Re-implement Oscar's GC scrub with a safer approach

Instead of trashing pointer-like values on the stack, we fill dead objects
that is possibly referred to from the stack with garbage.

src/gc-debug.c

@@ -262,8 +262,10 @@ static int gc_debug_alloc_check(jl_alloc_num_t *num)
     return ((num->num - num->min) % num->interv) == 0;
 }
 
+static char *gc_stack_lo;
 static void gc_debug_init()
 {
+    gc_stack_lo = (char*)gc_get_stack_ptr();
     char *env = getenv("JL_GC_NO_GENERATIONAL");
     if (env && strcmp(env, "0") != 0) {
         gc_debug_env.sweep_mask = GC_MARKED;
@@ -301,6 +303,27 @@ static inline void gc_debug_print()
     gc_debug_print_status();
 }
 
+static void gc_scrub_range(char *stack_lo, char *stack_hi)
+{
+    stack_lo = (char*)((uintptr_t)stack_lo & ~(uintptr_t)15);
+    for (char **stack_p = (char**)stack_lo;
+         stack_p > (char**)stack_hi;stack_p--) {
+        char *p = *stack_p;
+        size_t osize;
+        jl_taggedvalue_t *tag = jl_gc_find_taggedvalue_pool(p, &osize);
+        if (!tag || gc_marked(tag) || osize <= sizeof_jl_taggedvalue_t)
+            continue;
+        jl_value_t *value = jl_valueof(tag);
+        size_t value_size = osize - sizeof_jl_taggedvalue_t;
+        memset(value, 0xff, value_size);
+    }
+}
+
+static void gc_scrub(char *stack_hi)
+{
+    gc_scrub_range(gc_stack_lo, stack_hi);
+}
+
 #else
 
 static inline int gc_debug_check_other()
@@ -321,4 +344,9 @@ static inline void gc_debug_init()
 {
 }
 
+static void gc_scrub(char *stack_hi)
+{
+    (void)stack_hi;
+}
+
 #endif

src/gc.c

@@ -260,6 +260,15 @@ static gcpage_t *page_metadata(void *data);
 static void pre_mark(void);
 static void post_mark(arraylist_t *list, int dryrun);
 static region_t *find_region(void *ptr, int maybe);
+jl_taggedvalue_t *jl_gc_find_taggedvalue_pool(char *p, size_t *osize_p);
+
+NOINLINE static uintptr_t gc_get_stack_ptr()
+{
+    void *dummy = NULL;
+    // The mask is to suppress the compiler warning about returning
+    // address of local variable
+    return (uintptr_t)&dummy & ~(uintptr_t)15;
+}
 
 #include "gc-debug.c"
 
@@ -1997,6 +2006,7 @@ void jl_gc_collect(int full)
 {
     if (!is_gc_enabled) return;
     if (jl_in_gc) return;
+    char *stack_hi = (char*)gc_get_stack_ptr();
     gc_debug_print();
     JL_SIGATOMIC_BEGIN();
     jl_in_gc = 1;
@@ -2098,6 +2108,7 @@ void jl_gc_collect(int full)
 #endif
             estimate_freed = live_bytes - scanned_bytes - perm_scanned_bytes + actual_allocd;
 
+            gc_scrub(stack_hi);
             gc_verify();
 
 #if defined(MEMPROFILE)