[wasm] Improve performance of WasmCodeRefScope

Holding an unordered set is surprisingly inefficient for large sets.
Switching to just a vector makes e.g. deserialization of large modules
30% faster. We pay in terms of memory usage though, so if there is ever
a use case where we are storing the same code objects multiple times, we
might want do add a deduplication algorithm which cleans up the vector
every now and then.

R=thibaudm@chromium.org

Bug: v8:11164
Change-Id: I3983ee7f6f04ea7678b8da49fb5cec369693dbc3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2647260
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72325}

src/wasm/wasm-code-manager.cc

@@ -2106,10 +2106,7 @@ WasmCodeRefScope::WasmCodeRefScope()
 WasmCodeRefScope::~WasmCodeRefScope() {
   DCHECK_EQ(this, current_code_refs_scope);
   current_code_refs_scope = previous_scope_;
-  std::vector<WasmCode*> code_ptrs;
-  code_ptrs.reserve(code_ptrs_.size());
-  code_ptrs.assign(code_ptrs_.begin(), code_ptrs_.end());
-  WasmCode::DecrementRefCount(VectorOf(code_ptrs));
+  WasmCode::DecrementRefCount(VectorOf(code_ptrs_));
 }
 
 // static
@@ -2117,9 +2114,8 @@ void WasmCodeRefScope::AddRef(WasmCode* code) {
   DCHECK_NOT_NULL(code);
   WasmCodeRefScope* current_scope = current_code_refs_scope;
   DCHECK_NOT_NULL(current_scope);
-  auto entry = current_scope->code_ptrs_.insert(code);
-  // If we added a new entry, increment the ref counter.
-  if (entry.second) code->IncRef();
+  current_scope->code_ptrs_.push_back(code);
+  code->IncRef();
 }
 
 const char* GetRuntimeStubName(WasmCode::RuntimeStubId stub_id) {

src/wasm/wasm-code-manager.h

@@ -9,7 +9,6 @@
 #include <map>
 #include <memory>
 #include <set>
-#include <unordered_set>
 #include <utility>
 #include <vector>
 
@@ -926,7 +925,7 @@ class V8_EXPORT_PRIVATE V8_NODISCARD WasmCodeRefScope {
 
  private:
   WasmCodeRefScope* const previous_scope_;
-  std::unordered_set<WasmCode*> code_ptrs_;
+  std::vector<WasmCode*> code_ptrs_;
 };
 
 // Similarly to a global handle, a {GlobalWasmCodeRef} stores a single