NativeClient Forth

Saturday November 19, 2011

Motivation

• NativeClient allows machine code to run in the browser.
• Forth excels at bootstrapping new environments.
• My past Javascript Forths have been slow.

NativeClient (NaCl)

• Static Verification Sandbox
• Multi-threaded
• Execution in the Web Browser (currently requires web store install)
• Resource restrictions similar to Javascript
• Performance similar to machine code ~15%
• Only option for native code (machine code) under ChromeOS/ChromiumOS
• Sandbox versions for x86, x86-64, (ARM coming soon)
• Portable NaCl (PNaCl) coming soon

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NaCl Sandboxes

• Each 32-byte block of code can be straight line disassembled
• All jumps / calls / returns must be to a 32-byte boundary
• Enforced by requiring masking off bottom bits before control flow instructions
• Data writes restricted via page faults / segments
• I/O via system provided 'trampolines'
• Special compiler (gcc/llvm) - validation at load time
• Dynamic code (validated) via trampoline calls

Implementation

• Indirect-threaded kernel in ~750 lines of C code
• Uses gcc computed gotos for NEXT
• Avoids having to code x86-32 + x86-64 manually
• Kernel uses ~250 lines Javascript for simple I/O
• Google Accounts used to manage access to cloud storage
• Abandoned previous ColorForth based implementation 
• communication model to NaCl changed before release
• it was only x86-32
• given alignment constraints, its subroutine threading was probably slower than indirect threading

Moving Parts

Future Directions

• Implement PPAPI bindings
• Allows 2d/3d graphics + sound
• Allows direct HTTP
• Generate platform specific shims (create .EXEs etc for download)
• Turn into a per-page extension (allow inline Forth on webpages)
• Resurrect dynamic code generation for CODE words or subroutine-threading / optimization

Demo / Code Tour

Questions?