What is TinyGo exactly?

A new compiler and a new runtime implementation.

Specifically:

• A new compiler using (mostly) the standard library to parse Go programs and using LLVM to optimize the code and generate machine code for the target architecture.

• A new runtime library that implements some compiler intrinsics, like a memory allocator, a scheduler, and operations on strings. Also, some packages that are strongly connected to the runtime like the sync package and the reflect package have been or will be re-implemented for use with this new compiler.