[Fix] (Tessellation): compare the default patch levels by bit pattern, so a NaN level stops re-linking the program on every draw

MobileGL/MG_State/GLState/RenderState/RenderState.cpp

@@ -220,8 +220,13 @@ namespace MobileGL {
             // BumpVersions(), not just ++m_version, for the same reason SetPatchVertices does it:
             // these levels are compiled INTO the synthesized pass-through tessellation control
             // stage on both backends, so changing one makes an already-built program stale.
+            //
+            // The redundant-write guard compares BIT PATTERNS, not floats: glPatchParameterfv
+            // accepts NaN, and a float compare would let a re-set of the identical NaN tuple fall
+            // through and bump the pipeline-state version - invalidating DirectVulkan's pipeline
+            // memo and DirectGLES's render-state span - on every single call.
             void RenderState::SetPatchDefaultOuterLevel(const FloatVec4& levels) {
-                if (m_parameters.PatchDefaultOuterLevel == levels) return;
+                if (BitwiseEqual(m_parameters.PatchDefaultOuterLevel, levels)) return;
 
                 m_parameters.PatchDefaultOuterLevel = levels;
                 BumpVersions();
@@ -232,7 +237,7 @@ namespace MobileGL {
             }
 
             void RenderState::SetPatchDefaultInnerLevel(const FloatVec2& levels) {
-                if (m_parameters.PatchDefaultInnerLevel == levels) return;
+                if (BitwiseEqual(m_parameters.PatchDefaultInnerLevel, levels)) return;
 
                 m_parameters.PatchDefaultInnerLevel = levels;
                 BumpVersions();

MobileGL/MG_Util/Math/VectorTypes.h

@@ -10,6 +10,8 @@
 
 #include <Includes.h>
 
+#include <cstring>
+
 namespace MobileGL {
     template <typename Derived, typename T, SizeT N>
     struct VecBase {
@@ -84,6 +86,16 @@ namespace MobileGL {
         }
     };
 
+    // Bit-pattern equality, for a vector used as a cache or staleness KEY rather than as a
+    // number. IEEE `==` - which operator== above is - says a NaN never equals itself, so a single
+    // NaN component makes every comparison answer "changed" and whatever the key guards is
+    // rebuilt on every use, forever. Two zeros of opposite sign compare unequal here, which only
+    // ever costs one extra rebuild.
+    template <typename Derived, typename T, SizeT N>
+    Bool BitwiseEqual(const VecBase<Derived, T, N>& a, const VecBase<Derived, T, N>& b) {
+        return std::memcmp(a.data.data(), b.data.data(), sizeof(T) * N) == 0;
+    }
+
     template <typename T>
     struct Vec2 : public VecBase<Vec2<T>, T, 2> {
         using Base = VecBase<Vec2<T>, T, 2>;