[wasm] Reuse RefNull() node in WasmCompiler

Constant reloading of the null reference impacted some wasm-gc
benchmarks. This CL speeds some benchmarks by >5%.
Note: This solution is not ideal as it technically generates invalid
turbofan graphs. It is temporary until a proper optimization eliminating
excessive loads is implemented.

Change-Id: I7afa6fb8857f5dba3dde715bd30fe868ad06d92c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2704668
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72894}

src/compiler/wasm-compiler.cc

@@ -538,9 +538,20 @@ Node* WasmGraphBuilder::EffectPhi(unsigned count, Node** effects_and_control) {
 }
 
 Node* WasmGraphBuilder::RefNull() {
-  return LOAD_FULL_POINTER(
-      BuildLoadIsolateRoot(),
-      IsolateData::root_slot_offset(RootIndex::kNullValue));
+  // Technically speaking, this does not generate a valid graph since the effect
+  // of the last Load is not consumed.
+  // TODO(manoskouk): Remove this code once we implement Load elimination
+  // optimization for wasm.
+  if (!ref_null_node_.is_set()) {
+    Node* current_effect = effect();
+    Node* current_control = control();
+    SetEffectControl(mcgraph()->graph()->start());
+    ref_null_node_.set(LOAD_FULL_POINTER(
+        BuildLoadIsolateRoot(),
+        IsolateData::root_slot_offset(RootIndex::kNullValue)));
+    SetEffectControl(current_effect, current_control);
+  }
+  return ref_null_node_.get();
 }
 
 Node* WasmGraphBuilder::RefFunc(uint32_t function_index) {

src/compiler/wasm-compiler.h

@@ -713,6 +713,7 @@ class WasmGraphBuilder {
   WasmInstanceCacheNodes* instance_cache_ = nullptr;
 
   SetOncePointer<Node> instance_node_;
+  SetOncePointer<Node> ref_null_node_;
   SetOncePointer<Node> globals_start_;
   SetOncePointer<Node> imported_mutable_globals_;
   SetOncePointer<Node> stack_check_code_node_;