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 perhaps produce more efficient code by exploiting information that is not available to a traditional static compiler.

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 could also utilize it to reduce the space overhead incurred in most deoptimization techniques, and many other features such as dynamic thread migration.

Example

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:

beginning:
    mov A,[first string pointer]    ; Put location of first character of source string
                                    ; in register A
    mov B,[second string pointer]   ; Put location of first character of destination string
                                    ; in register B
loop:
    mov C,[A]            ; Copy byte at address in register A to register C
    mov [B],C            ; Copy byte in register C to the address in register B
    inc A                ; Increment the address in register A to point to
                         ; the next byte
    inc B                ; Increment the address in register B to point to
                         ; the next byte
    cmp C,#0             ; Compare the data we just copied to 0 (string end marker)
    jnz loop             ; If it wasn't 0 then we have more to copy, so go back
                         ; and copy the next byte
end:                     ; If we didn't loop then we must have finished,
                         ; so carry on with something else.

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.

Say there is an instruction on our new processor called movs, specifically designed to copy strings efficiently. Our theoretical movs instruction copies 16 bytes at a time, without having to load them into register C in between, but will stop if it copies a 0 byte (which marks the end of a string) and set the zero flag. It also knows that the addresses of the strings will be in registers A and B, so it increments A and B by 16 every time it executes, ready for the next copy.

Our new recompiled code might look something like this:

beginning:
    mov A,[first string pointer]    ; Put location of first character of source string
                                    ; in register A
    mov B,[second string pointer]   ; Put location of first character of destination string
                                    ; in register B
loop:
    movs [B],[A]            ; Copy 16 bytes at address in register A to address
                            ; in register B, then increment A and B by 16
    jnz loop                ; If the zero flag isn't set then we haven't reached
                            ; the end of the string, so go back and copy some more.
end:                        ; If we didn't loop then we must have finished,
                            ; so carry on with something else.

There is an immediate speed benefit simply because the processor doesn't have to load so many instructions to do the same task, but also because the movs instruction is likely to be optimized by the processor designer to be more efficient than the sequence used in the first example (for example it may make better use of parallel execution in the processor to increment A and B while it is still copying bytes).

Applications using dynamic recompilation

• Many Java virtual machines feature dynamic recompilation.
• MAME uses dynamic recompilation in its CPU emulators for MIPS, SuperH, PowerPC and even the Voodoo graphics processing units.
• 1964, a Nintendo 64 emulator for x86 hardware.
• Apple's Rosetta for Mac OS X on x86, allows PowerPC code to be run on the x86 architecture.
• Later versions of the Mac 68K emulator used in Mac OS to run 680x0 code on the PowerPC hardware.
• Psyco, a specializing compiler for Python.
• The HP Dynamo project, an example of a transparent binary dynamic optimizer.
• The Vx32 virtual machine employs dynamic recompilation to create OS-independent x86 architecture sandboxes for safe application plugins.
• Microsoft Virtual PC for Mac, used to run x86 code on PowerPC.
• NullDC, a Sega Dreamcast emulator for x86.
• QEMU, an open-source full system emulator.
• FreeKEYB, an international DOS keyboard and console driver with many usability enhancements utilized self-modifying code and dynamic dead code elimination to minimize its memory image based on its user configuration (selected features, languages, layouts) and actual runtime environment (OS variant and version, loaded drivers, underlying hardware), automatically resolving dependencies, dynamically relocating and recombining code sections on byte-level granularity and optimizing opstrings based on semantic information provided in the source code, relocation information generated by special tools during assembly and profile information obtained at load time.
• The backwards compatibility functionality of the Xbox 360 (i.e. running games written for the original Xbox) is widely assumed to use dynamic recompilation.
• PSEmu Pro, a Sony PlayStation emulator.
• Ultrahle, the first Nintendo 64 emulator to fully run commercial games.
• PCSX2, a Sony PlayStation 2 emulator, has a recompiler called "microVU", the successor of "SuperVU".
• Dolphin, a Nintendo GameCube and Wii emulator, has a dynarec option.
• GCemu, a Nintendo GameCube emulator.
• OVPsim, a freely-available full system emulator.
• VirtualBox uses dynamic recompilation
• Valgrind, a programming tool for memory debugging, memory leak detection, and profiling, uses dynamic recompilation.
• Wii64, a Nintendo 64 emulator for the Wii.
• WiiSX, a Sony PlayStation emulator for the Nintendo Wii.
• Mupen64Plus, a multi-platform Nintendo 64 emulator.
• Yabause, a multi-platform Saturn emulator.

See also

• Comparison of platform virtual machines
• Binary translation
• Just-in-time compilation
• Binary recompiler

External links

• Dynamic recompiler tutorial