Mercurial > org > blog / draft/a-bit-of-risc.org

 #+title: A Bit of RISC
 #+date: [2024-03-11 Mon]
 I recently picked up [[https://dl.acm.org/doi/10.5555/2462741][Hacker's Delight]] and having a lot of fun with
 it. It's a collection of bit-manipulation tricks collected by hackers
 over many years. You can flip open pretty much anywhere in the book
 and start learn something really cool.
 
 There's something about seeing bit strings and assembly code in a book
 that really catches my attention, this one goes a bit further by even
 describing a complete RISC (Reduced Instruction Set Computer) we can
 implement to play around with the various tricks.
 
 As an exercise and for fun, I'd like to employ some Lisp-fu here and
 implement a small VM specifically designed for mangling bits.
 
 As a fair warning, I'm not a mathematician and I don't write proofs
 often. If I get something wrong or there is a better way of doing
 things, let me know!
 
 You can find most of the code from the book [[https://github.com/hcs0/Hackers-Delight][here]].
 
 * Design
 ** Data Representation
 We'll be sticking with a 32-bit word length as recommended in the
 Preface. We will also represent registers as integers whenever
 possible, instead of say a bit-vector. Without going into too much
 detail, it's much more efficient to do bitwise ops on integers
 instead of bit-vectors in Lisp.
 
 We need a minimum of 16 general purpose registers, typically of word
 length, with R0 reserved for a constant 0. To address 16 different
 registers we actually only need 4 bits - 5-bits if we needed 32, 6 for
 64, etc.
 
 Floating-point support and special purpose registers are not required.
 
 ** Instructions
 The Hacker's Delight RISC architecture is described in two tables,
 denoted =basic RISC= and =full RISC= respectively.
 
 Most instructions take two source registers =RA= and =RB= with a
 destination register =RT=. The actual general-purpose registers are
 labelled =R0= (containg the constant 0) through =R15=.
 
 A 3-Address machine is assumed and some instructions take 16-bit
 signed or unsigned immediate values - denoted =I= and =Iu=
 respectively.
 
 #+tblname: Basic Instruction Set (basic RISC)
 | Opcode Mnemonic                                                 | Operands  | Description                                                                                                                             |
 |-----------------------------------------------------------------+-----------+-----------------------------------------------------------------------------------------------------------------------------------------|
 | add,sub,mul,div,divu,rem,remu                                   | RT,RA,RB  | RT <- (op RA RB)                                                                                                                        |
 | addi,muli                                                       | RT,RA,I   | RT <- (op RA I), I is a 16-bit signed immediate-value                                                                                   |
 | addis                                                           | RT,RA,I   | RT <- (+ RA (\vert\vert I 0x0000))                                                                                                      |
 | and,or,xor                                                      | RT,RA,RB  | RT <- (op RA RB)                                                                                                                        |
 | andi,ori,xori                                                   | RT,RA,Iu  | As above, expect the last operand is a 16-bit unsigned immediate-value                                                                  |
 | beq,bne,blt,ble,bgt,bge                                         | RT,target | Branch to target if (op RT)                                                                                                             |
 | bt,bf                                                           | RT,target | Branch true/false, same as bne/beq resp                                                                                                 |
 | cmpeq,cmpne,cmplt,cmple,cmpgt,cmpge,cmpltu,cmpleu,cmpgtu,cmpgeu | RT,RA,RB  | RT <- (if (op RA RB) 1 0)                                                                                                               |
 | cmpieq,cmpine,cmpilt,cmpile,cmpigt,cmpige                       | RT,RA,I   | Like cmpeq except second comparand is a 16-bit signed immediate-value                                                                   |
 | cmpiequ,cmpineu,cmpiltu,cmpileu,cmpigtu,cmpigeu                 | RT,RA,I   | Like cmpltu except second comparand is a 16-bit unsigned immediate-value                                                                |
 | ldbu,ldh,ldhu,ldw                                               | RT,d(RA)  | Load an unsigned-byte, signed-halfword, unsigned-halfword, or word into RT from (+ RA d) where d is a 16-bit signed immediate-value     |
 | mulhs,mulhu                                                     | RT,RA,RB  | RT gets the high-order 32 bits of (* RA RB)                                                                                             |
 | not                                                             | RT,RA     | RT <- bitwise one's-complement of RA                                                                                                    |
 | shl,shr,shrs                                                    | RT,RA,RB  | RT <- RA shifted left or right by rightmost six bits of RB; 0-fill except for shrs, which is sign-fill (shift amount treated modulo 64) |
 | shli,shri,shrsi                                                 | RT,RA,Iu  | RT <- RA shifted left or right by 5-bit immediate field                                                                                 |
 | stb,sth,stw                                                     | RS,d(RA)  | Store a byte,halfword,word from RS into memory at location (+ RA d) where d is a 16-bit signed immediate-value                          |
 
