Malloc is usually slower than GC-based allocation (which basically just increments a pointer). Of course, one can emulate this via custom allocators in C++. My guess is that Java development is just easier and quicker, and the JIT may even result in more effective optimizations than AOT compilation.
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In C++ you can link with your own version of malloc (one which just returns the next consecutive block, and can be implemented simply by adding the allocated size to a pointer).
in C++ they could also try using Profile-guided optimization to optimize hot code paths.