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

build: FPSemantics = Precise + clear the IWYU debt that blocked it

Browse Source 
Jahni: cross-platform play (Linux and Windows, either side hosting) is a product
requirement, so fix C9 at the cause instead of measuring the symptom.

Verified in UE 5.7 source rather than assumed:
  VCToolChain.cs:1264    Default -> /fp:fast          Precise -> /fp:precise
  ClangToolChain.cs:712  Default -> -ffp-contract=off Precise -> -ffp-contract=off
Default really did mean opposite float models per platform; Precise collapses them
onto the same one, so a Windows host and a Linux client agree by construction.

Losing the shared PCH is what the IWYU debt hid behind. Every use turned out to be a
pointer, TWeakObjectPtr or TSubclassOf parameter, so forward declarations suffice;
only the templates and macros needed real includes. Seven headers fixed.

VoxelDensityVolume.h was the one worth catching: it tests ENABLE_DRAW_DEBUG in an
#if, and an undefined macro there is silently 0 — the debug block would have
vanished without a warning rather than failing the build. Include paths verified
against the engine tree, not guessed.

Expect a residual tail; the shared PCH hid these for years and only a build
enumerates them all. Build.cs now says so, and says the fix is to add the include
rather than revert FPSemantics.

Also adds VoxelForge.Determinism.CrossPlatformDigest: SHAPE digest (sign of density
= the world) and FIELD digest (bit-for-bit) over a fixed integer grid, plus NearIso
to bound how many samples could flip sign at all. Reports rather than asserts until
pinned. The cross-platform comparison itself is deferred per Jahni.

Expect a perf regression from losing reassociation and contraction on a noise-heavy
hot path — measure against ARCHITECTURE 8.10.

UNVERIFIED: not compiled.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
This commit is contained in:
Fr0zkaandClaude Opus 5 committed 2026-07-27 15:53:23 +02:00
1 parent f1fd1e0b05
commit 6a60732c98
10 files changed
+392 -16
Download Patch File Download Diff File Expand all files Collapse all files

No files matched your search

Source/VoxelForge/VoxelForge.Build.cs
+42 -16
View File
@@ -12,25 +12,51 @@ public class VoxelForge : ModuleRules
PCHUsage = PCHUsageMode.UseExplicitOrSharedPCHs;
// ============================================================================
// ⚠️ DO NOT SET `FPSemantics` HERE — tried 2026-07-27, it does not build.
// FLOAT MODEL — pinned to Precise so Windows and Linux compute the SAME WORLD.
// ============================================================================
// Setting FPSemantics (or any other property that alters this module's compile
// environment) makes VoxelForge ineligible for the ENGINE'S SHARED PCH: UBT can only
// share a precompiled header between modules whose compile environments match. The
// build then fails with ~30 "undefined type" errors — UMaterialInterface, USoundBase,
// TSubclassOf<AActor>, APawn, ENABLE_DRAW_DEBUG — none of which are FP-related. They
// are includes this plugin has always relied on the shared PCH to provide for free.
// Jahni, 2026-07-27: the game must be playable on both Linux and Windows, either side
// hosting. The MP design replicates the SEED and has every peer regenerate the terrain, so
// a Windows host and a Linux client must agree on the density field.
//
// So the plugin has a latent IWYU (include-what-you-use) debt: several public headers
// use engine types they never include. That is worth fixing on its own terms one day
// (UE has been moving away from implicit shared-PCH includes for years), but it is a
// real chunk of work and must not be attempted inside an unrelated diagnostic.
// They did not, by construction. UBT resolves FPSemanticsMode.Default differently per
// toolchain (verified in UE 5.7 source, not assumed):
// VCToolChain.cs:1264 Default/Imprecise -> "/fp:fast" (Windows/MSVC)
// ClangToolChain.cs:712 Default/Precise -> "-ffp-contract=off" (Linux/Mac/Clang)
// So the same source was compiled under OPPOSITE float rules depending on who built it.
//
// The FP question it was meant to settle — whether identical source reassociates
// differently per translation unit under /fp:fast — is now answered inside
// VoxelForge.OpStack.MazeEquivalence instead, by compiling a verbatim copy of the
// Maze core into the TEST's translation unit and comparing all three. No build
// settings involved, and it cannot break anything.
// Precise resolves to "/fp:precise" on MSVC and "-ffp-contract=off" on Clang — both
// IEEE-754 compliant with no FMA contraction, so the two toolchains agree BY CONSTRUCTION
// rather than by luck. That is the fix for AUDIT-2026-07.md C9.
//
// COST: /fp:precise forbids the reassociation and contraction /fp:fast allowed, on a
// noise-heavy hot path. Expect a measurable perf regression and check it against
// ARCHITECTURE 8.10 — determinism across platforms is worth paying for, but the price
// should be known, not assumed.
//
// VERIFY: run VoxelForge.Determinism.CrossPlatformDigest on both platforms and compare the
// SHAPE digest (sign of density = the world) and the FIELD digest (bit-for-bit). Pin the
// values in that test once they agree, and it guards this forever after.
FPSemantics = FPSemanticsMode.Precise;
// ============================================================================
// ⚠️ HISTORY — why this took a second attempt (kept: it explains the includes below)
// ============================================================================
// Setting FPSemantics (or any property that alters this module's compile environment)
// makes VoxelForge ineligible for the ENGINE'S SHARED PCH — UBT can only share a
// precompiled header between modules whose compile environments match. The first attempt
// (2026-07-27) was therefore reverted: it failed with ~30 "undefined type" errors that
// were not FP-related at all — UMaterialInterface, USoundBase, TSubclassOf<AActor>,
// ENABLE_DRAW_DEBUG — i.e. includes this plugin had always taken from the shared PCH for
// free. That is a latent IWYU debt, not an FP problem.
//
// ⚠️ SO IF YOU SEE "undefined type" ERRORS HERE, THEY ARE IWYU, NOT FLOAT SETTINGS.
// The fix is to add the missing include or forward declaration to the header that needs
// it — never to revert FPSemantics, which is now load-bearing for cross-platform play.
// Headers fixed on 2026-07-27: VoxelBiomeDefinition, VoxelSettings, VoxelStrateDefinition,
// VoxelStrateTypes, VoxelContentManager, VoxelAtmosphereManager, VoxelDensityVolume.
// Expect a residual tail: the shared PCH hid these for years and only a build enumerates
// them all. VoxelDensityVolume's was the nasty one — ENABLE_DRAW_DEBUG is used in an #if,
// and an undefined macro there is silently 0 rather than an error.
// Modules we depend on:
// - Core: Basic types (TArray, FString, etc.)