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wag is a WebAssembly compiler implemented as a Go package.


  • The input is a wasm binary module.

  • The output is machine code.

  • It is only a compiler. A runtime environment for the compiled program, including all import functions, needs to be implemented separately. (But see wasys for an example runtime.)

  • Single-pass, fast ahead-of-time compilation. Early functions can be executed while the latter functions are still being compiled, even while the source is still being downloaded.

  • The generated code requires minimal runtime support; it's designed to be executed in an isolated environment. Calling standard library ABIs is not directly supported, but see wasys for an example which exposes syscalls as WebAssembly import functions.

  • Supports snapshot-and-restore across compiler versions and CPU architectures.

  • Supports breakpoint debugging via recompilation.

  • Cross-compilation is supported via Go build tags. If wagamd64 is specified, the x86-64 code generator is used regardless of host architecture, and CPU feature detection is disabled with pessimistic assumptions. Likewise for wagarm64 (but feature detection is not used for ARM64).


  • Supports WebAssembly version 1 (wasm32). No wasm extensions are supported.

  • Supports x86-64 and ARM64 code generation.

  • Generated x86-64 code requires SSE4.1 ROUNDSS/ROUNDSD instructions.


Spectre variant 1: Out-of-bounds linear memory access detection requires that addressable but unallocated memory is inaccessible. It naturally prevents conditional branch exploitation.

Spectre variant 2: On x86, Retpoline is used to protect the runtime environment (although user programs shouldn't be able to inject arbitrary addresses into the branch target buffer).


Requires Linux, Go, make, cmake, clang++ and libcapstone. About 75% of the WebAssembly spec testsuite is run, by first converting the tests to binary format:

  • git clone --recurse-submodules
  • cd wag
  • make check

Screenshot #1

$ go get
$ wasys -v $GOPATH/src/
import write(i32, i32, i32) i32
import openat(i32, i32, i32, i32) i32
import read(i32, i32, i32) i32
import close(i32) i32
import pipe2(i32, i32) i32
import _exit(i32)
hello, world

Screenshot #2

=== RUN   TestSnapshot
--- PASS: TestSnapshot (0.00s)
    snapshot_test.go:80: print output:
        --- snapshotting ---
        current memory limit: 0x6a96051ca000
        current stack ptr:    0x6a960533ffc0
        globals+memory addr:  0x6a96051ba000
        stack addr:           0x6a960533f000
        globals+memory size:  65536
        memory size:          65536
        stack offset:         4032
        #1  func-3
        #2  func-2
        --- shot snapped ---
    snapshot_test.go:88: resuming
    snapshot_test.go:100: print output:
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