[blog] PCSX2 64bit Recompilation
Many 64 bit architectures have been proposed; however, the x86-64 (aka AMD64) architecture has picked up a lot of speed since its initial proposal a couple of years ago. Most 64bit CPUs today support it, so it looks like a good candidate for 64bit recompilation. The x86-64 architecture offers many more registers and can potentially speed up games by a significant amount. Up to now, Pcsx2 has largely been ignoring the 64 bit arena because there have been massive compatability issues, the developers weren't sure if it was really worth it, and adding a new bug-free and fast recompiler to the existing code base is a very painful process. Anyone seriously suggesting this to a dev would have been laughed out of the chat room. However, the upcoming 0.9.2 release is looking very stable and after doing some research, we have decided to add support for x86-64 recompilation, both for 64bit versions of Linux and Windows (yes, Linux support is returning).

Before going into technical details, I want to cover the current Pcsx2 recompilation model.

Pcsx2 Recompilation

Every different instruction set requires either an interpreter or a recompiler to execute it on the PC. Both are important in emulation. Interpreters are implemented with regular high-level languages and are platform independent. They are easy to program, easy to debug, but slow. They are extremely important for testing and debugging purposes. For example, interpreting a simple 32bit EE MIPS instruction (code) might look like:

switch(code>>26) {
case 0x02: // J - jump to
  pc = (code & 0x03ffffff)*4; // change the program counter
case 0x23: // LW - load word, sign extend
  gpr[Rt] = (long long)*(long*)(memory+gpr[Rs]+(short)code);
Recompilers, on the other hand, try to cut as many corners as possible. For example, we know the instruction at address 0x1000 will never change, so there is no reason why the CPU needs to execute the switch statement and decode the instruction every single time it executes it. So recompilers generate the minimal amount of assembly the CPU needs to execute to emulate that instruction. Because we're working with assembly, recompilation is a very platform dependent process.

Simple recompilers look at one instruction at a time and keep all target platform (in this case, the PS2) registers in memory. For every new instruction, the used registers are read from memory and stored in real CPU registers, then some instructions are executed, and finally the register with the result is stored back in memory. Before 0.9, Pcsx2 used to employ this type of recompilation.

More complex recompilers divide the code into simple blocks (no jumps/branches) and try to preserve target platform registers across instructions in the real CPU registers. There are many different types of register allocation algorithms using graph coloring. Such compilers might also do constant propagation elimination. A common pattern in the MIPS Emotion Engine is something like:

lui s0, 0x1000
lw s0, 0x2000(s0)

If we propagated the constants at the lw, we know that the read address is 0x10002000.

A little more complex recompiler will know that 0x10002000 corresponds to the IPU, so the assembly will call the IPU straight away (without worrying about memory location translation).

There are many such local optimizations, however they aren't enough. At the end of every block, all the registers will have to be pushed to memory because the next simple block that needs to be executed can't be predicted at recompilation time (ie: branch if x >= 0 depends on the value of x at runtime).

An even more complex recompiler can work on the global scale by finding out which simple blocks are connected to which. Once it knows, it can get rid of the register flushing at the end of every simple block by simply telling the next blocks to allocate the same real CPU register to the same target platform register. This is called global register allocation and sometimes uses Markov blankets for block synchronization. For those people that know Bayes nets, this is very similar, except it applies to the global simple block graph. Just think about the nodes necessary for making a specific node independent with respect to the whole graph. This will include the node's parents, children, and the children's parents. For those that just got lost... don't worry.

The Pcsx2 recompilers also use MMX and SSE(1/2/3) interchangeably. So an EE register can be in an MMX, SSE, or regular x86 register at any point in time depending on the current types of instructions (this is a nightmare to manage).

Console emulators rarely need to go through such complex recompilers because up until a couple of years ago, consoles weren't that powerful. But starting with the PS2, consoles got powerful and the Pcsx2 recompilers for the EmotionEngine and Vectors Units got complex really fast. Pcsx2 0.9.1 supports all the above mentioned optimizations plus many more unmentioned ones. The VU recompiler (code named SuperVU) is by far the most complex and fastest. Anyone who wants to keep their sanity should stay away from it.

For those that remember what it was like in the 0.8.1 days can appreciate how powerful the 0.9.1 Pcsx2 optimizations are.


So why isn't x86-32 enough? Well, for starters the Playstation 2 EE has 32 128bit regular registers, 32 32bit floating point registers, and some COP0 registers. Most instructions work on 64 bits, the MMI instructions work on the full 128bits. On the other hand, the x86 CPU has 8 32bit general purpose registers (one is for stack), 8 64bit registers (MMX), and 8 128bit registers(SSE). And you can't combine the three that easily (ie: you can't add an x86 register with a SSE register before first transferring the x86 to SSE or vice versa). So there's a very big difference in registers sizes. Because of the small number of x86 registers, the recompiler does a lot of register thrashing (registers are spilled to memory very frequently). Each memory read/write is pretty slow, so the more thrashing, the slower the recompiler becomes. Also, x86-32 is inherently 32bit, so a 64bit add would require 2 32bit instructions and 4 regular x86 registers for the source and result (2 if reading from memory). The EE recompiler tries to alleviate the register pressure by using the 64bit arithmetic capabilities of MMX, but MMX has a pretty limited ISA and intra-register set transfers kill performance.

The registers on the x86-64 architecture are: 16 64bit general purpose registers, 8 64bit MMX registers, and 16 128bit SSE registers. This amounts to twice the number of register memory! This means much less register thrashing. On top of that, 64bit adds/shifts/etc can all be done in one instruction.

However, the story isn't as simple as it sounds. The recompiler has to interface with regular C++ code constantly (ie: calling plugin functions), so the calling conventions on the recompiler boundaries must be followed exactly. The x86-64 specification can be found here and is pretty straightforward. However, Microsoft decided that it wanted its own specification (for reasons not quite known to anyone else).. so now there are two different calling conventions with a different set of registers specifying arguments to functions and another different set acting as non-volatile data! (Thanks Microsoft, it wasn't difficult enough)

Because the size of the registers changed, all pointers are now 64 bits, which adds many difficulties to reading and writing from memory, incrementing the stack, etc.

Virtual memory is yet another obstacle to overcome with 64bit OSs. The AWE mapping trick (described in an early blog) has to be refined. But now that the address range is much bigger, there are less limitations. VM builds for Linux also need a completely new implemenation.

Finally, if anyone has seen Pcsx2 code, they would know that inline assembly is pretty frequent in the recompilers. The reasons we use inline assembly rather than C++ code are many. Actually, some things like dynamic dispatching become impossible to do with C++ code. So, inline is necessary... and it looks like Microsoft has disabled all functionality for inline assembly in 64bit editions of Visual C++!!!! (Thanks again Microsoft, you just know where to strike hardest)

With all the mentioned challenges, it will take a couple of months to get things working reasonably stable. By that time, more people would have switched to 64bit OSs. If we're even half right in our estimates, Pcsx2 will run much faster on a 64bit OS than on a 32bit OS on the same computer once x86-64 recompilation is done.


Moral of the blog Most recompiler theory discussed here actually comes straight from compiler theory. Compilers will always be necessary as long as engineers keep coming with new instruction set architectures (ISAs). Learn how a compiler works. I recommend Compilers: Principles, Techniques, and Tools by Alfred V. Aho, Ravi Sethi, and Jeffrey D. Ullman.

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