design: describe the dice bytecode interpreter

Updates #1.

Author: sbinet

design/1-bytecode-interpreter.md

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+# Proposal: Design of a bytecode interpreter for Go
+
+Author: Sebastien Binet
+
+Last updated: 2016-08-26
+
+Discussion at https://github.com/go-interpreter/proposal/issue/1.
+
+## Abstract
+
+We propose to design and implement a bytecode interpreter for Go,
+which will be the foundation for a Go [REPL](https://en.wikipedia.org/wiki/Read%E2%80%93eval%E2%80%93print_loop).
+
+## Background
+
+It is common in science or exploratory work to iterate on a piece of code
+to solve a given problem.
+Having an interactive conversation with your program, _via_ an interactive
+prompt (aka a REPL), greatly speeds up such exploratory work: one can easily
+iterate on various algorithms, modifying the state of your program and data,
+and write new types and functions to _e.g._ plot the new state of your data.
+
+A side benefit of such an interpreter is the ability to embed it inside
+a Go application and provide both scriptability and extensibility.
+Designing such an API is outside the perimeter of this proposal.
+
+There are currently already partial solutions or whole implementations
+of a Go REPL on the market but none of those meets the following requirements:
+
+- easy `go get` installation
+- implement the whole Go language
+- be a real REPL, not just an "on-the-fly re-compilation + re-run the whole snippet" approach
+- JIT-able
+- performant
+
+## Proposal
+
+We propose to break the complicated issue of bringing a complete interpreter
+for Go (interactivity, whole-program interpretation, runtime, native functions,
+external functions, JITing, parsing source code, ...) into small pieces.
+
+The current proposal only deals with describing the bytecode interpreter
+(its overall design and its components), the opcodes and instructions which
+can be found in a bytecode stream and how these bytecodes can be interpreted and
+acted upon by the interpreter.
+
+There are many ways to implement an interpreter and as many options
+for the interpretation process:
+
+1. directly interpret from the source code
+2. interpret the source code after it has been transformed into an AST
+3. compile statements into bytecode instructions that are then executed
+
+We propose to go with option 3).
+Option 1) doesn't lend itself to optimizations nor very efficient execution.
+Option 2) is somewhat better: there are ways to programmatically manipulate
+and transform an AST.
+But with option 3) we should be able to reuse the whole corpus of optimizations
+coming from the new SSA backend of the official `gc` Go compiler.
+As explained in Rob Pike's talk at GopherCon-2016: ["The Design of the Go Assembler"](https://talks.golang.org/2016/asm.slide),
+the `cmd/internal/obj` package can be seen as a rather portable assembly language.
+This paves the way for considering it as a portable intermediate representation
+(IR) of Go code.
+
+The proposal is thus to use this conduit as the general infrastructure to
+generate the opcodes and bytecode for the new Go VM.
+The concrete _modus_ _operandi_ for leveraging `cmd/internal/obj` and
+the whole `gc` compiler infrastructure might still need to be properly fleshed
+out, but here are the current options:
+
+- create a proper `GOARCH` architecture directly under `cmd/internal` like
+  the other `GOARCH=amd64`, `GOARCH=s390x`, etc... architectures and aim for
+  Go 1.8, (we would need to declare our plans [here](https://groups.google.com/forum/#!topic/golang-dev/098vr4999Tk))
+- vendor `cmd/compiler` at a given Go version (_e.g._ 1.7) and work off it,
+  aiming for integration at a later date (if at all possible),
+- ???
+
+### Instructions, opcodes and bytecode format
+
+We propose to reuse the opcodes and bytecode format as described in the [Dis VM](http://www.vitanuova.com/inferno/papers/dis.pdf)
+specification paper.
+The `Dis` VM was able to execute [Limbo](https://en.wikipedia.org/wiki/Limbo_%28programming_language%29)
+code.
+`Limbo` and `Go` share a common lineage and present similar features
+(channels, `select`, garbage collector, packages) so many (if not all) of
+the opcodes our VM will need are already present and the instruction set has
+been formally described.
+The on-disk object file format and overall organization has also been specified
+in the above paper.
+
+We intend to follow the general spirit of the specifications of the `Dis` VM
+and condense it inside a package named `dice`.
+The implementation of `dice` should be done from first principles,
+without looking at the `Dis` source code
+This is to ensure that `dice` can be licensed under `BSD-3`.
