Avoid many temporary allocations when serializing service protocol responses

runtime/vm/json_stream.cc

Index: runtime/vm/json_stream.cc
diff --git a/runtime/vm/json_stream.cc b/runtime/vm/json_stream.cc
index d95345b4953c184401adf3e477710531c5b33f80..3f50757364cfda4ba6cb9188184b4c86d3984aa0 100644
--- a/runtime/vm/json_stream.cc
+++ b/runtime/vm/json_stream.cc
@@ -490,21 +490,28 @@ void JSONStream::PrintValueNoEscape(const char* 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);
+  }
 }
 
 
@@ -712,20 +719,28 @@ void JSONStream::PrintProperty(const char* name,
 
 
 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);
+  }
 }
 
 
@@ -912,6 +927,8 @@ JSONObject::JSONObject(const JSONArray* arr) : stream_(arr->stream_) {
 
 
 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.
@@ -920,15 +937,20 @@ void JSONObject::AddFixedServiceId(const char* format, ...) const {
   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);
+  }
 }
 
 
@@ -989,38 +1011,52 @@ void JSONObject::AddUnresolvedLocation(
 
 
 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