Going Faster with Lisp

Tagged as lisp, emulation

For the first time in 3+ years, I'm working in earnest on a hobby project.

It feels like coming home, to be writing lisp and blogging again. Once again I'm playing with Nintendo emulation, don't ask why it's captured my imagination so. I'd like to briefly discuss what's brought me back and then we'll talk about what I learned writing lisp today.

My Absence

I haven't really worked on hobby projects since mid 2014. Even then my output was reduced substantially from 2012 when I lived alone and cl-6502/coleslaw had my full attention. I never stopped wanting to learn more, or loving lisp specifically, I just lost the energy to pursue it. That was due to (in rough order): Work, burnout, my relationship, and buying a house. Where burnout == a curiously strong blend of exhaustion, impostor syndrome, and unclear goals. It was scary at times when I wondered if I had lost my passion or commitment but ultimately good for me.

A lot of why I stalled out had to do with my old Nintendo emulator. I had made some bad assumptions, especially about how memory worked, due to not knowing much about systems programming or hardware when I started and didn't want to throw away everything I had to start fresh. cl-6502 had also felt very public so when progress had stalled before even being able to play a game that was quite embarrassing. I also didn't really know about test ROMs until way too late in the going.

But time heals all wounds and I have plenty of ideas. So here we are.

Design Changes

With cl-6502, I just focused on the CPU since that was something I had an inkling of how to approach. My biggest mistake was treating RAM as a 64k element array. The actual Nintendo used Memory Mapped I/O to talk to the graphics and sound cards. The only way to support that in famiclom was to overwrite the routines that read and wrote to RAM in cl-6502. It was unacceptable to me from both a design and performance perspective.

This time around, I'm using a separate object to represent the Memory Map so that when an CPU instruction reads or writes to an address, it'll actually get handled by the right part of the system: the RAM, Video Card, Sound, or cartridge. I'm also going to be focused on using test ROMs through as much of the process as I can. I'll write more about that in a future article but, long story short, TDD is hard to do when writing an emulator.

Lisp, Fast, OO: Pick 3

I managed to get cl-6502 running fast enough last time around but it was still 100x slower than Ian Piumarta's lib6502 written in C. There's no reason that has to be the case, I simply didn't know how to approach optimizing Lisp. I would use SBCL's statistical profiler, sprinkle compiler declarations in my code, re-profile, and pray. Today I'd like to focus on a few tricks for figuring out if declarations are helping or not and getting friendly with your disassembler. I'll also talk a little about why I wound up going with DEFSTRUCT over DEFCLASS.

lol, dis

Profilers are great for helping you figure out what parts of your code you spend the most time in. Once you've identified a function that needs to go fast, the next step is usually to add an optimize declaration. Something like:

( declare ( optimize ( speed 3 ) ( safety 1 ) ) )

Recompiling the function afterward will result in the compiler printing out notes about what tripped it up while compiling the code. One thing I didn't realize back when I was working on cl-6502 (but seems obvious in retrospect) is that you can include optimize and type declarations in methods too! That said, it can be a pain to constantly write out different optimize and type declarations, recompile, and call disassemble on the code to see differences in the output. Additionally, there is not a portable way to disassemble methods, only their generic functions which is really just the dispatch machinery and not the work that you're interested in.

While Doug Hoyte's book Let Over Lambda is a bit controversial among lispers, he offers some good advice and good tools for remedying these points in Chapter 7. In particular, he supplies a read macro to quickly enable maximum optimization in a method or function and a regular macro to allow testing out type declarations effect on an anonymous function quickly at the REPL. I've taken the liberty of adding both to my .sbclrc file so I have easy access to them when I'm trying things out.

( defun enable-sharpf-read-macro ( ) ( set-dispatch-macro-character #\# #\f ( lambda ( stream sub-char numarg ) ( declare ( ignore stream sub-char ) ) ( setf numarg ( or numarg 3 ) ) ( unless ( <= numarg 3 ) ( error "Invalid value for optimize declaration: ~a" numarg ) ) ` ( declare ( optimize ( speed ,numarg ) ( safety , ( - 3 numarg ) ) ) ) ) ) ) ( defmacro dis ( args &rest body ) ( flet ( ( arg-name ( arg ) ( if ( consp arg ) ( cadr arg ) arg ) ) ( arg-decl ( arg ) ( if ( consp arg ) ` ( type , ( car arg ) , ( cadr arg ) ) nil ) ) ) ( let ( ( arglist ( mapcar #'arg-name args ) ) ( declarations ( mapcar #'arg-decl args ) ) ) ` ( disassemble ( lambda ,arglist ( declare ,@ ( remove nil declarations ) ) ,@body ) ) ) ) )

I also dug around to see if there was a way to get disassembly for a single method and found a helpful thread on Google Groups from which I built a little function for disassembling the "fast-function" commonly invoked for a method.

