correctly track element pointer in heap snapshot (#90)

Backports JuliaLang#51592.

Author: d-netto

src/gc.c

@@ -2061,7 +2061,8 @@ STATIC_INLINE void gc_mark_objarray(jl_ptls_t ptls, jl_value_t *obj_parent, jl_v
         // the first young object before starting this chunk
         // (this also would be valid for young objects, but probably less beneficial)
         for (; obj_begin < obj_end; obj_begin += step) {
-            new_obj = *obj_begin;
+            jl_value_t **slot = obj_begin;
+            new_obj = *slot;
             if (new_obj != NULL) {
                 verify_parent2("obj array", obj_parent, obj_begin, "elem(%d)",
                                gc_slot_to_arrayidx(obj_parent, obj_begin));
@@ -2070,7 +2071,7 @@ STATIC_INLINE void gc_mark_objarray(jl_ptls_t ptls, jl_value_t *obj_parent, jl_v
                     nptr |= 1;
                 if (!gc_marked(o->header))
                     break;
-                gc_heap_snapshot_record_array_edge(obj_parent, &new_obj);
+                gc_heap_snapshot_record_array_edge(obj_parent, slot);
             }
         }
     }
@@ -2092,12 +2093,13 @@ STATIC_INLINE void gc_mark_objarray(jl_ptls_t ptls, jl_value_t *obj_parent, jl_v
         }
     }
     for (; obj_begin < scan_end; obj_begin += step) {
+        jl_value_t **slot = obj_begin;
         new_obj = *obj_begin;
         if (new_obj != NULL) {
             verify_parent2("obj array", obj_parent, obj_begin, "elem(%d)",
                         gc_slot_to_arrayidx(obj_parent, obj_begin));
             gc_try_claim_and_push(mq, new_obj, &nptr);
-            gc_heap_snapshot_record_array_edge(obj_parent, &new_obj);
+            gc_heap_snapshot_record_array_edge(obj_parent, slot);
         }
     }
     if (too_big) {
@@ -2128,7 +2130,8 @@ STATIC_INLINE void gc_mark_array8(jl_ptls_t ptls, jl_value_t *ary8_parent, jl_va
         for (; ary8_begin < ary8_end; ary8_begin += elsize) {
             int early_end = 0;
             for (uint8_t *pindex = elem_begin; pindex < elem_end; pindex++) {
-                new_obj = ary8_begin[*pindex];
+                jl_value_t **slot = &ary8_begin[*pindex];
+                new_obj = *slot;
                 if (new_obj != NULL) {
                     verify_parent2("array", ary8_parent, &new_obj, "elem(%d)",
                                 gc_slot_to_arrayidx(ary8_parent, ary8_begin));
@@ -2139,7 +2142,7 @@ STATIC_INLINE void gc_mark_array8(jl_ptls_t ptls, jl_value_t *ary8_parent, jl_va
                         early_end = 1;
                         break;
                     }
-                    gc_heap_snapshot_record_array_edge(ary8_parent, &new_obj);
+                    gc_heap_snapshot_record_array_edge(ary8_parent, slot);
                 }
             }
             if (early_end)
@@ -2165,12 +2168,13 @@ STATIC_INLINE void gc_mark_array8(jl_ptls_t ptls, jl_value_t *ary8_parent, jl_va
     }
     for (; ary8_begin < ary8_end; ary8_begin += elsize) {
         for (uint8_t *pindex = elem_begin; pindex < elem_end; pindex++) {
-            new_obj = ary8_begin[*pindex];
+            jl_value_t **slot = &ary8_begin[*pindex];
+            new_obj = *slot;
             if (new_obj != NULL) {
                 verify_parent2("array", ary8_parent, &new_obj, "elem(%d)",
                                gc_slot_to_arrayidx(ary8_parent, ary8_begin));
                 gc_try_claim_and_push(mq, new_obj, &nptr);
-                gc_heap_snapshot_record_array_edge(ary8_parent, &new_obj);
+                gc_heap_snapshot_record_array_edge(ary8_parent, slot);
             }
         }
     }
@@ -2193,7 +2197,34 @@ STATIC_INLINE void gc_mark_array16(jl_ptls_t ptls, jl_value_t *ary16_parent, jl_
     jl_gc_markqueue_t *mq = &ptls->mark_queue;
     jl_value_t *new_obj;
     size_t elsize = ((jl_array_t *)ary16_parent)->elsize / sizeof(jl_value_t *);
-    // Decide whether need to chunk ary16
+    assert(elsize > 0);
+    // Decide whether need to chunk objary
+    if ((nptr & 0x2) == 0x2) {
+        // pre-scan this object: most of this object should be old, so look for
+        // the first young object before starting this chunk
+        // (this also would be valid for young objects, but probably less beneficial)
+        for (; ary16_begin < ary16_end; ary16_begin += elsize) {
+            int early_end = 0;
+            for (uint16_t *pindex = elem_begin; pindex < elem_end; pindex++) {
+                jl_value_t **slot = &ary16_begin[*pindex];
+                new_obj = *slot;
+                if (new_obj != NULL) {
+                    verify_parent2("array", ary16_parent, &new_obj, "elem(%d)",
+                                gc_slot_to_arrayidx(ary16_parent, ary16_begin));
+                    jl_taggedvalue_t *o = jl_astaggedvalue(new_obj);
+                    if (!gc_old(o->header))
+                        nptr |= 1;
+                    if (!gc_marked(o->header)){
+                        early_end = 1;
+                        break;
+                    }
+                    gc_heap_snapshot_record_array_edge(ary16_parent, slot);
+                }
+            }
+            if (early_end)
+                break;
+        }
+    }
     size_t too_big = (ary16_end - ary16_begin) / GC_CHUNK_BATCH_SIZE > elsize; // use this order of operations to avoid idiv
     jl_value_t **scan_end = ary16_end;
     int pushed_chunk = 0;
@@ -2213,12 +2244,13 @@ STATIC_INLINE void gc_mark_array16(jl_ptls_t ptls, jl_value_t *ary16_parent, jl_
     }
     for (; ary16_begin < scan_end; ary16_begin += elsize) {
         for (uint16_t *pindex = elem_begin; pindex < elem_end; pindex++) {
-            new_obj = ary16_begin[*pindex];
+            jl_value_t **slot = &ary16_begin[*pindex];
+            new_obj = *slot;
             if (new_obj != NULL) {
                 verify_parent2("array", ary16_parent, &new_obj, "elem(%d)",
                                gc_slot_to_arrayidx(ary16_parent, ary16_begin));
                 gc_try_claim_and_push(mq, new_obj, &nptr);
-                gc_heap_snapshot_record_array_edge(ary16_parent, &new_obj);
+                gc_heap_snapshot_record_array_edge(ary16_parent, slot);
             }
         }
     }