From 240b63151250c0d495a4ef196e719482fd6c69cf Mon Sep 17 00:00:00 2001
From: Norman Feske <norman.feske@genode-labs.com>
Date: Wed, 4 Sep 2024 16:13:40 +0200
Subject: [PATCH] nitpicker: alloc initial slab block in session

This patch saves two sliced-heap allocations per session.

Issue #5340
---
 repos/os/src/server/nitpicker/gui_session.h | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/repos/os/src/server/nitpicker/gui_session.h b/repos/os/src/server/nitpicker/gui_session.h
index 6221d60fd9..03dc0c9f7a 100644
--- a/repos/os/src/server/nitpicker/gui_session.h
+++ b/repos/os/src/server/nitpicker/gui_session.h
@@ -132,9 +132,20 @@ class Nitpicker::Gui_session : public  Session_object<Gui::Session>,
 
 		List<Session_view_list_elem> _view_list { };
 
-		Tslab<View, 4000> _view_alloc { &_session_alloc };
+		/*
+		 * Slab allocator that includes an initial block as member
+		 */
+		template <size_t BLOCK_SIZE>
+		struct Initial_slab_block { uint8_t buf[BLOCK_SIZE]; };
+		template <typename T, size_t BLOCK_SIZE>
+		struct Slab : private Initial_slab_block<BLOCK_SIZE>, Tslab<T, BLOCK_SIZE>
+		{
+			Slab(Allocator &block_alloc)
+			: Tslab<T, BLOCK_SIZE>(block_alloc, Initial_slab_block<BLOCK_SIZE>::buf) { };
+		};
 
-		Tslab<View_ref, 4000> _view_ref_alloc { &_session_alloc };
+		Slab<View,     4000> _view_alloc     { _session_alloc };
+		Slab<View_ref, 4000> _view_ref_alloc { _session_alloc };
 
 		bool const _provides_default_bg;