Merge pull request #11603 from Dentomologist/fix_d3d_nan_regression

D3D: Restore workaround for erroneous NaN optimization

Source/Core/VideoCommon/ShaderGenCommon.cpp

@@ -100,7 +100,20 @@ std::string GetDiskShaderCacheFileName(APIType api_type, const char* type, bool
 
 void WriteIsNanHeader(ShaderCode& out, APIType api_type)
 {
-  out.Write("#define dolphin_isnan(f) isnan(f)\n");
+  if (api_type == APIType::D3D)
+  {
+    out.Write("bool dolphin_isnan(float f) {{\n"
+              "  // Workaround for the HLSL compiler deciding that isnan can never be true and\n"
+              "  // optimising away the call, even though the value can actually be NaN\n"
+              "  // Just look for the bit pattern that indicates NaN instead\n"
+              "  return (floatBitsToInt(f) & 0x7FFFFFFF) > 0x7F800000;\n"
+              "}}\n\n");
+    // If isfinite is needed, (floatBitsToInt(f) & 0x7F800000) != 0x7F800000 can be used
+  }
+  else
+  {
+    out.Write("#define dolphin_isnan(f) isnan(f)\n");
+  }
 }
 
 void WriteBitfieldExtractHeader(ShaderCode& out, APIType api_type,