GPU: Optimize GLStorageBuf::read performance

Add a separate persistent mapped buffer where the main SSBO can be
copied, so its contents can be read from the CPU without stalling the
GPU.

Pull Request: https://projects.blender.org/blender/blender/pulls/117521

source/blender/gpu/GPU_storage_buffer.h

@@ -63,9 +63,8 @@ void GPU_storagebuf_sync_to_host(GPUStorageBuf *ssbo);
  * If pending GPU updates to the storage buffer are not yet visible to the host, the command will
  * stall until dependent GPU work has completed.
  *
- * Otherwise, this command is unsynchronized and will return current visible storage buffer
- * contents immediately.
- * Alternatively, use appropriate barrier or GPU_finish before reading.
+ * Otherwise, this command is synchronized against this call and will stall the CPU until the
+ * buffer content can be read by the host.
  */
 void GPU_storagebuf_read(GPUStorageBuf *ssbo, void *data);
 

source/blender/gpu/opengl/gl_storage_buffer.cc

@@ -33,6 +33,25 @@ GLStorageBuf::GLStorageBuf(size_t size, GPUUsageType usage, const char *name)
 
 GLStorageBuf::~GLStorageBuf()
 {
+  if (read_fence_) {
+    glDeleteSync(read_fence_);
+  }
+
+  if (persistent_ptr_) {
+    if (GLContext::direct_state_access_support) {
+      glUnmapNamedBuffer(read_ssbo_id_);
+    }
+    else {
+      glBindBuffer(GL_SHADER_STORAGE_BUFFER, read_ssbo_id_);
+      glUnmapBuffer(GL_SHADER_STORAGE_BUFFER);
+      glBindBuffer(GL_SHADER_STORAGE_BUFFER, 0);
+    }
+  }
+
+  if (read_ssbo_id_) {
+    GLContext::buf_free(read_ssbo_id_);
+  }
+
   GLContext::buf_free(ssbo_id_);
 }
 
@@ -58,6 +77,7 @@ void GLStorageBuf::update(const void *data)
   if (ssbo_id_ == 0) {
     this->init();
   }
+
   glBindBuffer(GL_SHADER_STORAGE_BUFFER, ssbo_id_);
   glBufferSubData(GL_SHADER_STORAGE_BUFFER, 0, size_in_bytes_, data);
   glBindBuffer(GL_SHADER_STORAGE_BUFFER, 0);
@@ -162,25 +182,60 @@ void GLStorageBuf::copy_sub(VertBuf *src_, uint dst_offset, uint src_offset, uin
 }
 
 void GLStorageBuf::async_flush_to_host()
-{
-  GPU_memory_barrier(GPU_BARRIER_BUFFER_UPDATE);
-}
-
-void GLStorageBuf::read(void *data)
 {
   if (ssbo_id_ == 0) {
     this->init();
   }
 
+  if (read_ssbo_id_ == 0) {
+    glGenBuffers(1, &read_ssbo_id_);
+    glBindBuffer(GL_SHADER_STORAGE_BUFFER, read_ssbo_id_);
+    glBufferStorage(GL_SHADER_STORAGE_BUFFER,
+                    size_in_bytes_,
+                    nullptr,
+                    GL_MAP_PERSISTENT_BIT | GL_MAP_READ_BIT);
+    persistent_ptr_ = glMapBufferRange(
+        GL_SHADER_STORAGE_BUFFER, 0, size_in_bytes_, GL_MAP_PERSISTENT_BIT | GL_MAP_READ_BIT);
+    BLI_assert(persistent_ptr_);
+    debug::object_label(GL_SHADER_STORAGE_BUFFER, read_ssbo_id_, name_);
+  }
+
   if (GLContext::direct_state_access_support) {
-    glGetNamedBufferSubData(ssbo_id_, 0, size_in_bytes_, data);
+    glCopyNamedBufferSubData(ssbo_id_, read_ssbo_id_, 0, 0, size_in_bytes_);
   }
   else {
-    /* This binds the buffer to GL_ARRAY_BUFFER and upload the data if any. */
-    glBindBuffer(GL_SHADER_STORAGE_BUFFER, ssbo_id_);
-    glGetBufferSubData(GL_SHADER_STORAGE_BUFFER, 0, size_in_bytes_, data);
-    glBindBuffer(GL_SHADER_STORAGE_BUFFER, 0);
+    glBindBuffer(GL_COPY_READ_BUFFER, ssbo_id_);
+    glBindBuffer(GL_COPY_WRITE_BUFFER, read_ssbo_id_);
+    glCopyBufferSubData(GL_SHADER_STORAGE_BUFFER, GL_COPY_WRITE_BUFFER, 0, 0, size_in_bytes_);
+    glBindBuffer(GL_COPY_READ_BUFFER, 0);
+    glBindBuffer(GL_COPY_WRITE_BUFFER, 0);
   }
+
+  glMemoryBarrier(GL_CLIENT_MAPPED_BUFFER_BARRIER_BIT);
+
+  if (read_fence_) {
+    glDeleteSync(read_fence_);
+  }
+  read_fence_ = glFenceSync(GL_SYNC_GPU_COMMANDS_COMPLETE, 0);
+}
+
+void GLStorageBuf::read(void *data)
+{
+  if (data == nullptr) {
+    return;
+  }
+
+  if (!persistent_ptr_ || !read_fence_) {
+    this->async_flush_to_host();
+  }
+
+  while (glClientWaitSync(read_fence_, GL_SYNC_FLUSH_COMMANDS_BIT, 1000) == GL_TIMEOUT_EXPIRED) {
+    /* Repeat until the data is ready.*/
+  }
+  glDeleteSync(read_fence_);
+  read_fence_ = 0;
+
+  memcpy(data, persistent_ptr_, size_in_bytes_);
 }
 
 void GLStorageBuf::sync_as_indirect_buffer()

source/blender/gpu/opengl/gl_storage_buffer.hh

@@ -26,6 +26,10 @@ class GLStorageBuf : public StorageBuf {
   GLuint ssbo_id_ = 0;
   /** Usage type. */
   GPUUsageType usage_;
+  /* Read */
+  GLuint read_ssbo_id_ = 0;
+  GLsync read_fence_ = 0;
+  void *persistent_ptr_ = nullptr;
 
  public:
   GLStorageBuf(size_t size, GPUUsageType usage, const char *name);