Hi all
We think some of our 'leak' issues can be attributed to libgc's
false-positive identification of pointers.
Attached is a proof-of-concept patch to libgc (and a simple
demonstration program) that I hope will be the start of GC precise stack
scanning. The code should apply easily to sgen as well.
It basically adds an extra variable to the stack which contains specific
markers and references to all the pointers that will contain GC-alloced
memory. There is an optional failsafe mode that will fall back to the
current 'all stack is scanned' code if the markers are not seen.
This code will cover objects on unmanaged stacks but I don't know what
will be needed for managed code. I presume the JIT can add the same
sort of marker to the stack?
So, comments? Is this technique going to be workable?
- Dick
diff -urBbw gc6.6-orig/include/gc.h gc6.6/include/gc.h
--- gc6.6-orig/include/gc.h 2005-05-20 18:50:58.0 +0100
+++ gc6.6/include/gc.h 2009-09-28 11:17:45.0 +0100
@@ -859,6 +859,20 @@
# define GC_PTR_STORE(p, q) *((p) = (q))
#endif
+#ifdef GC_PRECISE_STACK
+/* Keep the least significant bit set so these don't look like pointers */
+# define GC_PRECISE_STACK_BEGIN_MARKER 0xF00FF00F
+# define GC_PRECISE_STACK_END_MARKER 0xD00DD00D
+/* This macro is very gcc specific, with the 'unused' attribute to
+ * shut up the unused-variable warning, the volatile placement to foil the
+ * optimiser, and the '##vars' to suppress the leading comma when the args
+ * list is empty
+ */
+# define GC_STACK_REFERENCE(x, vars...) __attribute__((unused)) void * volatile __GC_stack_references_ ## x[] = {(void *)GC_PRECISE_STACK_BEGIN_MARKER, ##vars, (void *)GC_PRECISE_STACK_END_MARKER};
+#else
+# define GC_STACK_REFERENCE(x, vars...)
+#endif
+
/* Functions called to report pointer checking errors */
GC_API void (*GC_same_obj_print_proc) GC_PROTO((GC_PTR p, GC_PTR q));
diff -urBbw gc6.6-orig/pthread_stop_world.c gc6.6/pthread_stop_world.c
--- gc6.6-orig/pthread_stop_world.c 2005-09-09 18:54:32.0 +0100
+++ gc6.6/pthread_stop_world.c 2009-09-28 11:11:48.0 +0100
@@ -256,6 +256,10 @@
/* On IA64, we also need to scan the register backing store. */
IF_IA64(ptr_t bs_lo; ptr_t bs_hi;)
pthread_t me = pthread_self();
+#if GC_PRECISE_STACK
+ptr_t stack_ptr;
+int found_markers;
+#endif
if (!GC_thr_initialized) GC_thr_init();
#if DEBUG_THREADS
@@ -284,7 +288,7 @@
hi = GC_stackbottom;
IF_IA64(bs_lo = BACKING_STORE_BASE;)
}
-#if DEBUG_THREADS
+#if defined(DEBUG_THREADS) || defined(DEBUG_PRECISE_STACK)
GC_printf3(Stack for thread 0x%lx = [%lx,%lx)\n,
(unsigned long) p - id,
(unsigned long) lo, (unsigned long) hi);
@@ -292,10 +296,70 @@
if (0 == lo) ABORT(GC_push_all_stacks: sp not set!\n);
# ifdef STACK_GROWS_UP
/* We got them backwards! */
+# ifdef GC_PRECISE_STACK
+ found_markers = 0;
+
+ for (stack_ptr = hi; stack_ptr = lo; stack_ptr += sizeof(ptr_t)) {
+ word content = *(word *)stack_ptr;
+ if (content == GC_PRECISE_STACK_BEGIN_MARKER) {
+ ptr_t stack_ptr_end = stack_ptr;
+# ifdef DEBUG_PRECISE_STACK
+ GC_printf1(Found precise begin marker at 0x%lx\n, stack_ptr);
+# endif
+ found_markers = 1;
+ do {
+stack_ptr_end += sizeof(ptr_t);
+content = *(word *)stack_ptr_end;
+if (content != GC_PRECISE_STACK_END_MARKER) {
+ GC_push_all_stack ((ptr_t)content, (ptr_t)(content + sizeof(ptr_t)));
+}
+ } while (content != GC_PRECISE_STACK_END_MARKER
+stack_ptr_end lo);
+ stack_ptr = stack_ptr_end;
+ }
+ }
+
+# ifdef GC_PRECISE_STACK_FAILSAFE
+ if (!found_markers) {
+ GC_push_all_stack(hi, lo);
+ }
+# endif
+# else
GC_push_all_stack(hi, lo);
+# endif
+# else
+# ifdef GC_PRECISE_STACK
+ found_markers = 0;
+
+ for (stack_ptr = lo; stack_ptr = hi; stack_ptr += sizeof(ptr_t)) {
+ word content = *(word *)stack_ptr;
+ if (content == GC_PRECISE_STACK_BEGIN_MARKER) {
+ ptr_t stack_ptr_end = stack_ptr;
+# ifdef DEBUG_PRECISE_STACK
+ GC_printf1(Found precise begin marker at 0x%lx\n, stack_ptr);
+# endif
+ found_markers = 1;
+ do {
+stack_ptr_end += sizeof(ptr_t);
+content = *(word *)stack_ptr_end;
+if (content != GC_PRECISE_STACK_END_MARKER) {
+ GC_push_all_stack ((ptr_t)content, (ptr_t)(content + sizeof(ptr_t)));
+}
+ } while (content != GC_PRECISE_STACK_END_MARKER
+stack_ptr_end hi);
+ stack_ptr = stack_ptr_end;
+ }
+ }
+
+# ifdef GC_PRECISE_STACK_FAILSAFE
+ if (!found_markers) {
+ GC_push_all_stack(lo, hi);
+ }
+# endif
# else
GC_push_all_stack(lo, hi);
# endif
+# endif
# ifdef IA64
# if DEBUG_THREADS
GC_printf3(Reg stack for thread 0x%lx = [%lx,%lx)\n,
#define GC_PRECISE_STACK
#include gc.h
#include stdio.h
#include stdlib.h
typedef struct {
int val;
void *ptr;
} Object;
void
