# WebAssembly Reference Interpreter This repository implements a interpreter for WebAssembly. It is written for clarity and simplicity, _not_ speed. It is intended as a playground for trying out ideas and a device for nailing down the exact semantics, and as a proxy for the (yet to be produced) formal specification of WebAssembly. For that purpose, the code is written in a fairly declarative, "speccy" way. The interpreter can * *decode*/*parse* and *validate* modules in binary or text format * *execute* scripts with module definitions, invocations, and assertions * *convert* between binary and text format (both directions) * *export* test scripts to self-contained JavaScript test cases * *run* as an interactive interpreter The text format defines modules in S-expression syntax. Moreover, it is generalised to a (very dumb) form of *script* that can define multiples module and a batch of invocations, assertions, and conversions between them. As such it is richer than the binary format, with the additional functionality purely intended as testing infrastructure. (See [below](#scripts) for details.) ## Building You'll need OCaml 4.08 or higher. Instructions for installing a recent version of OCaml on multiple platforms are available [here](https://ocaml.org/docs/install.html). On most platforms, the recommended way is through [OPAM](https://ocaml.org/docs/install.html#OPAM). Once you have OCaml, simply do ``` make ``` You'll get an executable named `./wasm`. This is a byte code executable. If you want a (faster) native code executable, do ``` make opt ``` To run the test suite, ``` make test ``` To do everything: ``` make all ``` Before committing changes, you should do ``` make land ``` That builds `all`, plus updates `winmake.bat`. #### Building on Windows The instructions depend on how you [installed OCaml on Windows](https://ocaml.org/docs/install.html#Windows). 1. *Cygwin*: If you want to build a native code executable, or want to hack on the interpreter (i.e., use incremental compilation), then you need to install the Cygwin core that is included with the OCaml installer. Then you can build the interpreter using `make` in the Cygwin terminal, as described above. 2. *Windows Subsystem for Linux* (WSL): You can build the interpreter using `make`, as described above. 3. *From source*: If you just want to build the interpreter and don't care about modifying it, you don't need to install the Cygwin core that comes with the installer. Just install OCaml itself and run ``` winmake.bat ``` in a Windows shell, which creates a program named `wasm`. Note that this will be a byte code executable only, i.e., somewhat slower. In any way, in order to run the test suite you'll need to have Python installed. If you used Option 3, you can invoke the test runner `runtests.py` directly instead of doing it through `make`. #### Cross-compiling the Interpreter to JavaScript #### The Makefile also provides a target to compile (parts of) the interpreter into a [JavaScript library](#javascript-library): ``` make wast.js ``` Building this target requires `js_of_ocaml`, which can be installed with OPAM: ``` opam install js_of_ocaml js_of_ocaml-ppx ``` ## Synopsis #### Running Modules or Scripts You can call the executable with ``` wasm [option | file ...] ``` where `file`, depending on its extension, either should be a binary (`.wasm`) or textual (`.wat`) module file to be loaded, or a script file (`.wast`, see below) to be run. By default, the interpreter validates all modules. The `-u` option selects "unchecked mode", which skips validation and runs code as is. Runtime type errors will be captured and reported appropriately. #### Converting Modules or Scripts A file prefixed by `-o` is taken to be an output file. Depending on its extension, this will write out the preceding module definition in either S-expression or binary format. This option can be used to convert between the two in both directions, e.g.: ``` wasm -d module.wat -o module.wasm wasm -d module.wasm -o module.wat ``` In the second case, the produced script contains exactly one module definition. The `-d` option selects "dry mode" and ensures that the input module is not run, even if it has a start section. In addition, the `-u` option for "unchecked mode" can be used to convert even modules that do not validate. The interpreter can also convert entire test scripts: ``` wasm -d script.wast -o script.bin.wast wasm -d script.wast -o script2.wast wasm -d script.wast -o script.js ``` The first creates a new test scripts where all