Automatic Java to C++ conversion [duplicate]

Question

Has anyone tried automatic Java to C++ conversion for speed improvements? Is it a maintenance nightmare in the long run? Just read that is used to generate the HTML5 parsing engine in Gecko http://ejohn.org/blog/html-5-parsing/

Answer 1

In general, automatic conversions from one language to another will not be an improvement. Different languages have different idioms that affect performance.

The simplest example is with loops and variable creation. In a Java GC world, creating objects with new is almost free, and they dive into oblivion just as easily. In C++ memory allocation is (generally speaking) expensive:

// Sample java code
for ( int i = 0; i < 10000000; ++i )
{
   String str = new String( "hi" ); // new is free, GC is almost free for young objects
}

Direct conversion to C++ will result in bad performance (use of TR1 shared_ptr as memory handler instead of GC):

for ( int i = 0; i < 10000000; ++i )
{
   std::shared_ptr< std::string > str( new std::string( "hi" ) );
}

The equivalent loop written in C++ would be:

for ( int i = 0; i < 10000000; ++i )
{
   std::string str( "hi" );
}

Direct translation from a language to another usually ends with the worst of both worlds and harder to maintain code.