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"
@@ skipping 472 matching lines @@
 }
 
 
 void JSONStream::PrintValueNoEscape(const char* s) {
   PrintCommaIfNeeded();
   buffer_.Printf("%s", s);
 }
 
 
 void JSONStream::PrintfValue(const char* format, ...) {
+  const intptr_t kOnStackBufferCapacity = 4096;

[zra, 2017/03/31 17:47:09]

Maybe something like:

class MaybeOnStackBuffer {
 public:
  MaybeOnStackBuffer(intptr_t size) {
    if (size > kOnStackBufferCapacity) {
      p_ = malloc(size);
    } else {
      p_ = &buffer_[0];
    }
  }
  ~MaybeOnStackBuffer() {
    if (p_ != &buffer_[0]) free(p_);
  }

  char* p() { return p_; }

 private:
  char* p_;
  char buffer_[kOnStackBufferCapacity];
}

[Cutch, 2017/03/31 18:04:47]

Done.

+  char on_stack_buffer[kOnStackBufferCapacity];
   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));
+  char* p = &on_stack_buffer[0];
+  if (len + 1 >= kOnStackBufferCapacity) {

[siva, 2017/03/31 17:59:33]

((len + 1) >= kOnStackBufferCapacity) here and below.

[Cutch, 2017/03/31 18:04:47]

Acknowledged.

+    p = reinterpret_cast<char*>(malloc(len + 1));
+  }
   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);
+  if (p != &on_stack_buffer[0]) {
+    free(p);
+  }
 }
 
 
 void JSONStream::PrintValue(const Object& o, bool ref) {
   PrintCommaIfNeeded();
   o.PrintJSON(this, ref);
 }
 
 
 void JSONStream::PrintValue(Breakpoint* bpt) {
@@ skipping 187 matching lines @@
 
 
 void JSONStream::PrintProperty(const char* name,
                                const TimelineEventBlock* timeline_event_block) {
   PrintPropertyName(name);
   PrintValue(timeline_event_block);
 }
 
 
 void JSONStream::PrintfProperty(const char* name, const char* format, ...) {
+  const intptr_t kOnStackBufferCapacity = 4096;
+  char on_stack_buffer[kOnStackBufferCapacity];
   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));
+  char* p = &on_stack_buffer[0];
+  if (len + 1 >= kOnStackBufferCapacity) {
+    p = reinterpret_cast<char*>(malloc(len + 1));
+  }
+
   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);
+  if (p != &on_stack_buffer[0]) {
+    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 166 matching lines @@
   return (offset > 0) || (limit < length);
 }
 
 
 JSONObject::JSONObject(const JSONArray* arr) : stream_(arr->stream_) {
   stream_->OpenObject();
 }
 
 
 void JSONObject::AddFixedServiceId(const char* format, ...) const {
+  const intptr_t kOnStackBufferCapacity = 4096;
+  char on_stack_buffer[kOnStackBufferCapacity];
   // 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));
+  char* p = &on_stack_buffer[0];
+  if (len + 1 >= kOnStackBufferCapacity) {
+    p = reinterpret_cast<char*>(malloc(len + 1));
+  }
   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);
+  if (p != &on_stack_buffer[0]) {
+    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 40 matching lines @@
       location.AddProperty("column", bpt_loc->requested_column_number());
     }
   } else {
     // This unresolved breakpoint was requested at some function entry.
     location.AddProperty("tokenPos", token_pos);
   }
 }
 
 
 void JSONObject::AddPropertyF(const char* name, const char* format, ...) const {
+  const intptr_t kOnStackBufferCapacity = 4096;
+  char on_stack_buffer[kOnStackBufferCapacity];
   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));
+  char* p = &on_stack_buffer[0];
+  if (len + 1 >= kOnStackBufferCapacity) {
+    p = reinterpret_cast<char*>(malloc(len + 1));
+  }
   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);
+  if (p != &on_stack_buffer[0]) {
+    free(p);
+  }
 }
 
 
 void JSONArray::AddValueF(const char* format, ...) const {
+  const intptr_t kOnStackBufferCapacity = 4096;
+  char on_stack_buffer[kOnStackBufferCapacity];
   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));
+  char* p = &on_stack_buffer[0];
+  if (len + 1 >= kOnStackBufferCapacity) {
+    p = reinterpret_cast<char*>(malloc(len + 1));
+  }
   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);
+  if (p != &on_stack_buffer[0]) {
+    free(p);
+  }
 }
 
 #endif  // !PRODUCT
 
 }  // namespace dart