 
 #+name: Addition Instructions (full RISC)
 | Opcode Mnemonic                                                 | Operands  | Description                                                                                            |
 |-----------------------------------------------------------------+-----------+--------------------------------------------------------------------------------------------------------|
 | abs,nabs                                                        | RT,RA     | RT <- (op RA)                                                                                          |
 | andc,eqv,nand,nor,orc                                           | RT,RA,RB  | RT <- (op RA RB)                                                                                       |
 | extr                                                            | RT,RA,I,L | extract bits I through I+L-1 of RA and place them right-adjusted in RT, with 0-fill                    |
 | extrs                                                           | RT,RA,I,L | Like extr, but sign-fill                                                                               |
 | ins                                                             | RT,RA,I,L | Insert bits 0 through L-1 of RA into bits I through I+L-1 of RT                                        |
 | nlz                                                             | RT,RA     | RT gets count of leading 0's in RA (0 to 32)                                                           |
 | pop                                                             | RT,RA     | RT gets the number of 1-bits in RA (0 to 32)                                                           |
 | ldb                                                             | RT,d(RA)  | Load a signed byte into RT from memory at location (+ RA d) where d is a 16-bit signed immediate value |
 | moveq,movne,movlt,movle,movgt,movge                             | RT,RA,RB  | RT <- RA rotate-shifted left or right by the rightmost 5-bits of RB                                    |
 | shlr,shrr                                                       | RT,RA,RB  | RT <- RA rotate-shifted left or right by the rightmost 5-bits of RB                                    |
 | shlri,shrri                                                     | RT,RA,Iu  | RT <- RA rotate-shifted left or right by the 5-bit immediate field                                     |
 | trpeq,trpne,trplt,trple,trpgt,trpge,trpltu,trpleu,trpgtu,trpgeu | RA,RB     | Trap (interrupt) if (op RA RB)                                                                         |
 | trpieq,trpine,trpilt,trpile,trpigt,trpige                       | RA,I      | Trap if (op RA I) where I is a 16-bit signed immediate-value                                           |
 | trpiequ,trpineu,trpiltu,trpileu,trpigtu,trpigeu                 | RA,Iu     | Trap if (op RA Iu) where Iu is a 16-bit unsigned immediate-value                                       |
 
 There is also some extensions, which are like macros that usually
 expand to a single instruction.
 
 #+name: Extended Mnemonics
 | Extended Mnemonic | Expansion        | Description               |
 |-------------------+------------------+---------------------------|
 | b target          | beq R0,target    | Unconditional branch      |
 | li RT,I           | (addi,addis,ori) | Load immediate            |
 | mov RT,RA         | ori RT,RA,0      | Move register RA to RT    |
 | neg RT,RA         | sub RT,R0,RA     | Negate (two's-complement) |
 | subi RT,RA,I      | addi RT,RA,-I    | Subtract immediate        |
 
 All of these instructions are available on x86,arm,riscv and the likes
 so no real surprises. We will implement the basic set in Lisp, mapping
 instructions directly to Lisp functions using macros.
 
 ** Execution Model
 We'll build this machine in Lisp and use plenty intrinsics from
 SBCL. As a starting point I followed Paul Khuong's excellent blog
 post: [[https://pvk.ca/Blog/2014/03/15/sbcl-the-ultimate-assembly-code-breadboard/][SBCL: The ultimate assembly code breadboard]].
 
