fix(vu0): pin vf0 to (0,0,0,1) in every constructed R5900Context

ps2xRuntime/include/ps2_runtime.h

@@ -147,6 +147,13 @@ struct alignas(16) R5900Context
         // Initialize VU0 registers
         vu0_q = 1.0f; // Q register usually initialized to 1.0
 
+        // vf0 is hardwired on real VU0 hardware to (x,y,z,w) = (0,0,0,1) and reads
+        // back that constant regardless of writes; it is the vector-unit analogue of
+        // GPR r0. memset above leaves it (0,0,0,0), so w reads 0 instead of 1. Pin it
+        // here so every context this runtime constructs starts hardware-correct.
+        // (_mm_set_ps arguments are in (w, z, y, x) order, so this is x=0,y=0,z=0,w=1.)
+        vu0_vf[0] = _mm_set_ps(1.0f, 0.0f, 0.0f, 0.0f);
+
         // Reset COP0 registers
         cop0_random = 47; // Start at maximum value
         // cop0_status = 0x400000; // BEV set, ERL clear, kernel mode

ps2xRuntime/src/lib/ps2_runtime.cpp

@@ -524,6 +524,10 @@ PS2Runtime::PS2Runtime()
     // R0 is always zero in MIPS
     m_cpuContext.r[0] = _mm_set1_epi32(0);
 
+    // vf0 is hardwired to (0,0,0,1) on VU0 hardware; the memset above cleared
+    // the constructor's pin, so repin it here, mirroring r0.
+    m_cpuContext.vu0_vf[0] = _mm_set_ps(1.0f, 0.0f, 0.0f, 0.0f);
+
     // Stack pointer (SP) and global pointer (GP) will be set by the loaded ELF
 
     m_loadedModules.clear();

ps2xTest/src/ps2_runtime_expansion_tests.cpp

@@ -2244,5 +2244,28 @@ void register_ps2_runtime_expansion_tests()
             std::memcpy(&directCmd, rdram.data() + kBaseAddr + 12u, sizeof(directCmd));
             t.Equals(directCmd, 0x50000001u, "sceVif1PkCloseDirectCode should store a 1-QW DIRECT length");
         });
+
+        tc.Run("R5900Context constructor pins vf0 to hardware (0,0,0,1)", [](TestCase &t)
+        {
+            R5900Context ctx;                       // default constructor is under test
+            alignas(16) float vf0[4]{};
+            _mm_storeu_ps(vf0, ctx.vu0_vf[0]);      // vf0[0..3] = x,y,z,w
+            t.Equals(vf0[0], 0.0f, "vf0.x must be 0");
+            t.Equals(vf0[1], 0.0f, "vf0.y must be 0");
+            t.Equals(vf0[2], 0.0f, "vf0.z must be 0");
+            t.Equals(vf0[3], 1.0f, "vf0.w must be 1 (VU0 hardware homogeneous constant)");
+        });
+
+        tc.Run("PS2Runtime's own CPU context has vf0 pinned to hardware (0,0,0,1)", [](TestCase &t)
+        {
+            PS2Runtime runtime;                     // runtime constructor's memset/repin is under test
+            const R5900Context &ctx = runtime.cpu();
+            alignas(16) float vf0[4]{};
+            _mm_storeu_ps(vf0, ctx.vu0_vf[0]);      // vf0[0..3] = x,y,z,w
+            t.Equals(vf0[0], 0.0f, "runtime vf0.x must be 0");
+            t.Equals(vf0[1], 0.0f, "runtime vf0.y must be 0");
+            t.Equals(vf0[2], 0.0f, "runtime vf0.z must be 0");
+            t.Equals(vf0[3], 1.0f, "runtime vf0.w must be 1 (VU0 hardware homogeneous constant)");
+        });
     });
 }