tinygo/README.markdown

TinyGo - Go compiler for microcontrollers

We never expected Go to be an embedded language and so it's got serious problems [...].

-- Rob Pike, GopherCon 2014 Opening Keynote

TinyGo is a project to bring Go to microcontrollers and small systems with a single processor core. It is similar to emgo but a major difference is that I want to keep the Go memory model (which implies garbage collection of some sort). Another difference is that TinyGo uses LLVM internally instead of emitting C, which hopefully leads to smaller and more efficient code and certainly leads to more flexibility.

My original reasoning was: if Python can run on microcontrollers, then certainly Go should be able to and run on even lower level micros.

Example program (blinky):

import "machine"

func main() {
	led := machine.GPIO{machine.LED}
	led.Configure(machine.GPIOConfig{Mode: machine.GPIO_OUTPUT})
	for {
		led.Low()
		runtime.Sleep(runtime.Millisecond * 1000)

		led.High()
		runtime.Sleep(runtime.Millisecond * 1000)
	}
}

Currently supported features:

• control flow
• many (but not all) basic types: most ints, floats, strings, structs
• function calling
• interfaces for basic types (with type switches and asserts)
• goroutines (very initial support)
• function pointers (non-blocking)
• interface methods
• standard library (but most packages won't work due to missing language features)
• slices (partially)
• maps (very rough, unfinished)

Not yet supported:

• complex numbers
• garbage collection
• defer
• closures
• channels
• introspection (if it ever gets implemented)
• ...

Supported targets

Most targets that are supported by LLVM should be supported by this compiler. This means amd64 (where most of the testing happens), ARM, and Cortex-M microcontrollers.

The AVR platform (as used by the Arduino, for example) is also supported when support for it is enabled in LLVM. However, because it is a Harvard style architecture with different address spaces for code and data and because LLVM turns globals into const for you (moving them to PROGMEM) most real programs don't work unfortunately. This can be fixed but that can be difficult to do efficiently and hasn't been implemented yet.

Analysis and optimizations

The goal is to reduce code size (and increase performance) by performing all kinds of whole-program analysis passes. The official Go compiler doesn't do a whole lot of analysis (except for escape analysis) becauses it needs to be fast, but embedded programs are necessarily smaller so it becomes practical. And I think especially program size can be reduced by a large margin when actually trying to optimize for it.

Implemented compiler passes:

• Analyse which functions are blocking. Blocking functions are functions that call sleep, chan send, etc. Its parents are also blocking.
• Analyse whether the scheduler is needed. It is only needed when there are go statements for blocking functions.
• Analyse whether a given type switch or type assert is possible with type-based alias analysis. I would like to use flow-based alias analysis in the future, if feasible.
• Do basic dead code elimination of functions. This pass makes later passes better and probably improves compile time as well.

Building

Requirements:

• Go 1.11+
• LLVM dependencies, see the Software section in the LLVM build guide

First download the sources (this takes a while):

go get -u github.com/aykevl/tinygo

You'll get an error like the following, this is expected:

src/github.com/aykevl/llvm/bindings/go/llvm/analysis.go:17:10: fatal error: 'llvm-c/Analysis.h' file not found
#include "llvm-c/Analysis.h" // If you are getting an error here read bindings/go/README.txt
         ^~~~~~~~~~~~~~~~~~~
1 error generated.

To continue, you'll need to build LLVM for Go. This will take about an hour and require a fair bit of RAM. In fact, I would recommend setting your ld binary to gold to speed up linking, especially on systems with less than 16GB RAM.

Also, I would recommend editing build.sh and set cmake_flags to:

cmake_flags="../../../../.. $@ -DLLVM_EXPERIMENTAL_TARGETS_TO_BUILD=AVR -DLLVM_LINK_LLVM_DYLIB=ON"

This will enable the experimental AVR backend (for Arduino support) and will make sure tinygo links to a shared library instead of a static library, greatly improving link time on every rebuild. This is especially useful during development.

After LLVM has been built, you can run an example with:

make run-hello

For a blinky example on the PCA10040 development board, do this:

make flash-blinky TARGET=pca10040

License

This project is licensed under the BSD 3-clause license, just like the Go project itself.