( defun disasm-method ( name specializers ) "E.g. (disasm-method 'package:generic-fun '(class t))" ( let* ( ( method ( find-method name nil specializers ) ) ( function ( sb-mop:method-function method ) ) ( fast-function ( sb-pcl::%method-function-fast-function function ) ) ) ( disassemble fast-function ) ) )

A "Practical" Example

All code for this section is on the ground-floor branch on Github

Today I was working on memory mappers / cartridges for the NES emulator. Let's look at how I used these tools to optimize a method on the simplest mapper NROM. (Used in titles like Donkey Kong and the original Super Mario Brothers.) The method we'll be looking at is called load-prg . Put simply, it takes an address and loads a byte from the PRG section of the game cartridge.

Since any game will load data from the cartridge a lot we really want this to be a fast operation. And since it's loading from a static array, we would hope we can get this down to a handful of assembly instructions. Here's my initial implementation:

( defmethod load-prg ( ( mapper nrom ) address ) ( let ( ( rom ( mapper-rom mapper ) ) ) ( if ( = 1 ( rom-prg-count rom ) ) ( aref ( rom-prg rom ) ( logand address #x3fff ) ) ( aref ( rom-prg rom ) ( logand address #x7fff ) ) ) ) )

You can see it takes an NROM mapper and an address and, based on the number of PRG banks in the cartridge, does a little math on the address and accesses the PRG with AREF . Let your eyes skim over the unoptimized disassembly:

