[asm.js] Tune initial buffer sizes and growing strategy.

This simplifies the growing strategy used in {ZoneBuffer} and also tunes
the initial sizes used for various instances of these buffers. Note that
such a {ZoneBuffer} is used for entire modules and individual function
bodies.

R=clemensh@chromium.org

Change-Id: I99a0898589984e1830c681845fabb0ed5f8317ab
Reviewed-on: https://chromium-review.googlesource.com/508711
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45419}

src/wasm/wasm-module-builder.cc

@@ -47,7 +47,7 @@ WasmFunctionBuilder::WasmFunctionBuilder(WasmModuleBuilder* builder)
       locals_(builder->zone()),
       signature_index_(0),
       func_index_(static_cast<uint32_t>(builder->functions_.size())),
-      body_(builder->zone()),
+      body_(builder->zone(), 256),
       i32_temps_(builder->zone()),
       i64_temps_(builder->zone()),
       f32_temps_(builder->zone()),

src/wasm/wasm-module-builder.h

@@ -20,7 +20,7 @@ namespace wasm {
 
 class ZoneBuffer : public ZoneObject {
  public:
-  static const uint32_t kInitialSize = 4096;
+  static constexpr size_t kInitialSize = 1024;
   explicit ZoneBuffer(Zone* zone, size_t initial = kInitialSize)
       : zone_(zone), buffer_(reinterpret_cast<byte*>(zone->New(initial))) {
     pos_ = buffer_;
@@ -125,7 +125,7 @@ class ZoneBuffer : public ZoneObject {
 
   void EnsureSpace(size_t size) {
     if ((pos_ + size) > end_) {
-      size_t new_size = 4096 + size + (end_ - buffer_) * 3;
+      size_t new_size = size + (end_ - buffer_) * 2;
       byte* new_buffer = reinterpret_cast<byte*>(zone_->New(new_size));
       memcpy(new_buffer, buffer_, (pos_ - buffer_));
       pos_ = new_buffer + (pos_ - buffer_);