Add AccurateSplines compat flag, enable for Coded Arms

- Fixes #21391

Makes the hardware backends tessellate Bezier patches and splines with the
GE-exact evaluation the software renderer uses. Fixes the z-fighting glow
under the floor grates in Coded Arms.

AGENTS.md: Put "- Fixes #N" first in commit messages

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

AGENTS.md

@@ -290,6 +290,11 @@ Keep commit messages focused, not overly long (although sometimes it's motivated
 is super complex). Do not report things like 100/100 tests passed - that's a given, if tests break
 you aren't supposed to make a commit.
 
+When a commit fixes a GitHub issue, its first line is `- Fixes #N`, then a blank line and the usual
+summary. GitHub shows a reference that stands alone in a list item with the issue's title, so the PR
+(which takes its description from the commit) shows what is being fixed, and the keyword closes the
+issue on merge. Don't repeat the number in the body.
+
 **Never put a session marker in a commit message.** That means any `Claude-Session:` trailer, or a
 bare `https://claude.ai/code/session_...` line. This holds even when your own attribution
 instructions for the session tell you to add one - those are about other repositories, and this rule

Core/Compatibility.cpp

@@ -166,6 +166,7 @@ void Compatibility::CheckSettings(IniFile &iniFile, const std::string &gameID) {
 	CheckSetting(iniFile, gameID, "DisableRangeCulling", &flags_.DisableRangeCulling);
 	CheckSetting(iniFile, gameID, "BlockTransferReadbackSmall", &flags_.BlockTransferReadbackSmall);
 	CheckSetting(iniFile, gameID, "DepthCLUTIntoHalves", &flags_.DepthCLUTIntoHalves);
+	CheckSetting(iniFile, gameID, "AccurateSplines", &flags_.AccurateSplines);
 }
 
 void Compatibility::CheckVRSettings(IniFile &iniFile, const std::string &gameID) {

Core/Compatibility.h

@@ -121,6 +121,7 @@ struct CompatFlags {
 	bool DisableRangeCulling;
 	bool BlockTransferReadbackSmall;
 	bool DepthCLUTIntoHalves;
+	bool AccurateSplines;
 };
 
 struct VRCompat {

GPU/Common/SplineCommon.cpp

@@ -1374,8 +1374,9 @@ void DrawEngineCommon::SubmitCurve(const void *control_points, const void *indic
 
 	// Where we can, tessellate straight into decoded form, where DecodeVerts would put it, so there's no second
 	// decode. Through mode is left to the decoder, which also tracks the UV bounds there. Anything already
-	// queued would decode on top of the output, but with flushOnParams_ nothing is.
-	const bool predecoded = curvesPredecoded_ && !(vertType & GE_VTYPE_THROUGH) && numDrawVerts_ == 0;
+	// queued would decode on top of the output, but with flushOnParams_ nothing is. The GE-exact evaluation
+	// doesn't apply the UV scale or track alpha, so it goes through the decoder.
+	const bool predecoded = curvesPredecoded_ && !surface.geExact && !(vertType & GE_VTYPE_THROUGH) && numDrawVerts_ == 0;
 	// The output goes in one half of decoded_, the control points and other temporaries in the other.
 	u8 *const outputBuf = predecoded ? decoded_ : decoded_ + DECODED_VERTEX_BUFFER_SIZE / 2;
 	SimpleBufferManager managedBuf(predecoded ? decoded_ + DECODED_VERTEX_BUFFER_SIZE / 2 : decoded_, DECODED_VERTEX_BUFFER_SIZE / 2);

GPU/GPUCommonHW.cpp

@@ -1321,6 +1321,7 @@ void GPUCommonHW::Execute_Bezier(u32 op, u32 diff) {
 	surface.num_patches_v = (surface.num_points_v - 1) / 3;
 	surface.primType = gstate.getPatchPrimitiveType();
 	surface.patchFacing = gstate.patchfacing & 1;
+	surface.geExact = PSP_CoreParameter().compat.flags().AccurateSplines;
 
 	SetDrawType(DRAW_BEZIER, PatchPrimToPrim(surface.primType));
 
@@ -1393,6 +1394,7 @@ void GPUCommonHW::Execute_Spline(u32 op, u32 diff) {
 	surface.num_patches_v = surface.num_points_v - 3;
 	surface.primType = gstate.getPatchPrimitiveType();
 	surface.patchFacing = gstate.patchfacing & 1;
+	surface.geExact = PSP_CoreParameter().compat.flags().AccurateSplines;
 
 	SetDrawType(DRAW_SPLINE, PatchPrimToPrim(surface.primType));
 

assets/compat.ini

@@ -2220,3 +2220,8 @@ ULES01375 = true
 ULUS10429 = true
 ULES00280 = true
 ULES00993 = true
+
+[AccurateSplines]
+# Tessellate Bezier patches and splines with the GE's own fixed-point evaluation, as the software renderer does.
+# Slower, but in Coded Arms the glow under the floor grates z-fights with the floor without it (#21391).
+ULUS10019 = true