CL-USER> (disasm-method #'clones.mappers::load-prg '(clones.mappers::nrom t)) ; disassembly for (SB-PCL::FAST-METHOD CLONES.MAPPERS:LOAD-PRG (CLONES.MAPPERS::NROM T)) ; Size: 280 bytes. Origin: #x2290E8F5 ; 8F5: 498B4C2460 MOV RCX, [R12+96] ; no-arg-parsing entry point ; thread.binding-stack-pointer ; 8FA: 48894DF8 MOV [RBP-8], RCX ; 8FE: 498B5805 MOV RBX, [R8+5] ; 902: 48895DE0 MOV [RBP-32], RBX ; 906: 8D43FD LEA EAX, [RBX-3] ; 909: A80F TEST AL, 15 ; 90B: 0F85F3000000 JNE L11 ; 911: 8B4B01 MOV ECX, [RBX+1] ; 914: 4881F903FD5020 CMP RCX, #x2050FD03 ; #<SB-KERNEL:LAYOUT for CLONES.ROM:ROM {2050FD03}> ; 91B: 0F85C8000000 JNE L10 ; 921: L0: 488B531D MOV RDX, [RBX+29] ; 925: BF02000000 MOV EDI, 2 ; 92A: E8411C1FFF CALL #x21B00570 ; GENERIC-= ; 92F: 488B5DE0 MOV RBX, [RBP-32] ; 933: 488B75E8 MOV RSI, [RBP-24] ; 937: 4C8B45F0 MOV R8, [RBP-16] ; 93B: 7456 JEQ L5 ; 93D: 488B4B0D MOV RCX, [RBX+13] ; 941: 8D46F1 LEA EAX, [RSI-15] ; 944: A801 TEST AL, 1 ; 946: 750A JNE L1 ; 948: A80F TEST AL, 15 ; 94A: 7542 JNE L4 ; 94C: 807EF111 CMP BYTE PTR [RSI-15], 17 ; 950: 753C JNE L4 ; 952: L1: 488BFE MOV RDI, RSI ; 955: 40F6C701 TEST DIL, 1 ; 959: 7407 JEQ L2 ; 95B: 488B7FF9 MOV RDI, [RDI-7] ; 95F: 48D1E7 SHL RDI, 1 ; 962: L2: 4881E7FEFF0000 AND RDI, 65534 ; 969: 8D41F1 LEA EAX, [RCX-15] ; 96C: A80F TEST AL, 15 ; 96E: 7519 JNE L3 ; 970: 8B41F1 MOV EAX, [RCX-15] ; 973: 2C85 SUB AL, -123 ; 975: 3C74 CMP AL, 116 ; 977: 7710 JNBE L3 ; 979: 488BD1 MOV RDX, RCX ; 97C: B904000000 MOV ECX, 4 ; 981: FF7508 PUSH QWORD PTR [RBP+8] ; 984: E9AFEDA1FD JMP #x2032D738 ; #<FDEFN SB-KERNEL:HAIRY-DATA-VECTOR-REF/CHECK-BOUNDS> ; 989: L3: 0F0B0A BREAK 10 ; error trap ; 98C: 36 BYTE #X36 ; OBJECT-NOT-VECTOR-ERROR ; 98D: 08 BYTE #X08 ; RCX ; 98E: L4: 0F0B0A BREAK 10 ; error trap ; 991: 41 BYTE #X41 ; OBJECT-NOT-INTEGER-ERROR ; 992: 30 BYTE #X30 ; RSI ; 993: L5: 488B4B0D MOV RCX, [RBX+13] ; 997: 8D46F1 LEA EAX, [RSI-15] ; 99A: A801 TEST AL, 1 ; 99C: 750A JNE L6 ; 99E: A80F TEST AL, 15 ; 9A0: 7542 JNE L9 ; 9A2: 807EF111 CMP BYTE PTR [RSI-15], 17 ; 9A6: 753C JNE L9 ; 9A8: L6: 488BFE MOV RDI, RSI ; 9AB: 40F6C701 TEST DIL, 1 ; 9AF: 7407 JEQ L7 ; 9B1: 488B7FF9 MOV RDI, [RDI-7] ; 9B5: 48D1E7 SHL RDI, 1 ; 9B8: L7: 4881E7FE7F0000 AND RDI, 32766 ; 9BF: 8D41F1 LEA EAX, [RCX-15] ; 9C2: A80F TEST AL, 15 ; 9C4: 7519 JNE L8 ; 9C6: 8B41F1 MOV EAX, [RCX-15] ; 9C9: 2C85 SUB AL, -123 ; 9CB: 3C74 CMP AL, 116 ; 9CD: 7710 JNBE L8 ; 9CF: 488BD1 MOV RDX, RCX ; 9D2: B904000000 MOV ECX, 4 ; 9D7: FF7508 PUSH QWORD PTR [RBP+8] ; 9DA: E959EDA1FD JMP #x2032D738 ; #<FDEFN SB-KERNEL:HAIRY-DATA-VECTOR-REF/CHECK-BOUNDS> ; 9DF: L8: 0F0B0A BREAK 10 ; error trap ; 9E2: 36 BYTE #X36 ; OBJECT-NOT-VECTOR-ERROR ; 9E3: 08 BYTE #X08 ; RCX ; 9E4: L9: 0F0B0A BREAK 10 ; error trap ; 9E7: 41 BYTE #X41 ; OBJECT-NOT-INTEGER-ERROR ; 9E8: 30 BYTE #X30 ; RSI ; 9E9: L10: 488B512D MOV RDX, [RCX+45] ; 9ED: 4883FA04 CMP RDX, 4 ; 9F1: 7E11 JLE L11 ; 9F3: 488B4125 MOV RAX, [RCX+37] ; 9F7: 81781103FD5020 CMP DWORD PTR [RAX+17], #x2050FD03 ; #<SB-KERNEL:LAYOUT for CLONES.ROM:ROM {2050FD03}> ; 9FE: 0F841DFFFFFF JEQ L0 ; A04: L11: 0F0B0A BREAK 10 ; error trap ; A07: 0A BYTE #X0A ; OBJECT-NOT-TYPE-ERROR ; A08: 18 BYTE #X18 ; RBX ; A09: 23 BYTE #X23 ; 'CLONES.ROM:ROM ; A0A: 0F0B10 BREAK 16 ; Invalid argument count trap

WOOF! 280 bytes of assembly, including a full CALL to a generic equality test, and two JMP instructions to other functions. Even without knowing any assembly, this seems like an awful lot of junk just for a measly array lookup! I think one valuable insight I got from Chapter 7 of Let Over Lambda was to disregard what I thought I know or didn't about assembly and just use my damn eyes. Doesn't this seem like a silly amount of code? Let's crank the optimization up:

( defmethod load-prg ( ( mapper nrom ) address ) #f ( let ( ( rom ( mapper-rom mapper ) ) ) ( if ( = 1 ( rom-prg-count rom ) ) ( aref ( rom-prg rom ) ( logand address #x3fff ) ) ( aref ( rom-prg rom ) ( logand address #x7fff ) ) ) ) )

