[wasm] Grow table entries exponentially

In order to implement {dlsym} like functionality, toolchains might
generate code that grows the table by one element at a time (e.g.
Emscripten currently does that). To improve performance in such a case,
we over-allocate the backing store of the {WasmTableObject}. Whenever
the backing store grows, it grows at least by the old size of the table.
This ensures exponentially growth, avoiding too many re-allocations.

R=mstarzinger@chromium.org
CC=​ecmziegler@chromium.org

Bug: v8:10018
Change-Id: I502d590a89f7804363938a157b7ed2189283227a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1939051Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65210}

src/wasm/wasm-objects.cc

@@ -485,9 +485,20 @@ int WasmTableObject::Grow(Isolate* isolate, Handle<WasmTableObject> table,
   if (max_size - old_size < count) return -1;
 
   uint32_t new_size = old_size + count;
+  // Even with 2x over-allocation, there should not be an integer overflow.
+  STATIC_ASSERT(wasm::kV8MaxWasmTableSize <= kMaxInt / 2);
+  DCHECK_GE(kMaxInt, new_size);
+  int old_capacity = table->entries().length();
+  if (new_size > static_cast<uint32_t>(old_capacity)) {
+    int grow = static_cast<int>(new_size) - old_capacity;
+    // Grow at least by the old capacity, to implement exponential growing.
+    grow = std::max(grow, old_capacity);
+    // Never grow larger than the max size.
+    grow = std::min(grow, static_cast<int>(max_size - old_capacity));
     auto new_store = isolate->factory()->CopyFixedArrayAndGrow(
-      handle(table->entries(), isolate), count);
+        handle(table->entries(), isolate), grow);
     table->set_entries(*new_store, WriteBarrierMode::UPDATE_WRITE_BARRIER);
+  }
   table->set_current_length(new_size);
 
   Handle<FixedArray> dispatch_tables(table->dispatch_tables(), isolate);

src/wasm/wasm-objects.h

@@ -203,6 +203,8 @@ class V8_EXPORT_PRIVATE WasmTableObject : public JSObject {
  public:
   DECL_CAST(WasmTableObject)
 
+  // The entries array is at least as big as {current_length()}, but might be
+  // bigger to make future growth more efficient.
   DECL_ACCESSORS(entries, FixedArray)
   DECL_INT_ACCESSORS(current_length)
   // TODO(titzer): introduce DECL_I64_ACCESSORS macro