Avoid many temporary allocations when serializing service protocol responses

runtime/vm/json_stream.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "platform/assert.h"
 
 #include "include/dart_native_api.h"
 #include "vm/dart_entry.h"
 #include "vm/debugger.h"
 #include "vm/json_stream.h"
 #include "vm/message.h"
 #include "vm/metrics.h"
 #include "vm/object.h"
 #include "vm/safepoint.h"
 #include "vm/service.h"
 #include "vm/service_event.h"
 #include "vm/thread_registry.h"
 #include "vm/timeline.h"
 #include "vm/unicode.h"
 
 
 namespace dart {
 
 #ifndef PRODUCT
 
+class MaybeOnStackBuffer {
+ public:
+  explicit MaybeOnStackBuffer(intptr_t size) {
+    if (size > kOnStackBufferCapacity) {
+      p_ = reinterpret_cast<char*>(malloc(size));
+    } else {
+      p_ = &buffer_[0];
+    }
+  }
+  ~MaybeOnStackBuffer() {
+    if (p_ != &buffer_[0]) free(p_);
+  }
+
+  char* p() { return p_; }
+
+ private:
+  static const intptr_t kOnStackBufferCapacity = 4096;
+  char* p_;
+  char buffer_[kOnStackBufferCapacity];
+};
+
+
 void AppendJSONStreamConsumer(Dart_StreamConsumer_State state,
                               const char* stream_name,
                               const uint8_t* buffer,
                               intptr_t buffer_length,
                               void* user_data) {
   if ((state == Dart_StreamConsumer_kStart) ||
       (state == Dart_StreamConsumer_kFinish)) {
     // Ignore.
     return;
   }
@@ skipping 453 matching lines @@
 }
 
 
 void JSONStream::PrintfValue(const char* format, ...) {
   PrintCommaIfNeeded();
 
   va_list args;
   va_start(args, format);
   intptr_t len = OS::VSNPrint(NULL, 0, format, args);
   va_end(args);
-  char* p = reinterpret_cast<char*>(malloc(len + 1));
+  MaybeOnStackBuffer mosb(len + 1);
+  char* p = mosb.p();
   va_start(args, format);
   intptr_t len2 = OS::VSNPrint(p, len + 1, format, args);
   va_end(args);
   ASSERT(len == len2);
   buffer_.AddChar('"');
-  AddEscapedUTF8String(p);
+  AddEscapedUTF8String(p, len);
   buffer_.AddChar('"');
-  free(p);
 }
 
 
 void JSONStream::PrintValue(const Object& o, bool ref) {
   PrintCommaIfNeeded();
   o.PrintJSON(this, ref);
 }
 
 
 void JSONStream::PrintValue(Breakpoint* bpt) {
@@ skipping 192 matching lines @@
   PrintValue(timeline_event_block);
 }
 
 
 void JSONStream::PrintfProperty(const char* name, const char* format, ...) {
   PrintPropertyName(name);
   va_list args;
   va_start(args, format);
   intptr_t len = OS::VSNPrint(NULL, 0, format, args);
   va_end(args);
-  char* p = reinterpret_cast<char*>(malloc(len + 1));
+  MaybeOnStackBuffer mosb(len + 1);
+  char* p = mosb.p();
   va_start(args, format);
   intptr_t len2 = OS::VSNPrint(p, len + 1, format, args);
   va_end(args);
   ASSERT(len == len2);
   buffer_.AddChar('"');
-  AddEscapedUTF8String(p);
+  AddEscapedUTF8String(p, len);
   buffer_.AddChar('"');
-  free(p);
 }
 
 
 void JSONStream::Steal(char** buffer, intptr_t* buffer_length) {
   ASSERT(buffer != NULL);
   ASSERT(buffer_length != NULL);
   *buffer_length = buffer_.length();
   *buffer = buffer_.Steal();
 }
 
@@ skipping 174 matching lines @@
 
 void JSONObject::AddFixedServiceId(const char* format, ...) const {
   // Mark that this id is fixed.
   AddProperty("fixedId", true);
   // Add the id property.
   stream_->PrintPropertyName("id");
   va_list args;
   va_start(args, format);
   intptr_t len = OS::VSNPrint(NULL, 0, format, args);
   va_end(args);
-  char* p = reinterpret_cast<char*>(malloc(len + 1));
+  MaybeOnStackBuffer mosb(len + 1);
+  char* p = mosb.p();
   va_start(args, format);
   intptr_t len2 = OS::VSNPrint(p, len + 1, format, args);
   va_end(args);
   ASSERT(len == len2);
   stream_->buffer_.AddChar('"');
-  stream_->AddEscapedUTF8String(p);
+  stream_->AddEscapedUTF8String(p, len);
   stream_->buffer_.AddChar('"');
-  free(p);
 }
 
 
 void JSONObject::AddLocation(const Script& script,
                              TokenPosition token_pos,
                              TokenPosition end_token_pos) const {
   JSONObject location(this, "location");
   location.AddProperty("type", "SourceLocation");
   location.AddProperty("script", script);
   location.AddProperty("tokenPos", token_pos);
@@ skipping 45 matching lines @@
   }
 }
 
 
 void JSONObject::AddPropertyF(const char* name, const char* format, ...) const {
   stream_->PrintPropertyName(name);
   va_list args;
   va_start(args, format);
   intptr_t len = OS::VSNPrint(NULL, 0, format, args);
   va_end(args);
-  char* p = reinterpret_cast<char*>(malloc(len + 1));
+  MaybeOnStackBuffer mosb(len + 1);
+  char* p = mosb.p();
   va_start(args, format);
   intptr_t len2 = OS::VSNPrint(p, len + 1, format, args);
   va_end(args);
   ASSERT(len == len2);
   stream_->buffer_.AddChar('"');
-  stream_->AddEscapedUTF8String(p);
+  stream_->AddEscapedUTF8String(p, len);
   stream_->buffer_.AddChar('"');
-  free(p);
 }
 
 
 void JSONArray::AddValueF(const char* format, ...) const {
   stream_->PrintCommaIfNeeded();
   va_list args;
   va_start(args, format);
   intptr_t len = OS::VSNPrint(NULL, 0, format, args);
   va_end(args);
-  char* p = reinterpret_cast<char*>(malloc(len + 1));
+  MaybeOnStackBuffer mosb(len + 1);
+  char* p = mosb.p();
   va_start(args, format);
   intptr_t len2 = OS::VSNPrint(p, len + 1, format, args);
   va_end(args);
   ASSERT(len == len2);
   stream_->buffer_.AddChar('"');
-  stream_->AddEscapedUTF8String(p);
+  stream_->AddEscapedUTF8String(p, len);
   stream_->buffer_.AddChar('"');
-  free(p);
 }
 
 #endif  // !PRODUCT
 
 }  // namespace dart