embedded modules are converted to binary, the second one where all are converted to textual. The last invocation produces an equivalent, self-contained JavaScript test file. The flag `-h` can be used to omit the test harness from the converted file; it then is the client's responsibility to provide versions of the necessary functions. #### Command Line Expressions Finally, the option `-e` allows to provide arbitrary script commands directly on the command line. For example: ``` wasm module.wasm -e '(invoke "foo")' ``` #### Interactive Mode If neither a file nor any of the previous options is given, you'll land in the REPL and can enter script commands interactively. You can also get into the REPL by explicitly passing `-` as a file name. You can do that in combination to giving a module or script file, so that you can then invoke its exports interactively, e.g.: ``` wasm module.wat - ``` See `wasm -h` for (the few) additional options. #### JavaScript Library #### The file `wast.js` generated by the respective [Makefile target](#cross-compiling-the-interpreter-to-javascript) is a self-contained JavaScript library for making the [S-expression syntax](#s-expression-syntax) available directly within JavaScript. It provides a global object named `WebAssemblyText` that currently provides two methods, ``` WebAssemblyText.encode(source) ``` which turns a module in S-expression syntax into a WebAssembly binary, and ``` WebAssemblyText.decode(binary, width) ``` which pretty-prints a binary back into a canonicalised S-expression string. For example: ``` let source = '(module (func (export "f") (param i32 i32) (result i32) (i32.add (local.get 0) (local.get 1))))' let binary = WebAssemblyText.encode(source) (new WebAssembly.Instance(new WebAssembly.Module(binary))).exports.f(3, 4) // => 7 WebAssemblyText.decode(binary, 80) // => // (module // (type $0 (func (param i32 i32) (result i32))) // (func $0 (type 0) (local.get 0) (local.get 1) (i32.add)) // (export "f" (func 0)) // ) ``` Depending on how you load the library, the object may be accessed in different ways. For example, using `require` in node.js: ``` let wast = require("./wast.js"); let binary = wast.WebAssemblyText.encode("(module)"); ``` Or using `load` from a JavaScript shell: ``` load("./wast.js"); let binary = WebAssemblyText.encode("(module)"); ``` ## S-Expression Syntax The implementation consumes a WebAssembly AST given in S-expression syntax. Here is an overview of the grammar of types, expressions, functions, and modules, mirroring what's described in the [design doc](https://github.com/WebAssembly/design/blob/main/Semantics.md). Note: The grammar is shown here for convenience, the definite source is the [specification of the text format](https://webassembly.github.io/spec/core/text/). ``` num: (_? )* hexnum: (_? )* nat: | 0x int: | + | - float: .?(e|E )? | 0x.?(p|P )? name: $( | | _ | . | + | - | * | / | \ | ^ | ~ | = | < | > | ! | ? | @ | # | $ | % | & | | | : | ' | `)+ string: "( | \n | \t | \\ | \' | \" | \ | \u{+})*" num_type: i32 | i64 | f32 | f64 vec_type: v128 vec_shape: i8x16 | i16x8 | i32x4 | i64x2 | f32x4 | f64x2 | v128 ref_kind: func | extern ref_type: funcref | externref val_type: | | block_type : ( result * )* func_type: ( type )? * * global_type: | ( mut ) table_type: ? memory_type: ? num: | var: | unop: ctz | clz | popcnt | ... binop: add | sub | mul | ... testop: eqz relop: eq | ne | lt | ... sign: s | u offset: offset= align: align=(1|2|4|8|...) cvtop: trunc | extend | wrap | ... vecunop: abs | neg | ... vecbinop: add | sub | min_ | ... vecternop: bitselect vectestop: all_true | any_true vecrelop: eq | ne | lt | ... veccvtop: extend_low | extend_high | trunc_sat | ... vecshiftop: shl | shr_ expr: ( ) ( + ) ;; = + () ( block ? * ) ( loop ? * ) ( if ? ( then * ) ( else * )? ) ( if ? + ( then * ) ( else * )? ) ;; = + (if ? (then *) (else *)?) instr: ;; = () block ? * end ? ;; = (block ? *) loop ? * end ? ;; = (loop ? *) if ? * end ? ;; = (if ? (then *)) if ? * else ? * end ? ;; = (if ? (then *) (else *)) op: unreachable nop br br_if br_table + return call call_indirect ? drop select local.get local.set local.tee global.get global.set table.get ? table.set ? table.size ? table.grow ? table.fill ? table.copy ? ? table.init ? elem.drop .load((8|16|32)_)? ? ? .store(8|16|32)? ? ? .load((8x8|16x4|32x2)_)? ? ? .store ? ? .load(8|16|32|64)_(lane|splat|zero) ? ? .store(8|16|32|64)_lane ? ? memory.size memory.grow memory.fill memory.copy memory.init data.drop ref.null ref.is_null ref.func .const . . . . ._(_)? .const + . . . . . ._(_)?