 Some things to keep in mind for our machine:
 - every instruction requires at most two register reads and one
   register write - good for compilers
 - every instruction counts as a single cycle
 - we pay no attention to instruction-level parallelism
 
 * The HAKMEM VM
 :properties:
 :header-args: :session t :results none
 :end:
 #+name: defpackage
 #+begin_src lisp
   (ql:quickload :prelude)
   (in-package :std-user)
   (defpackage :hakmem
     (:use :cl :std)
     (:import-from :sb-assem :inst)
     (:import-from :sb-vm :immediate-constant :registers :zero :ea))
   (in-package :hakmem)
   ;; (in-package :sb-x86-64-asm)
   (in-readtable :std)
   (declaim (optimize (speed 3) (safety 1)))
   (defconstant +word-size+ 32 "default word size and register length.")
 #+end_src
 
 #+name: vars
 #+begin_src lisp :package hakmem
   (declaim (type (unsigned-byte #.+word-size+) +ro+))
   (defconstant +r0+ 0 "constant value for register 0")
   (defvar *stack* (make-array 8 :initial-contents (list sb-vm::r8-tn
                                                         sb-vm::r9-tn
                                                         sb-vm::r10-tn
                                                         sb-vm::r11-tn
                                                         sb-vm::r12-tn
                                                         sb-vm::r13-tn
                                                         sb-vm::r14-tn
                                                         sb-vm::r15-tn)))
   (defvar *stack-pointer*)
 
   (defvar *rax* sb-vm::rax-tn)
   (defvar *rbx* sb-vm::rax-tn)
   (defvar *rcx* sb-vm::rax-tn)
   (defvar *rdx* sb-vm::rax-tn)
 
   ;; (@ 0) returns the (current) register for TOS, (@ 1) returns
   ;; the one just below, etc.
   (defun @ (i)
     (aref *stack* (mod (+ i *stack-pointer*) (length *stack*))))
 
   (defvar *code-base* sb-vm::rsi-tn)
   (defvar *virtual-ip* sb-vm::rdi-tn)
   (sb-x86-64-asm::get-gpr :qword 4)
   ;; (sb-vm::immediate-constant-sc 10000)
   ;; arena vector or list?
   (defvar *instructions* (make-hash-table :test #'equal))
 
   (defvar *primitive-code-offset* (* 64 67))
 
   (defstruct code-page
     (alloc 0) ;; next free byte
     (code (make-array *primitive-code-offset* :element-type 'octet)))
 
   (defun emit-code (pages emitter)
     ;; there must be as many code pages as there are stack slots
     (assert (= (length *stack*) (length pages)))
     ;; find the rightmost starting point, and align to 16 bytes
     (let* ((alloc (logandc2 (+ 15 (reduce #'max pages :key #'code-page-alloc))
                             15))
            (bytes (loop for i below (length pages)
                         for page = (elt pages i)
                         collect (let ((segment (sb-assem:make-segment))
                                       (*stack-pointer* i))
                                   ;; assemble the variant for this value
                                   ;; of *stack-pointer* in a fresh code
                                   ;; segment
                                   (sb-assem:assemble (segment)
                                     ;; but first, insert padding
                                     (sb-vm::emit-long-nop segment (- alloc (code-page-alloc page)))
                                     (funcall emitter))
                                   ;; tidy up any backreference
                                   (sb-assem:finalize-segment segment)
                                   ;; then get the (position-independent) machine
                                   ;; code as a vector of bytes
                                   (sb-assem:segment-contents-as-vector segment)))))
       ;; finally, copy each machine code sequence to the right code page
       (map nil (lambda (page bytes)
                  (let ((alloc (code-page-alloc page)))
                    (replace (code-page-code page) bytes :start1 alloc)
                    (assert (<= (+ alloc (length bytes)) (length (code-page-code page))))
                    (setf (code-page-alloc page) (+ alloc (length bytes)))))
            pages bytes)
       ;; and return the offset for that code sequence
       alloc))
 
   (defun emit-all-code (&rest emitters)
     (let ((pages (loop repeat (length *stack*)
                        for page = (make-code-page)
                        ;; prefill everything with one-byte NOPs
                        do (fill (code-page-code page) #x90)
                        collect page)))
       (values (mapcar (lambda (emitter)
                         (emit-code pages emitter))
                       emitters)
               pages)))
 
