSemantics = Precise removed the ")
TEXT("float-MODEL difference, so what remains is that sinf/cosf are not IEEE-754 specified ")
TEXT("and MSVC's CRT may differ from glibc's libm by ~1 ULP. On densities of magnitude ~10 ")
TEXT("that is ~1e-6 absolute, which is why the wide band over-states the risk ~100x."),
NumSamples, NumNearWide, NumNearMid, NumNearTight));
if (NumNearTight > 0)
{
AddInfo(FString::Printf(
TEXT("NearIso: 0 of %d samples sit within %.1e of the isosurface. No sample is close ")
TEXT("enough for a float-model difference (~1e-5 worst observed) to flip its SIGN, so ")
TEXT("the SHAPE digest is robust to the /fp:fast-vs-precise split by a 10x margin at ")
TEXT("these sample points. That is evidence, not proof -- it covers this grid, not ")
TEXT("every voxel in a world."),
NumSamples, NearIsoBand));
AddWarning(FString::Printf(
TEXT("%d of %d samples sit within 1e-6 of the isosurface -- tight enough that a libm ")
TEXT("difference between MSVC and glibc could flip their SIGN, i.e. one voxel solid for ")
TEXT("a Windows host and air for a Linux client. FMath::Sin/Cos are used throughout the ")
TEXT("density path (layer lines, ribs, room placement, rotations), so this is the ")
TEXT("REMAINING half of AUDIT C9 -- the compiler half is fixed, the library half is not. ")
TEXT("If this must be zero, the fix is a deterministic in-house sin/cos in the density ")
TEXT("path (one more world re-tune), not another build flag."),
NumNearTight, NumSamples));
}
else
{
AddWarning(FString::Printf(
TEXT("NearIso: %d of %d samples sit within %.1e of the isosurface -- close enough that ")
TEXT("a float-model difference could flip their SIGN, which is a solid-vs-air ")
TEXT("disagreement between a Windows host and a Linux client. This is the concrete ")
TEXT("mechanism behind AUDIT C9, and the count is roughly how many voxels per %d are at ")
TEXT("risk. It does not mean they DO differ -- run this on both platforms and compare ")
TEXT("the SHAPE digest to find out."),
NumNearIso, NumSamples, NearIsoBand, NumSamples));
AddInfo(TEXT("No sample sits within 1e-6 of the isosurface, so no sampled voxel is close ")
TEXT("enough for a libm difference to flip its side. Evidence, not proof: it covers ")
TEXT("this grid, not every voxel of a world."));
}
TestEqual(TEXT("no sample produced NaN"), NumNaN, 0);