As soon as I recompiled this code, I got 6 notes from the compiler stating that it wasn't confident about the return value of (rom-prg-count rom) hence the generic equality test. It also wasn't confident what kind of array (rom-prg rom) was or if all the elements even shared a type! That will cause AREF to be slow. Even so, the generated assembly drops to 116 bytes since the #f read macro expands to a declaration with maximum speed (3) and minimum safety (0). It should go without saying that you only want to do this in code that A) really needs to be fast and for which, B) you're very confident about who will call it and how. Here's the disassembly:

CL-USER> (disasm-method #'clones.mappers::load-prg '(clones.mappers::nrom t)) ; disassembly for (SB-PCL::FAST-METHOD CLONES.MAPPERS:LOAD-PRG (CLONES.MAPPERS::NROM T)) ; Size: 116 bytes. Origin: #x2290F6CB ; 6CB: 48895DF0 MOV [RBP-16], RBX ; no-arg-parsing entry point ; 6CF: 488B4605 MOV RAX, [RSI+5] ; 6D3: 488945F8 MOV [RBP-8], RAX ; 6D7: 488B501D MOV RDX, [RAX+29] ; 6DB: BF02000000 MOV EDI, 2 ; 6E0: E88B0E1FFF CALL #x21B00570 ; GENERIC-= ; 6E5: 488B5DF0 MOV RBX, [RBP-16] ; 6E9: 488B45F8 MOV RAX, [RBP-8] ; 6ED: 7528 JNE L1 ; 6EF: 488B500D MOV RDX, [RAX+13] ; 6F3: 488BFB MOV RDI, RBX ; 6F6: 40F6C701 TEST DIL, 1 ; 6FA: 7407 JEQ L0 ; 6FC: 488B7FF9 MOV RDI, [RDI-7] ; 700: 48D1E7 SHL RDI, 1 ; 703: L0: 4881E7FE7F0000 AND RDI, 32766 ; 70A: B904000000 MOV ECX, 4 ; 70F: FF7508 PUSH QWORD PTR [RBP+8] ; 712: E9E166A2FD JMP #x20335DF8 ; #<FDEFN SB-KERNEL:HAIRY-DATA-VECTOR-REF> ; 717: L1: 488B500D MOV RDX, [RAX+13] ; 71B: 488BFB MOV RDI, RBX ; 71E: 40F6C701 TEST DIL, 1 ; 722: 7407 JEQ L2 ; 724: 488B7FF9 MOV RDI, [RDI-7] ; 728: 48D1E7 SHL RDI, 1 ; 72B: L2: 4881E7FEFF0000 AND RDI, 65534 ; 732: B904000000 MOV ECX, 4 ; 737: FF7508 PUSH QWORD PTR [RBP+8] ; 73A: E9B966A2FD JMP #x20335DF8 ; #<FDEFN SB-KERNEL:HAIRY-DATA-VECTOR-REF>

A Fork in the Road

Those two JMP instructions and the generic equality CALL are still in the assembly though as you can see from the comments on the right hand side. Why? Because we didn't actually resolve any of the compiler's uncertainties about the code. We have to help it know what type of values it will be working with. The question is how to best do that. One way would be to just add a bunch of local type declarations in the method:

( defmethod load-prg ( ( mapper nrom ) address ) #f ( let* ( ( rom ( mapper-rom mapper ) ) ( prg ( rom-prg rom ) ) ( prg-count ( rom-prg-count rom ) ) ) ( declare ( type byte-vector prg ) ( type fixnum prg-count ) ) ( if ( = 1 prg-count ) ( aref prg ( logand address #x3fff ) ) ( aref prg ( logand address #x7fff ) ) ) ) )

That will work and does generately substantially nicer code (82 bytes and no CALLs or JMPs). But boy, it forced us to completely restructure the method and, well, the new version feels a bit disjointed. The declarations stick out and distract from the underlying ideas. The alternative is to try and teach the compiler what types are returned by the accessor functions we're using to pull data out of the ROM. And this is where we come to the important difference about DEFCLASS and DEFSTRUCT from where I'm sitting as an emulator author.

Optimizing with Structs is Easier

(Ed. note 09/19/2017: Rainer Joswig left a very informative comment about Structs vs Classes and Optimizing with CLOS on reddit.)

