[wasm][gc] Add stress GC mode

Add a flag which causes wasm code gc to be triggered whenever any code
is found to be potentially dead. This mode found several bugs already,
and I plan to enable it in 'gc-stress' mode once all issues are fixed.

R=mstarzinger@chromium.org

Bug: v8:8217
Change-Id: If28d980ded98b77b9efe7446da74d857e3c5e1b7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1585720
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61039}

src/flag-definitions.h

@@ -694,6 +694,8 @@ DEFINE_NEG_IMPLICATION(wasm_interpret_all, wasm_lazy_compilation)
 DEFINE_NEG_IMPLICATION(wasm_interpret_all, wasm_tier_up)
 DEFINE_BOOL(wasm_code_gc, false, "enable garbage collection of wasm code")
 DEFINE_IMPLICATION(future, wasm_code_gc)
+DEFINE_BOOL(stress_wasm_code_gc, false,
+            "stress test garbage collection of wasm code")
 
 // Profiler flags.
 DEFINE_INT(frame_count, 1, "number of stack frames inspected by the profiler")

src/wasm/wasm-engine.cc

@@ -703,12 +703,17 @@ bool WasmEngine::AddPotentiallyDeadCode(WasmCode* code) {
   auto added = it->second->potentially_dead_code.insert(code);
   if (!added.second) return false;  // An entry already existed.
   new_potentially_dead_code_size_ += code->instructions().size();
-  // Trigger a GC if 1MiB plus 10% of committed code are potentially dead.
-  size_t dead_code_limit = 1 * MB + code_manager_.committed_code_space() / 10;
-  if (FLAG_wasm_code_gc && new_potentially_dead_code_size_ > dead_code_limit &&
-      !current_gc_info_) {
-    new_potentially_dead_code_size_ = 0;
-    TriggerGC();
+  if (FLAG_wasm_code_gc) {
+    // Trigger a GC if 1MiB plus 10% of committed code are potentially dead.
+    size_t dead_code_limit =
+        FLAG_stress_wasm_code_gc
+            ? 0
+            : 1 * MB + code_manager_.committed_code_space() / 10;
+    bool need_gc = new_potentially_dead_code_size_ > dead_code_limit;
+    if (need_gc && !current_gc_info_) {
+      new_potentially_dead_code_size_ = 0;
+      TriggerGC();
+    }
   }
   return true;
 }