GPUShaderCreateInfo: Add DepthWrite option

This option lets specify explicitely how the fragment shader will change
the fragment's depth.

source/blender/gpu/intern/gpu_shader_create_info.cc

@@ -65,6 +65,9 @@ void ShaderCreateInfo::finalize()
     if (info.early_fragment_test_) {
       early_fragment_test_ = true;
     }
+    if (info.depth_write_ != DepthWrite::ANY) {
+      depth_write_ = info.depth_write_;
+    }
 
     validate(info);
 

source/blender/gpu/intern/gpu_shader_create_info.hh

@@ -130,6 +130,17 @@ enum class BuiltinBits {
 };
 ENUM_OPERATORS(BuiltinBits, BuiltinBits::WORK_GROUP_SIZE);
 
+/**
+ * Follow convention described in:
+ * https://www.khronos.org/registry/OpenGL/extensions/ARB/ARB_conservative_depth.txt
+ */
+enum class DepthWrite {
+  ANY = 0,
+  GREATER,
+  LESS,
+  UNCHANGED,
+};
+
 /* Samplers & images. */
 enum class ImageType {
   /** Color samplers/image. */
@@ -273,6 +284,8 @@ struct ShaderCreateInfo {
   bool auto_resource_location_ = false;
   /** If true, force depth and stencil tests to always happen before fragment shader invocation. */
   bool early_fragment_test_ = false;
+  /** Allow optimisation when fragment shader writes to gl_FragDepth. */
+  DepthWrite depth_write_ = DepthWrite::ANY;
   /**
    * Maximum length of all the resource names including each null terminator.
    * Only for names used by gpu::ShaderInterface.
@@ -695,6 +708,13 @@ struct ShaderCreateInfo {
     return *(Self *)this;
   }
 
+  /* Defines how the fragment shader will write to gl_FragDepth. */
+  Self &depth_write(DepthWrite value)
+  {
+    depth_write_ = value;
+    return *(Self *)this;
+  }
+
   Self &auto_resource_location(bool value)
   {
     auto_resource_location_ = value;

source/blender/gpu/opengl/gl_shader.cc

@@ -217,6 +217,20 @@ static const char *to_string(const PrimitiveOut &layout)
   }
 }
 
+static const char *to_string(const DepthWrite &value)
+{
+  switch (value) {
+    case DepthWrite::ANY:
+      return "depth_any";
+    case DepthWrite::GREATER:
+      return "depth_greater";
+    case DepthWrite::LESS:
+      return "depth_less";
+    default:
+      return "depth_unchanged";
+  }
+}
+
 static void print_image_type(std::ostream &os,
                              const ImageType &type,
                              const ShaderCreateInfo::Resource::BindType bind_type)
@@ -585,6 +599,9 @@ std::string GLShader::fragment_interface_declare(const ShaderCreateInfo &info) c
   if (info.early_fragment_test_) {
     ss << "layout(early_fragment_tests) in;\n";
   }
+  if (GLEW_VERSION_4_2 || GLEW_ARB_conservative_depth) {
+    ss << "layout(" << to_string(info.depth_write_) << ") out float gl_FragDepth;\n";
+  }
   ss << "\n/* Outputs. */\n";
   for (const ShaderCreateInfo::FragOut &output : info.fragment_outputs_) {
     ss << "layout(location = " << output.index;