Getting struct-related code to go fast is easier for a very specific reason. Both DEFCLASS and DEFSTRUCT allow you to optionally specify the types of their slots. Unfortunately, DEFCLASS does absolutely no optimization with this information, while DEFSTRUCT treats it as a guarantee and propagates it through the auto-generated slot accessors and, therefore, the rest of your code.

Now there's a good reason for this and I am certainly not advocating for using DEFSTRUCT by default. The reason is that DEFSTRUCT is not designed to be interactively redefined or changed at runtime unlike most of the language. DEFCLASS could have the types of its slots (or even the slots themselves) change at any time including runtime and so it isn't reasonable for it to treat the type declaration as a fact.

DEFSTRUCT has other downsides as well, including auto-generating a bunch of symbols in the current package among other things. It's clunkier to work with in several ways than DEFCLASS but for truly performance intensive stuff, the type declaration behavior makes it worth it from where I'm sitting. Just don't default to DEFSTRUCT in general. This message from the Rob Warnock Archive may also prove interesting.

This is something I always had questions about though and it was compounded a bit due to the fact that DEFSTRUCT is barely mentioned by Practical Common Lisp or Common Lisp Recipes. Practical Common Lisp is still the best way to learn the language in my opinion. I also honestly enjoy the things that are in the Common Lisp standard due to history but I'd never quite found an answer to "When should I use structs vs classes?" that I liked. Hopefully future lispers will be able to stumble on these notes (or parse the spec better than I did).

Modifying the ROM definition

Here's what our ROM struct looks like with the added type declarations:

( defstruct rom ( pathname nil :read-only t ) ( prg # ( ) :read-only t :type byte-vector ) ( chr # ( ) :read-only t :type byte-vector ) ( prg-count 0 :read-only t :type ub8 ) ( chr-count 0 :read-only t :type ub8 ) ( mirroring nil :read-only t ) ( mapper-name nil :read-only t ) )

The previous version had no :type options and the default values were all nil . After changing the struct and recompiling, we can write the same version of load-prg as before but get much better generated assembly since the compiler knows the types returned by the struct accessors (and thus the array element type):

( defmethod load-prg ( ( mapper nrom ) address ) #f ( let ( ( rom ( mapper-rom mapper ) ) ) ( if ( = 1 ( rom-prg-count rom ) ) ( aref ( rom-prg rom ) ( logand address #x3fff ) ) ( aref ( rom-prg rom ) ( logand address #x7fff ) ) ) ) )

Finally, we can improve things just a bit by promising that the address we call the load-prg method with will be an unsigned 16-bit value since the 6502 only has a 64k address space:

( defmethod load-prg ( ( mapper nrom ) address ) #f ( declare ( type ub16 address ) ) ( let ( ( rom ( mapper-rom mapper ) ) ) ( if ( = 1 ( rom-prg-count rom ) ) ( aref ( rom-prg rom ) ( logand address #x3fff ) ) ( aref ( rom-prg rom ) ( logand address #x7fff ) ) ) ) )

Updates

(Ed. note 09/19/2017: Some additional speedups have been made since this article was published.)

Paul Khuong was kind enough to note that SBCL was unable to hoist the (logand address xxx) computation out of the conditional. This duplication can be seen in the disassembly from the two MOV .. AND .. SAR .. MOVZX blocks. Doing so improved the assembly a bit further to 51 bytes. Reflecting on it further, I realized there's no need for a conditional expression at all!

In NROM cartridges, they can either have 1 or 2 PRG banks each of which are 16k. Because the 6502 has a 64k address space and the cartridge data begins at 32k, an NROM cartridge with only 1 PRG bank doesn't actually fill the address space. In our load-prg method, we just want to make sure that if we're given a higher address like 54321 that we wrap that around to not run off the end of our 16k worth of PRG. To do that, we can just logical AND the address with (1- (length array)) .

Doing that eliminates the branch and results in a nice, lean 40 bytes for our final disassembly:

( defmethod load-prg ( ( mapper nrom ) address ) #f ( declare ( type ub16 address ) ) ( let* ( ( rom ( mapper-rom mapper ) ) ( end-of-rom ( 1- ( length ( rom-prg rom ) ) ) ) ( wrapped-address ( logand address end-of-rom ) ) ) ( aref ( rom-prg rom ) wrapped-address ) ) )

Wrap Up

There's a lot of work left to do on the (new) emulator but I'm writing code again, having fun, learning, and using lisp and that's the most important part to me. If you made it this far, thanks for reading. Let me know what you think and happy hacking!