[Vulkan] Port ac6_ground_fix vertex-fetch nudge to SPIR-V

src/xenia/gpu/dxbc_shader_translator_fetch.cc

@@ -14,14 +14,9 @@
 #include "xenia/base/math.h"
 #include "xenia/base/string.h"
 #include "xenia/gpu/dxbc_shader_translator.h"
+#include "xenia/gpu/gpu_flags.h"
 #include "xenia/gpu/render_target_cache.h"
 
-DEFINE_bool(
-    ac6_ground_fix, false,
-    "This fixes(hide) issues with black ground in AC6. Use only in AC6. "
-    "Might cause issues in other titles.",
-    "HACKS");
-
 namespace xe {
 namespace gpu {
 using namespace ucode;

src/xenia/gpu/gpu_flags.cc

@@ -108,3 +108,9 @@ DEFINE_bool(
     "artifacts while pipelines are being created. When disabled, pipelines are "
     "created synchronously which causes stutter but no visual artifacts.",
     "GPU");
+
+DEFINE_bool(
+    ac6_ground_fix, false,
+    "This fixes(hide) issues with black ground in AC6. Use only in AC6. "
+    "Might cause issues in other titles.",
+    "HACKS");

src/xenia/gpu/gpu_flags.h

@@ -42,6 +42,8 @@ DECLARE_bool(async_shader_compilation);
 
 DECLARE_bool(gpu_3d_to_2d_texture);
 
+DECLARE_bool(ac6_ground_fix);
+
 #define XE_GPU_FINE_GRAINED_DRAW_SCOPES 1
 
 #endif  // XENIA_GPU_GPU_FLAGS_H_

src/xenia/gpu/spirv_shader_translator_fetch.cc

@@ -16,6 +16,7 @@
 #include "third_party/glslang/SPIRV/GLSL.std.450.h"
 #include "xenia/base/assert.h"
 #include "xenia/base/math.h"
+#include "xenia/gpu/gpu_flags.h"
 #include "xenia/gpu/render_target_cache.h"
 #include "xenia/gpu/spirv_compatibility.h"
 
@@ -81,6 +82,12 @@ void SpirvShaderTranslator::ProcessVertexFetchInstruction(
       if (instr.attributes.is_index_rounded) {
         index = builder_->createNoContractionBinOp(
             spv::OpFAdd, type_float_, index, builder_->makeFloatConstant(0.5f));
+      } else if (cvars::ac6_ground_fix) {
+        // UGLY HACK for AC6 copied from the DXBC translator.
+        // Proper fix requires accurate RCP implementation.
+        index = builder_->createNoContractionBinOp(
+            spv::OpFAdd, type_float_, index,
+            builder_->makeFloatConstant(0.00025f));
       }
       index = builder_->createUnaryOp(
           spv::OpConvertFToS, type_int_,