   (defun next (&optional offset)
     (setf offset (or offset 0)) ; accommodate primops that frob IP
     (let ((rotation (mod *stack-pointer* (length *stack*))))
       (inst movzx *rax* (make-ea :dword :base *virtual-ip*
                                         :disp offset))
       (unless (= -4 offset)
         (inst add *virtual-ip* (+ 4 offset)))
       (if (zerop rotation)
           (inst add *rax* *code-base*)
           (inst lea *rax* (make-ea :qword :base *code-base*
                                           :index *rax*
                                           :disp (* rotation *primitive-code-offset*))))
       (inst jmp *rax*)))
 
   (defun swap ()
     (inst xchg (@ 0) (@ 1)) ; exchange top of stack and stack[1]
     (next))
 
 #+end_src
 
 #+name: instructions
 #+begin_src lisp :package hakmem
   ;; todo
   (defun %parse-reg3 (rt ra rb))
   (defun %parse-reg2i (rt ra i))
   (defun %parse-reg2ui (rt ra ui))
   (defmacro def-inst (name args &body body)
       ;; todo: compose a function based on regs+args+body
       `(let ((sc *scratch*)
              (r0 +r0+)
              (ra 0)
              (rb 0)
              (rt 0))
          (declare (ignorable sc r0 ra rb rt))
          (setf (gethash ',name *instructions*) (lambda ,args (progn ,@body)))))
 
   (defmacro def-prim (name op)
     `(def-inst ,name () (setf rt (,op ra rb))))
 #+end_src
 
 #+name: prims
 #+begin_src lisp :package hakmem
   (def-prim add +)
   (def-prim sub -)
   (def-prim mul *)
   (def-prim div /)
   ;; divu
   (def-prim rem mod)
   ;; remu
   (def-prim cmpeq =)
   (def-prim cmpne /=)
   (def-prim cmplt <)
   (def-prim cmple <=)
   (def-prim cmpgt >)
   (def-prim cmpge >=)
   ;; ltu leu gtu geu
   (def-inst addi (i)
     (setf rt (+ ra i)))
   (def-inst muli (i)
     (setf rt (* ra i)))
   (def-prim and logand)
   (def-prim or logior)
   (def-prim xor logxor)
 
   (defun get-inst (i) (gethash i *instructions*))
 
   (defmacro %inst (i &body args)
     `(funcall (get-inst ',i) ,@args))
 
   (defun list-instructions (&optional (tbl *instructions*))
     (hash-table-alist tbl))
 
 #+end_src
 #+name: instruction-list
 #+begin_src lisp :results replace :exports both :package hakmem
   (list-instructions)
 #+end_src
 
 #+RESULTS: instruction-list
 #+begin_example
 ((XOR . #<FUNCTION (LAMBDA ()) {10077C774B}>)
  (OR . #<FUNCTION (LAMBDA ()) {10077C777B}>)
  (AND . #<FUNCTION (LAMBDA ()) {10077C77AB}>)
  (MULI . #<FUNCTION (LAMBDA (I)) {10077C77DB}>)
  (ADDI . #<FUNCTION (LAMBDA (I)) {10077C780B}>)
  (CMPGE . #<FUNCTION (LAMBDA ()) {10077C783B}>)
  (CMPGT . #<FUNCTION (LAMBDA ()) {10077C786B}>)
  (CMPLE . #<FUNCTION (LAMBDA ()) {10077C789B}>)
  (CMPLT . #<FUNCTION (LAMBDA ()) {10077C78CB}>)
  (CMPNE . #<FUNCTION (LAMBDA ()) {10077C78FB}>)
  (CMPEQ . #<FUNCTION (LAMBDA ()) {10077C792B}>)
  (REM . #<FUNCTION (LAMBDA ()) {10077C795B}>)
  (DIV . #<FUNCTION (LAMBDA ()) {10077C79AB}>)
  (MUL . #<FUNCTION (LAMBDA ()) {10077C79EB}>)
  (SUB . #<FUNCTION (LAMBDA ()) {10077C7A1B}>)
  (ADD . #<FUNCTION (LAMBDA ()) {10077C7A4B}>))
 #+end_example