src: avoid common case heap allocation

Optimize three functions that pass on (part of) their JS arguments to
the JS function they call by stack-allocating the storage in the common
case.

PR-URL: https://github.com/nodejs/node/pull/21409
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Eugene Ostroukhov <eostroukhov@google.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>

src/inspector_js_api.cc

@@ -131,11 +131,7 @@ void CallAndPauseOnStart(const FunctionCallbackInfo<v8::Value>& args) {
   Environment* env = Environment::GetCurrent(args);
   CHECK_GT(args.Length(), 1);
   CHECK(args[0]->IsFunction());
-  std::vector<v8::Local<v8::Value>> call_args;
-  for (int i = 2; i < args.Length(); i++) {
-    call_args.push_back(args[i]);
-  }
-
+  SlicedArguments call_args(args, /* start */ 2);
   env->inspector_agent()->PauseOnNextJavascriptStatement("Break on start");
   v8::MaybeLocal<v8::Value> retval =
       args[0].As<v8::Function>()->Call(env->context(), args[1],
@@ -150,10 +146,7 @@ void InspectorConsoleCall(const FunctionCallbackInfo<Value>& info) {
   HandleScope handle_scope(isolate);
   Local<Context> context = isolate->GetCurrentContext();
   CHECK_LT(2, info.Length());
-  std::vector<Local<Value>> call_args;
-  for (int i = 3; i < info.Length(); ++i) {
-    call_args.push_back(info[i]);
-  }
+  SlicedArguments call_args(info, /* start */ 3);
   Environment* env = Environment::GetCurrent(isolate);
   if (InspectorEnabled(env)) {
     Local<Value> inspector_method = info[0];

src/node_internals.h

@@ -40,6 +40,7 @@
 #include <stdlib.h>
 
 #include <string>
+#include <vector>
 
 // Custom constants used by both node_constants.cc and node_zlib.cc
 #define Z_MIN_WINDOWBITS 8
@@ -315,6 +316,39 @@ class FatalTryCatch : public v8::TryCatch {
   Environment* env_;
 };
 
+class SlicedArguments {
+ public:
+  inline explicit SlicedArguments(
+      const v8::FunctionCallbackInfo<v8::Value>& args,
+      size_t start = 0);
+  inline size_t size() const { return size_; }
+  inline v8::Local<v8::Value>* data() { return data_; }
+
+ private:
+  size_t size_;
+  v8::Local<v8::Value>* data_;
+  v8::Local<v8::Value> fixed_[64];
+  std::vector<v8::Local<v8::Value>> dynamic_;
+};
+
+SlicedArguments::SlicedArguments(
+    const v8::FunctionCallbackInfo<v8::Value>& args,
+    size_t start) : size_(0), data_(fixed_) {
+  const size_t length = static_cast<size_t>(args.Length());
+  if (start >= length) return;
+  const size_t size = length - start;
+
+  if (size > arraysize(fixed_)) {
+    dynamic_.resize(size);
+    data_ = dynamic_.data();
+  }
+
+  for (size_t i = 0; i < size; ++i)
+    data_[i] = args[i + start];
+
+  size_ = size;
+}
+
 void ReportException(Environment* env,
                      v8::Local<v8::Value> er,
                      v8::Local<v8::Message> message);

src/node_perf.cc

@@ -1,8 +1,6 @@
 #include "node_internals.h"
 #include "node_perf.h"
 
-#include <vector>
-
 #ifdef __POSIX__
 #include <sys/time.h>  // gettimeofday
 #endif
@@ -309,10 +307,7 @@ void TimerFunctionCall(const FunctionCallbackInfo<Value>& args) {
   Local<Function> fn = args.Data().As<Function>();
   size_t count = args.Length();
   size_t idx;
-  std::vector<Local<Value>> call_args;
-  for (size_t i = 0; i < count; ++i)
-    call_args.push_back(args[i]);
-
+  SlicedArguments call_args(args);
   Utf8Value name(isolate, GetName(fn));
 
   uint64_t start;