Patch message

gcc_component_ref.patch

@@ -1,7 +1,45 @@
 From ab8884cbf3712f939556fc7789bbf32132c7ff1a Mon Sep 17 00:00:00 2001
 From: York Jasper Niebuhr <yjnworkstation@gmail.com>
 Date: Fri, 17 Oct 2025 22:04:41 +0200
-Subject: [PATCH] PLUGIN_BUILD_COMPONENT_REF callback
+Subject: [PATCH] Add PLUGIN_BUILD_COMPONENT_REF callback
+
+This patch adds the PLUGIN_BUILD_COMPONENT_REF callback, which is invoked
+by the C front end when a COMPONENT_REF node is built. The callback
+receives a pointer to the COMPONENT_REF tree (of type 'tree *'). Plugins
+may replace the node by assigning through the pointer, but any
+replacement must be type-compatible with the original node.
+
+The callback allows plugins to observe or instrument struct member
+accesses that would otherwise be lost due to folding before the earliest
+possible plugin pass or hook. In particular, the fold_offsetof
+functionality removes all traces of type and member information in
+offsetof-like trees, leaving only an integer constant for plugins to
+inspect.
+
+A considered alternative was to disable fold_offsetof altogether.
+However, that prevents offsetof expressions from qualifying as
+constant-expressions; for example, static assertions can no longer be
+evaluated if they contain non-folded offsetof expressions. The callback
+provides fine-grained control over individual COMPONENT_REFs instead of
+universally changing folding behavior.
+
+A typical use case would be to replace a select set of COMPONENT_REF
+nodes with type-compatible expressions calling a placeholder function,
+e.g. __deferred_offsetof(type, member). These calls cannot be folded
+away and thus remain available for plugin analysis in later passes.
+Offsets not of interest can be left untouched, preserving their const
+qualification and use in static assertions.
+
+Allowing PLUGIN_BUILD_COMPONENT_REF to alter COMPONENT_REF nodes required
+minor adjustments to fold_offsetof, which assumes a specific input
+format. Code paths that cannot guarantee that format should now use
+fold_offsetof_maybe(), which attempts to fold normally but, on failure,
+casts the unfolded expression to the desired output type.
+
+If the callback is not used to alter COMPONENT_REF trees, there is **no
+change** in GCC’s behavior.
+
+Signed-off-by: York Jasper Niebuhr <yjnworkstation@gmail.com>
 
 ---
  gcc/c-family/c-common.cc | 48 +++++++++++++++++++++++++++++++---------