Interesting. I think numerical computing is a narrow enough domain where programs have very well-behaved control flow, which avoids most of the problems of trace compilation. Loops over branchy code, which are really common in general programs, are very difficult to make work well with tracing.

Numerical programs being very stable in terms of control is what enables GPU parallelization and loop optimizations in the long tradition of Fortran compilers. Optimizations like loop tiling, interchange, strip mining, etc aren't going to be easy to do with trace compilation.

Anyway my comment was more directed toward trace compilation in the context of dynamic languages, and there I think it's pretty well established it only works well for small programs.

and PyPy right?