Bug 1266573 - Add the `JS::ubi::dumpPaths` debug utility; r=jimb

MozReview-Commit-ID: Gx37kFnasVl

--HG--
extra : rebase_source : b5ba6bf7853a4ab1753b168c0c304eace5739d5d

js/public/UbiNodeShortestPaths.h

@@ -330,6 +330,19 @@ struct JS_PUBLIC_API(ShortestPaths)
     }
 };
 
+#ifdef DEBUG
+// A helper function to dump the first `maxNumPaths` shortest retaining paths to
+// `node` from the GC roots. Useful when GC things you expect to have been
+// reclaimed by the collector haven't been!
+//
+// Usage:
+//
+//     JSObject* foo = ...;
+//     JS::ubi::dumpPaths(rt, JS::ubi::Node(foo));
+JS_PUBLIC_API(void)
+dumpPaths(JSRuntime* rt, Node node, uint32_t maxNumPaths = 10);
+#endif
+
 } // namespace ubi
 } // namespace JS
 

js/src/vm/UbiNodeShortestPaths.cpp

@@ -6,6 +6,9 @@
 
 #include "js/UbiNodeShortestPaths.h"
 
+#include "mozilla/Maybe.h"
+#include "mozilla/Move.h"
+
 #include "jsstr.h"
 
 namespace JS {
@@ -27,5 +30,49 @@ BackEdge::clone() const
     return mozilla::Move(clone);
 }
 
+#ifdef DEBUG
+JS_PUBLIC_API(void)
+dumpPaths(JSRuntime* rt, Node node, uint32_t maxNumPaths /* = 10 */)
+{
+    mozilla::Maybe<AutoCheckCannotGC> nogc;
+
+    JS::ubi::RootList rootList(rt, nogc);
+    MOZ_ASSERT(rootList.init());
+
+    NodeSet targets;
+    bool ok = targets.init() && targets.putNew(node);
+    MOZ_ASSERT(ok);
+
+    auto paths = ShortestPaths::Create(rt, nogc.ref(), maxNumPaths, &rootList, mozilla::Move(targets));
+    MOZ_ASSERT(paths.isSome());
+
+    int i = 0;
+    ok = paths->forEachPath(node, [&](Path& path) {
+        fprintf(stderr, "Path %d:\n", i++);
+        for (auto backEdge : path) {
+            fprintf(stderr, "    predecessor = %p\n", (void*) backEdge->predecessor().identifier());
+            fprintf(stderr, "            |\n");
+            fprintf(stderr, "            |\n");
+            fprintf(stderr, "        '");
+
+            const char16_t* name = backEdge->name().get();
+            if (!name)
+                name = (const char16_t*) MOZ_UTF16("<no edge name>");
+            auto len = js_strlen(name);
+            for (size_t i = 0; i < len; i++)
+                fprintf(stderr, "%c", char(name[i]));
+            fprintf(stderr, "'\n");
+
+            fprintf(stderr, "            |\n");
+            fprintf(stderr, "            V\n");
+        }
+
+        fprintf(stderr, "    target = %p\n\n\n", (void*) node.identifier());
+        return true;
+    });
+    MOZ_ASSERT(ok);
+}
+#endif
+
 } // namespace ubi
 } // namespace JS