(_)? . .bitmask .splat .extract_lane(_)? .replace_lane func: ( func ? * * ) ( func ? ( export ) <...> ) ;; = (export (func )) (func ? <...>) ( func ? ( import ) ) ;; = (import (func ? )) param: ( param * ) | ( param ) result: ( result * ) local: ( local * ) | ( local ) global: ( global ? * ) ( global ? ( export ) <...> ) ;; = (export (global )) (global ? <...>) ( global ? ( import ) ) ;; = (import (global ? )) table: ( table ? ) ( table ? ( export ) <...> ) ;; = (export (table )) (table ? <...>) ( table ? ( import ) ) ;; = (import (table ? )) ( table ? ( export )* ( elem * ) ) ;; = (table ? ( export )* ) (elem (i32.const 0) *) elem: ( elem ? (offset * ) * ) ( elem ? * ) ;; = (elem ? (offset ) *) ( elem ? declare * ) elem: ( elem ? ( table )? * ) ( elem ? ( table )? func * ) ;; = (elem ? ( table )? funcref (ref.func )*) ( elem ? declare? * ) ( elem ? declare? func * ) ;; = (elem ? declare? funcref (ref.func )*) offset: ( offset * ) ;; = ( offset ) item: ( item * ) ;; = ( item ) memory: ( memory ? ) ( memory ? ( export ) <...> ) ;; = (export (memory ))+ (memory ? <...>) ( memory ? ( import ) ) ;; = (import (memory ? )) ( memory ? ( export )* ( data * ) ) ;; = (memory ? ( export )* ) (data (i32.const 0) *) data: ( data ? ( memory )? * ) start: ( start ) typedef: ( type ? ( func * * ) ) import: ( import ) imkind: ( func ? ) ( global ? ) ( table ? ) ( memory ? ) export: ( export ) exkind: ( func ) ( global ) ( table ) ( memory ) module: ( module ? * * * * * ? * * * ? ) * * * *
* ? * * * ? ;; = ( module * * * *
* ? * * * ? ) ``` Here, productions marked with respective comments are abbreviation forms for equivalent expansions (see the explanation of the AST below). In particular, WebAssembly is a stack machine, so that all expressions of the form `( +)` are merely abbreviations of a corresponding post-order sequence of instructions. For raw instructions, the syntax allows omitting the parentheses around the operator name and its immediate operands. In the case of control operators (`block`, `loop`, `if`), this requires marking the end of the nested sequence with an explicit `end` keyword. Any form of naming via `` and `` (including expression labels) is merely notational convenience of this text format. The actual AST has no names, and all bindings are referred to via ordered numeric indices; consequently, names are immediately resolved in the parser and replaced by indices. Indices can also be used directly in the text format. The segment strings in the memory field are used to initialize the consecutive memory at the given offset. The `` in the expansion of the two short-hand forms for `table` and `memory` is the minimal size that can hold the segment: the number of ``s for tables, and the accumulative length of the strings rounded up to page size for memories. In addition to the grammar rules above, the fields of a module may appear in any order, except that all imports must occur before the first proper definition of a function, table, memory, or global. Comments can be written in one of two ways: ``` comment: ;; * (; ( | )* ;) ``` In particular, comments of the latter form nest properly. ## Scripts In order to be able to check and run modules for testing purposes, the S-expression format is interpreted as a very simple and dumb notion of "script", with commands as follows: ``` script: * cmd: ;; define, validate, and initialize module ( register ? ) ;; register module for imports ;; perform action and print results ;; assert result of an action ;; meta command module: ... ( module ? binary * ) ;; module in binary format (may be malformed) ( module ? quote * ) ;; module quoted in text (may be malformed) action: ( invoke ? * ) ;; invoke function export ( get ? ) ;; get global export const: ( .const ) ;; number value ( + ) ;; vector value ( ref.null ) ;; null reference ( ref.extern ) ;; host reference assertion: ( assert_return * ) ;; assert action has expected results ( assert_trap ) ;; assert action traps with given failure string ( assert_exhaustion ) ;; assert action exhausts system resources ( assert_malformed ) ;; assert module cannot be decoded with given failure string ( assert_invalid ) ;; assert module is invalid with given failure string ( assert_unlinkable ) ;; assert module fails to link ( assert_trap ) ;; assert module traps on instantiation result: ( .const ) ( .const + ) ( ref.extern ) ( ref.func ) num_pat: ;; literal result nan:canonical ;; NaN in canonical form nan:arithmetic ;; NaN with 1 in MSB of payload meta: ( script ?