Dynamic recompilation

In computer science, dynamic recompilation (sometimes abbreviated to dynarec or the pseudo-acronym DRC) is a feature of some emulators and virtual machines, where the system may recompile some part of a program during execution. By compiling during execution, the system can tailor the generated code to reflect the program's run-time environment, and potentially produce more efficient code by exploiting information that is not available to a traditional static compiler.

Most dynamic recompilers are used to convert machine code between architectures at runtime. This is a task often needed in the emulation of legacy gaming platforms. In other cases, a system may employ dynamic recompilation as part of an adaptive optimization strategy to execute a portable program representation such as Java or .NET Common Language Runtime bytecodes. Full-speed debuggers also utilize dynamic recompilation to reduce the space overhead incurred in most deoptimization techniques, and other features such as dynamic thread migration.

The main tasks a dynamic recompiler has to perform are:

A dynamic recompiler may also perform some auxiliary tasks:

Suppose a program is being run in an emulator and needs to copy a null-terminated string. The program is compiled originally for a very simple processor. This processor can only copy a byte at a time, and must do so by first reading it from the source string into a register, then writing it from that register into the destination string. The original program might look something like this:

The emulator might be running on a processor which is similar, but extremely good at copying strings, and the emulator knows it can take advantage of this. It might recognize the string copy sequence of instructions and decide to rewrite them more efficiently just before execution, to speed up the emulation.

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