GPU: UniformBuffer: Add possibility to bind as SSBO

This way UBOs can be modified directly in shader just like VBOs and IBOs.

source/blender/gpu/GPU_uniform_buffer.h

@@ -39,6 +39,7 @@ void GPU_uniformbuf_free(GPUUniformBuf *ubo);
 void GPU_uniformbuf_update(GPUUniformBuf *ubo, const void *data);
 
 void GPU_uniformbuf_bind(GPUUniformBuf *ubo, int slot);
+void GPU_uniformbuf_bind_as_ssbo(GPUUniformBuf *ubo, int slot);
 void GPU_uniformbuf_unbind(GPUUniformBuf *ubo);
 void GPU_uniformbuf_unbind_all(void);
 

source/blender/gpu/intern/gpu_uniform_buffer.cc

@@ -218,6 +218,11 @@ void GPU_uniformbuf_bind(GPUUniformBuf *ubo, int slot)
   unwrap(ubo)->bind(slot);
 }
 
+void GPU_uniformbuf_bind_as_ssbo(GPUUniformBuf *ubo, int slot)
+{
+  unwrap(ubo)->bind_as_ssbo(slot);
+}
+
 void GPU_uniformbuf_unbind(GPUUniformBuf *ubo)
 {
   unwrap(ubo)->unbind();

source/blender/gpu/intern/gpu_uniform_buffer_private.hh

@@ -39,6 +39,7 @@ class UniformBuf {
 
   virtual void update(const void *data) = 0;
   virtual void bind(int slot) = 0;
+  virtual void bind_as_ssbo(int slot) = 0;
   virtual void unbind() = 0;
 
   /** Used to defer data upload at drawing time.

source/blender/gpu/metal/mtl_uniform_buffer.hh

@@ -35,6 +35,7 @@ class MTLUniformBuf : public UniformBuf {
 
   void update(const void *data) override;
   void bind(int slot) override;
+  void bind_as_ssbo(int slot) override;
   void unbind() override;
 
   id<MTLBuffer> get_metal_buffer(int *r_offset);

source/blender/gpu/metal/mtl_uniform_buffer.mm

@@ -113,6 +113,16 @@ void MTLUniformBuf::bind(int slot)
   }
 }
 
+void MTLUniformBuf::bind_as_ssbo(int slot)
+{
+  if (slot < 0) {
+    MTL_LOG_WARNING("Failed to bind UBO %p as SSBO. uniform location %d invalid.\n", this, slot);
+    return;
+  }
+
+  BLI_assert_msg(0, "Not implemented yet");
+}
+
 void MTLUniformBuf::unbind()
 {
   /* Unbind in debug mode to validate missing binds.

source/blender/gpu/opengl/gl_uniform_buffer.cc

@@ -92,6 +92,19 @@ void GLUniformBuf::bind(int slot)
 #endif
 }
 
+void GLUniformBuf::bind_as_ssbo(int slot)
+{
+  if (ubo_id_ == 0) {
+    this->init();
+  }
+  if (data_ != nullptr) {
+    this->update(data_);
+    MEM_SAFE_FREE(data_);
+  }
+
+  glBindBufferBase(GL_SHADER_STORAGE_BUFFER, slot, ubo_id_);
+}
+
 void GLUniformBuf::unbind()
 {
 #ifdef DEBUG

source/blender/gpu/opengl/gl_uniform_buffer.hh

@@ -30,6 +30,7 @@ class GLUniformBuf : public UniformBuf {
 
   void update(const void *data) override;
   void bind(int slot) override;
+  void bind_as_ssbo(int slot) override;
   void unbind() override;
 
  private: