[Profile] Add Heap Snapshot tool (#46862)

We expose a function `Profile.take_heap_snapshot(file)`, which writes a
heap snapshot in Chrome's .heapsnapshot JSON format to the given IO
stream.

This can be loaded into Chrome Devtools' snapshot viewer to explore the
heap and find memory leaks.

Co-Authored-By: Dean De Leo <dean.deleo@relational.ai>
Co-Authored-By: Nathan Daly <NHDaly@gmail.com>
Co-Authored-By: Pete Vilter <7341+vilterp@users.noreply.github.com>
Co-Authored-By: Valentin Churavy <v.churavy@gmail.com>
Co-Authored-By: Jameson Nash <vtjnash@gmail.com>

src/Makefile

@@ -45,7 +45,7 @@ SRCS := \
 	dlload sys init task array dump staticdata toplevel jl_uv datatype \
 	simplevector runtime_intrinsics precompile jloptions \
 	threading partr stackwalk gc gc-debug gc-pages gc-stacks gc-alloc-profiler method \
-	jlapi signal-handling safepoint timing subtype rtutils \
+	jlapi signal-handling safepoint timing subtype rtutils gc-heap-snapshot \
 	crc32c APInt-C processor ircode opaque_closure codegen-stubs coverage runtime_ccall
 
 RT_LLVMLINK :=
@@ -293,7 +293,9 @@ $(BUILDDIR)/disasm.o $(BUILDDIR)/disasm.dbg.obj: $(SRCDIR)/debuginfo.h $(SRCDIR)
 $(BUILDDIR)/dump.o $(BUILDDIR)/dump.dbg.obj: $(addprefix $(SRCDIR)/,common_symbols1.inc common_symbols2.inc builtin_proto.h serialize.h)
 $(BUILDDIR)/gc-debug.o $(BUILDDIR)/gc-debug.dbg.obj: $(SRCDIR)/gc.h
 $(BUILDDIR)/gc-pages.o $(BUILDDIR)/gc-pages.dbg.obj: $(SRCDIR)/gc.h
-$(BUILDDIR)/gc.o $(BUILDDIR)/gc.dbg.obj: $(SRCDIR)/gc.h $(SRCDIR)/gc-alloc-profiler.h
+$(BUILDDIR)/gc.o $(BUILDDIR)/gc.dbg.obj: $(SRCDIR)/gc.h $(SRCDIR)/gc-heap-snapshot.h $(SRCDIR)/gc-alloc-profiler.h
+$(BUILDDIR)/gc-heap-snapshot.o $(BUILDDIR)/gc-heap-snapshot.dbg.obj: $(SRCDIR)/gc.h $(SRCDIR)/gc-heap-snapshot.h
+$(BUILDDIR)/gc-alloc-profiler.o $(BUILDDIR)/gc-alloc-profiler.dbg.obj: $(SRCDIR)/gc.h $(SRCDIR)/gc-alloc-profiler.h
 $(BUILDDIR)/init.o $(BUILDDIR)/init.dbg.obj: $(SRCDIR)/builtin_proto.h
 $(BUILDDIR)/interpreter.o $(BUILDDIR)/interpreter.dbg.obj: $(SRCDIR)/builtin_proto.h
 $(BUILDDIR)/jitlayers.o $(BUILDDIR)/jitlayers.dbg.obj: $(SRCDIR)/jitlayers.h $(SRCDIR)/codegen_shared.h $(SRCDIR)/debug-registry.h

src/gc-debug.c

@@ -1227,20 +1227,17 @@ void gc_count_pool(void)
     jl_safe_printf("************************\n");
 }
 
-int gc_slot_to_fieldidx(void *obj, void *slot)
+int gc_slot_to_fieldidx(void *obj, void *slot, jl_datatype_t *vt) JL_NOTSAFEPOINT
 {
-    jl_datatype_t *vt = (jl_datatype_t*)jl_typeof(obj);
     int nf = (int)jl_datatype_nfields(vt);
-    for (int i = 0; i < nf; i++) {
-        void *fieldaddr = (char*)obj + jl_field_offset(vt, i);
-        if (fieldaddr >= slot) {
-            return i;
-        }
+    for (int i = 1; i < nf; i++) {
+        if (slot < (void*)((char*)obj + jl_field_offset(vt, i)))
+            return i - 1;
     }
-    return -1;
+    return nf - 1;
 }
 
-int gc_slot_to_arrayidx(void *obj, void *_slot)
+int gc_slot_to_arrayidx(void *obj, void *_slot) JL_NOTSAFEPOINT
 {
     char *slot = (char*)_slot;
     jl_datatype_t *vt = (jl_datatype_t*)jl_typeof(obj);
@@ -1258,8 +1255,6 @@ int gc_slot_to_arrayidx(void *obj, void *_slot)
     }
     else if (vt->name == jl_array_typename) {
         jl_array_t *a = (jl_array_t*)obj;
-        if (!a->flags.ptrarray)
-            return -1;
         start = (char*)a->data;
         len = jl_array_len(a);
         elsize = a->elsize;