x86 vs x64

[omega_rugal]

This has been bothering me for a while, maybe a developer can explain to me in detail:

whats the diference between the x86 and the x64 builds? x64 is faster but why? is there some specific optimization for x64?

i don`t think that the amount of adressable memory provides the performance boost

or does it?

[Qaazavaca Qaanic]

More efficient JIT. Emulates the Wii CPU faster. Why? I don't know, the Wii CPU is 32-bit.

[Verix]

Because x64 code is more efficient?
Because the x86 code has been worked on less than x64, hence it will be phased out?

[lamedude]

You get 16 GPR and 16 XMM registers in 64bit as opposed to 8 in 32bit. That probably makes emulating a 32GPR CPU easier.

[xemnas]

JIT64 is better but I don't know why. According to my knowledge, Dolphin is the only emulator whose 64-bit build is significantly faster than 32-bit build. Please correct me if I'm wrong.

[pauldacheez]

lamedude and Verix have it right – JIT64 has received a lot more attention than JIT32, and x86-64 is inherently better at emulating the Gekko/Broadway, plus it's what the whole computing industry has already migrated over to.

I'm honestly surprised that other emulators haven't embraced x86-64 as fast as Dolphin has. PCSX2 explains that they're 32-bit-only because it'd be too much work to rewrite their dynarec for 64-bit (and yet they're already using OpenGL 4 features?), while clearly N64 emudevs seem to love rewriting things from scratch unless it's to support new computing hardware...

[xemnas]

x86-64 is inherently better at emulating the Gekko/Broadway

Could you please explain why?

while clearly N64 emudevs seem to love rewriting things from scratch unless it's to support new computing hardware...

How does that have anything to do with 32-bit/64-bit?

PCSX2 explains that they're 32-bit-only because it'd be too much work to rewrite their dynarec for 64-bit (and yet they're already using OpenGL 4 features?)

No. That might only be one of the reasons. The more important reason they explained was that even if they made the 64-bit work it probably wouldn't be faster than 32-bit. The performance would be pretty much the same.

Regarding the registers someone mentioned, PCSX2 team also explained that the number of usable registers weren't that much different. 32-bit can use MMX registers or something if I remember correctly.

[NaturalViolence]

Could you please explain why?

Lamedude already did. There are a number of notable improvements made in x86-64 over x86-32 that improve performance but the main one is the larger ISA register set. More GPRs, more XMM, and instruction pointer relative data access all probably improve performance in dolphin. You end up with better code optimization and more efficient compilation.

PCSX2 team also explained that the number of usable registers weren't that much different.

Please find me a quote for that. Because I have trouble believing anyone with knowledge of x86 GPRs would say such a thing. Being able to store twice as many variables in the cpu at once is a big deal for an emulator.

32-bit can use MMX registers or something if I remember correctly.

MMX is grossly obsolete to SSE anyways so why does this even matter?

[delroth]

Also we don't support fastmem on x86, only on x86_64 (address space too small). That's something like 10-15% performance difference.

[xemnas]

Lamedude already did. There are a number of notable improvements made in x86-64 over x86-32 that improve performance but the main one is the larger ISA register set. More GPRs, more XMM, and instruction pointer relative data access all probably improve performance in dolphin. You end up with better code optimization and more efficient compilation.

Thanks for the information. But I'm not quite convinced why larger ISA register set, more GPRs, more XMM, etc. make the 64-bit build faster because I don't see why they make things faster. Larger doesn't always mean faster like if we increase the amount of RAM, it doesn't make the emulation faster. You may be right but I don't see the clear connection here. I don't understand how they impact the performance.

Just an another opinion, PCSX2 team doesn't agree that just go 64-bit will certainly/significantly increase the performance:

There are many reasons for this change. Just naming a few...
- Big increase in complexity of maintaining both versions
- No increase in performances as the emulator is structured now
- Dubious increase in performance in general even if we were to restructure it...

http://forums.pcsx2.net/Thread-We-are-dr...5#pid12935

Please find me a quote for that. Because I have trouble believing anyone with knowledge of x86 GPRs would say such a thing. Being able to store twice as many variables in the cpu at once is a big deal for an emulator.

But i cant believe there is no performance advantages in X64 LMA mode , there is almost x4 the register space than in compatibility mode or 32Bits.

For traditional coding, yes, there's performance advantages to x64. But from a recompiler aspect -- where the optimization assumptions and code reordering options available to us are significantly limited -- there's not so much advantage. Additionally the x64 does not have that many more registers over the x86. x86 has the 8 MMX registers (which are all 64 bit), and x64 modes do not. So whenever you do register counting between the two CPUs, you need to factor those in as well.

http://forums.pcsx2.net/Thread-We-are-dr...1#pid13171
I believe he is very knowledgeable.

MMX is grossly obsolete to SSE anyways so why does this even matter?

Please read the above quote.