+
+The various `opcode`s are listed here:
+
+```
+00 nop    20 headb  40 mulw  60 blew    80 shrl
+01 alt    21 headw  41 mulf  61 bgtw    81 bnel
+02 nbalt  22 headp  42 divb  62 bgew    82 bltl
+03 goto   23 headf  43 divw  63 beqf    83 blel
+04 call   24 headm  44 divf  64 bnef    84 bgtl
+05 frame  25 headmp 45 modw  65 bltf    85 bgel
+06 spawn  26 tail   46 modb  66 blef    86 beql
+07 runt   27 lea    47 andb  67 bgtf    87 cvtlf
+08 load   28 indx   48 andw  68 bgef    88 cvtfl
+09 mcall  29 movp   49 orb   69 beqc    89 cvtlw
+0A mspawn 2A movm   4A orw   6A bnec    8A cvtwl
+0B mframe 2B movmp  4B xorb  6B bltc    8B cvtlc
+0C ret    2C movb   4C xorw  6C blec    8C cvtcl
+0D jmp    2D movw   4D shlb  6D bgtc    8D headl
+0E case   2E movf   4E shlw  6E bgec    8E consl
+0F exit   2F cvtbw  4F shrb  6F slicea  8F newcl
+10 new    30 cvtwb  50 shrw  70 slicela 90 casec
+11 newa   31 cvtfw  51 insc  71 slicec  91 indl
+12 newcb  32 cvtwf  52 indc  72 indw    92 movpc
+13 newcw  33 cvtca  53 addc  73 indf    93 tcmp
+14 newcf  34 cvtac  54 lenc  74 indb    94 mnewz
+15 newcp  35 cvtwc  55 lena  75 negf    95 cvtrf
+16 newcm  36 cvtcw  56 lenl  76 movl    96 cvtfr
+17 newcmp 37 cvtfc  57 beqb  77 addl    97 cvtws
+18 send   38 cvtcf  58 bneb  78 subl    98 cvtsw
+19 recv   39 addb   59 bltb  79 divl    99 lsrw
+1A consb  3A addw   5A bleb  7A modl    9A lsrl
+1B consw  3B addf   5B bgtb  7B mull    9B eclr
+1C consp  3C subb   5C bgeb  7C andl    9C newz
+1D consf  3D subw   5D beqw  7D orl     9D newaz
+1E consm  3E subf   5E bnew  7E xorl
+1F consmp 3F mulb   5F bltw  7F shll
+```
+
+We reserve the right to rename some of these `opcode`s to better reflect
+the naming conventions of our source language, Go.
+
+### Virtual Machine
+
+Once a Go package, command or code snippet has been compiled to our `dice` bytecode,
+that bytecode needs to be somehow executed.
+This job is performed by the `dice.VM` virtual machine:
+
+```go
+package dice
+
+type VM struct {
+	frame   *frame
+	globals []reflect.Value
+}
+
+type frame struct {
+	vm     *VM
+	caller *frame
+	locals []reflect.Value
+	pc     int // program counter
+	code   []instruction
+}
+
+type instruction struct {
+	opcode byte
+	amode  byte   // address mode
+	addrs  uint64 // operands (src1, src2, dst)
+}
+
+func (vm *VM) run() {
+	run(vm.frame)
+}
+
+func run(fr *frame) {
+	for {
+code:
+		for _, code := range fr.code {
+			switch exec(fr, code) {
+				case cfReturn:
+					return
+				case cfNext:
+					// fetching next instruction
+				case cfJump:
+					break code
+			}
+		}
+	}
+}
+
+func exec(fr *frame, code instruction) cfKind {
+	switch code.opcode {
+		case opADDF:
+			// dst = src1 + src2
+			fr.pc++
+		case opCALL:
+			run(&frame{caller:fr, pc:0, code: from(src)})
+		case opRET:
+			// fetch result if any
+			return cfReturn
+		case opGO:
+			go func() {
+				run(&frame{caller:fr})
+			}()
+		// etc...
+	}
+}
+```
+
+At this moment, the proposal is to be able to byte compile this simple Go package:
+
+```go
+package main
+
+func add(i, j int) int {
+	return i+j
+}
+
+func main() {}
+```
+
+and in a later stage, be able to run `add(40, 2)`.
+
+## Rationale
+
+Why do we implement yet another Go interpreter and a REPL?
+Aren't there already enough of them?
+
+Here is a list of alternatives:
+
+- [llgoi](https://github.com/llvm-mirror/llgo/blob/master/cmd/llgoi/llgoi.go) is a JIT-enabled interpreter built on top of `LLVM` and `llgo`.
+  The first issue with `llgoi` is the somewhat painfull installation process.
+  This pain point should be resorbed with time (and also by providing [snap based](https://groups.google.com/forum/#!msg/llgo-dev/ny8MgDlNkng/8kEvgzfuCQAJ)
+  isntallations of `llgoi`.
+  But the main issue is that `llgo` development is behind that of the reference
+  implementation of `Go`: `gc`.
+  Also, the pace of development of `LLVM` itself (very fast) and the version skew
+  that may result on users' machines *might* set the scene for difficult user
+  support and debugging sessions.
+
+- [ssainterp](https://github.com/go-interpreter/ssainterp) and [ssadump -run](https://godoc.org/golang.org/x/tools/cmd/ssadump)
+  are based on the SSA suite developped at [golang.org/x/tools/go/ssa](https://godoc.org/golang.org/x/tools/go/ssa).
