[D3D12] DXBC: Fix two uninitialized register usages in ROV code, better comments

src/xenia/gpu/dxbc_shader_translator.cc

@@ -1333,14 +1333,16 @@ void DxbcShaderTranslator::StartTranslation() {
     system_temp_position_ = PushSystemTemp(0b1111);
   } else if (IsDxbcPixelShader()) {
     if (edram_rov_used_) {
+      // Will be initialized unconditionally.
       system_temp_rov_params_ = PushSystemTemp();
-      // If the shader doesn't write to depth, StartPixelShader will load the
-      // depth/stencil for early test, so no need to initialize. If it does,
-      // initialize it to something consistent - depth must be written on every
-      // shader execution path (at least in PC ps_3_0 and later shader models)
-      // and to make compilation easier.
+      // If the shader doesn't write to oDepth, each component will be written
+      // to if depth/stencil is enabled and the respective sample is covered -
+      // so need to initialize now because the first writes will be conditional.
+      // If the shader writes to oDepth, this is oDepth of the shader, written
+      // by the guest code, so initialize because assumptions can't be made
+      // about the integrity of the guest code.
       system_temp_rov_depth_stencil_ =
-          PushSystemTemp(writes_depth() ? 0b0001 : 0);
+          PushSystemTemp(writes_depth() ? 0b0001 : 0b1111);
     }
     for (uint32_t i = 0; i < 4; ++i) {
       if (writes_color_target(i)) {
@@ -1377,7 +1379,10 @@ void DxbcShaderTranslator::StartTranslation() {
       }
     }
 
-    // Allocate system temporary variables for the translated code.
+    // Allocate system temporary variables for the translated code. Since access
+    // depends on the guest code (thus no guarantees), initialize everything
+    // now (except for pv, it's an internal temporary variable, not accessible
+    // by the guest).
     system_temp_pv_ = PushSystemTemp();
     system_temp_ps_pc_p0_a0_ = PushSystemTemp(0b1111);
     system_temp_aL_ = PushSystemTemp(0b1111);

src/xenia/gpu/dxbc_shader_translator.h

@@ -49,6 +49,10 @@ namespace gpu {
 // optimization possibilities as this may result in false dependencies. Always
 // mov l(0, 0, 0, 0) to such components before potential branching -
 // PushSystemTemp accepts a zero mask for this purpose.
+//
+// Clamping must be done first to the lower bound (using max), then to the upper
+// bound (using min), to match the saturate modifier behavior, which results in
+// 0 for NaN.
 class DxbcShaderTranslator : public ShaderTranslator {
  public:
   DxbcShaderTranslator(uint32_t vendor_id, bool edram_rov_used);
@@ -1626,7 +1630,8 @@ class DxbcShaderTranslator : public ShaderTranslator {
                        uint32_t temp2_component);
   // Packs a float32x4 color value to 32bpp or a 64bpp in color_temp to
   // packed_temp.packed_temp_components, using 2 temporary VGPR. color_temp and
-  // packed_temp may be the same if packed_temp_components is 0.
+  // packed_temp may be the same if packed_temp_components is 0. If the format
+  // is 32bpp, will still the high part to break register dependency.
   void ROV_PackPreClampedColor(uint32_t rt_index, uint32_t color_temp,
                                uint32_t packed_temp,
                                uint32_t packed_temp_components, uint32_t temp1,
@@ -2002,7 +2007,8 @@ class DxbcShaderTranslator : public ShaderTranslator {
   // W - Base-relative resolution-scaled EDRAM offset for 64bpp color data, in
   //     dwords.
   uint32_t system_temp_rov_params_;
-  // ROV only - new depth/stencil data. 4 VGPRs.
+  // ROV only - new depth/stencil data. 4 VGPRs when not writing to oDepth, 1
+  // VGPR when writing to oDepth.
   // When not writing to oDepth: New per-sample depth/stencil values, generated
   // during early depth/stencil test (actual writing checks coverage bits).
   // When writing to oDepth: X also used to hold the depth written by the

src/xenia/gpu/dxbc_shader_translator_om.cc

@@ -880,6 +880,10 @@ void DxbcShaderTranslator::ROV_PackPreClampedColor(
   DxbcDest temp2_dest(DxbcDest::R(temp2, 1 << temp2_component));
   DxbcSrc temp2_src(DxbcSrc::R(temp2).Select(temp2_component));
 
+  // Break register dependency after 32bpp cases.
+  DxbcOpMov(DxbcDest::R(packed_temp, 1 << (packed_temp_components + 1)),
+            DxbcSrc::LU(0));
+
   // Choose the packing based on the render target's format.
   system_constants_used_ |= 1ull << kSysConst_EDRAMRTFormatFlags_Index;
   DxbcOpSwitch(DxbcSrc::CB(cbuffer_index_system_constants_,