; src/alloc.c: Add a GC reg spill mechanism and Bug#41357 related commentary.

1 files changed, 17 insertions, 3 deletions

diff --git a/src/alloc.c b/src/alloc.c
index b7ebaa63a5b..d5a6d9167ea 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -4998,8 +4998,9 @@ mark_stack (char const *bottom, char const *end)
 #endif
 }
 
-/* This is a trampoline function that flushes registers to the stack,
-   and then calls FUNC.  ARG is passed through to FUNC verbatim.
+/* flush_stack_call_func is the trampoline function that flushes
+   registers to the stack, and then calls FUNC.  ARG is passed through
+   to FUNC verbatim.
 
    This function must be called whenever Emacs is about to release the
    global interpreter lock.  This lets the garbage collector easily
@@ -5007,7 +5008,20 @@ mark_stack (char const *bottom, char const *end)
    Lisp.
 
    It is invalid to run any Lisp code or to allocate any GC memory
-   from FUNC.  */
+   from FUNC.
+
+   Note: all register spilling is done in flush_stack_call_func before
+   flush_stack_call_func1 is activated.
+
+   flush_stack_call_func1 is responsible for identifying the stack
+   address range to be scanned.  It *must* be carefully kept as
+   noinline to make sure that registers has been spilled before it is
+   called, otherwise given __builtin_frame_address (0) typically
+   returns the frame pointer (base pointer) and not the stack pointer
+   [1] GC will miss to scan callee-saved registers content
+   (Bug#41357).
+
+   [1] <https://gcc.gnu.org/onlinedocs/gcc/Return-Address.html>.  */
 
 NO_INLINE void
 flush_stack_call_func1 (void (*func) (void *arg), void *arg)