+  They are able to parse and interpret a vast majority of valid Go code,
+  but lack an interactive interpreter mode.
+  `ssadump` code is also clearly stated as *NOT* meant to be used as a
+  production-grade interpreter for Go but merely as an adjunct for testing
+  the SSA construction algorithm.
+
+- [igo](https://github.com/sbinet/igo) and [go-eval](https://github.com/sbinet/go-eval)
+  are projects salvaged from the pre `Go-1` era.
+  `go-eval` does not lend itself easily to compilation optimizations and lacks
+  support for `imports`, `goroutines`, type creation, ...
+
+- [gore](https://github.com/motemen/gore) supports the whole Go language but
+  does not (completely cleanly) preserve state or side effects between
+  2 interactive commands: `gore` recompiles on-the-fly your Go snippets and
+  re-executes them.
+
+It seems necessary to implement some kind of a virtual machine to be able
+to provide an efficient and truly interactive interpreter for Go.
+
+The same question can be also raised about reimplementing a whole new VM.
+Couldn't we have somehow reused an already existing VM?
+`Python`, `Lua`, `JVM` and `Dis` come to mind.
+`Dis` is LGPL and thus not easily integrable in the usual Go ecosystem.
+`Python` and `Lua` have more permissive licenses, but their reference
+implementation are written in `C`, bringing either performance issues on the
+table (`cgo`) or throwing `go-get`-ability out of the window.
+There are however `Go` implementations (partial or complete) of these VMs:
+
+- https://github.com/Shopify/go-lua/blob/master/vm.go
+- https://github.com/flowlo/gothon/blob/master/frame.go
+
+The following issue at this point is the adequacy of their respective VM
+instructions sets with the Go language.
+
+Finally, why do we use the `Dis` VM instructions set, instead of a more recent
+or more in vogue set, such as [LLVM bitcode](http://llvm.org/docs/BitCodeFormat.html)
+and its associated [LLVM assembly](http://llvm.org/docs/LangRef.html), or the
+nascent [`wasm` bytecode](https://webassembly.github.io/) format?
+
+The `LLVM` solution suffers (to a lesser extent) from the same issues than the `llgoi` approach.
+We should note though there exists a pure-Go project to interact with the `LLVM` `IR`:
+[llir/llvm](https://github.com/llir/llvm).
+This project is still a work in progress at this time of writing (August 2016).
+
+`wasm` is probably a very strong and sensible option, and poised to take over
+the whole web industry.
+Unfortunately, there is only a work in progress `C/C++` project at the moment (August 2016),
+so it is probably a bit early to write code to target it.
+However, `wasm` is definitely a backend to monitor: `gopherjs`, a project transpiling
+Go code into `JavaScript` will probably target it at some point.
+
+## Compatibility - Open issues
+
+There are a few interesting issues when interpreting Go code in an interactive
+fashion.
+
+1. Should we allow mid-way imports of packages ?
+   ```
+   go> slice := []string{"HELLO", "GO"}
+   go> import "strings"
+   go> println(strings.ToLower(slice[0]))
+   ```
+
+   What if `slice` was instead named `strings`?
+   Should we allow shadowing of variables by package identifiers?
+   Should we instead re-shadow the package identifier with the variable
+   identifier?
+   The latter seems like the more idiomatic Go behaviour, or at least the
+   behaviour a gopher would expect if she were to write the program in
+   a compiled environment (_i.e.:_ with `goimports` putting the `import`
+   statement at the top)
+
+2. Support for `cgo` and `import "C"` ?
+3. Support for packages with assembly ? (from the `stdlib` or otherwise)
+4. Calls to `syscalls` ? Should they be somehow recognized and performed
+   on a dedicated `goroutine`? What should `os.Exit` do? and how?
+5. How to efficiently implement iteration over maps?
+6. How to implement `unsafe`? Should we?
+7. How to implement the definition of new types?
+   Package `reflect` has some support for this (`StructOf`, `ArrayOf`, ...) but
+   it currently has no support for defining new interface types nor any new
+   named types.
+8. In an interactive interpreter, how do we define methods for a named type?
+   When, and how, do we tell the interpreter that the method set of a given
+   named type is done?
+9. What is the most efficient way to write the `opcode` dispatch loop?
+   A huge switch? ([go-lua](https://engineering.shopify.com/79963844-announcing-go-lua)
+   reported issues with huge switches and migrated to a jump table.)
+
+## Implementation
+
+1. `dice.{VM,frame,instruction}` implementation leading to the execution
+   of already decoded instructions,
+2. implementation of the bytecode stream decoder,
+3. implementation of the bytecode encoder,
+4. implementation of the interactive prompt of the REPL (with limitations),
+5. implementation of dynamically importing packages at the REPL level.
+   This probably needs either a working `buildmode=plugin` from the `go` tool,
+   or a complete handling of dynamically loading bytecode object files.
+