Cold part of make_space() code moved to reallocate member function. (#4130)

Change makes make_space() code much friendlier for the compiler to inline which makes significant (measurable) performance improvements.

include/flatbuffers/flatbuffers.h

@@ -539,17 +539,7 @@ class vector_downward {
 
   uint8_t *make_space(size_t len) {
     if (len > static_cast<size_t>(cur_ - buf_)) {
-      auto old_size = size();
-      auto largest_align = AlignOf<largest_scalar_t>();
-      reserved_ += (std::max)(len, growth_policy(reserved_));
-      // Round up to avoid undefined behavior from unaligned loads and stores.
-      reserved_ = (reserved_ + (largest_align - 1)) & ~(largest_align - 1);
-      auto new_buf = allocator_.allocate(reserved_);
-      auto new_cur = new_buf + reserved_ - old_size;
-      memcpy(new_cur, cur_, old_size);
-      cur_ = new_cur;
-      allocator_.deallocate(buf_);
-      buf_ = new_buf;
+      reallocate(len);
     }
     cur_ -= len;
     // Beyond this, signed offsets may not have enough range:
@@ -593,6 +583,20 @@ class vector_downward {
   uint8_t *buf_;
   uint8_t *cur_;  // Points at location between empty (below) and used (above).
   const simple_allocator &allocator_;
+
+  void reallocate(size_t len) {
+    auto old_size = size();
+    auto largest_align = AlignOf<largest_scalar_t>();
+    reserved_ += (std::max)(len, growth_policy(reserved_));
+    // Round up to avoid undefined behavior from unaligned loads and stores.
+    reserved_ = (reserved_ + (largest_align - 1)) & ~(largest_align - 1);
+    auto new_buf = allocator_.allocate(reserved_);
+    auto new_cur = new_buf + reserved_ - old_size;
+    memcpy(new_cur, cur_, old_size);
+    cur_ = new_cur;
+    allocator_.deallocate(buf_);
+    buf_ = new_buf;
+  }
 };
 
 // Converts a Field ID to a virtual table offset.