Most Truffle runtimes interpret source code, but there’s nothing that says you have to do that. The Sulong project is creating a Truffle interpreter for LLVM bitcode.
So I found this definition of what are bit codes at the LLVM documentation (IR = Intermediate Representation):
What is commonly known as the LLVM bitcode file format (also, sometimes anachronistically known as bytecode) is actually two things: a bitstream container format and an encoding of LLVM IR into the container format.
The bitstream format is an abstract encoding of structured data, very similar to XML in some ways. Like XML, bitstream files contain tags, and nested structures, and you can parse the file without having to understand the tags. Unlike XML, the bitstream format is a binary encoding, and unlike XML it provides a mechanism for the file to self-describe “abbreviations”, which are effectively size optimizations for the content.
LLVM IR files may be optionally embedded into a wrapper structure, or in a native object file. Both of these mechanisms make it easy to embed extra data along with LLVM IR files
So looking further I found helpful these two bits of information at Stack Overflow:
The format is literally a bitstream, not a bytestream. See this document for more details: http://llvm.org/docs/BitCodeFormat.html
The biggest difference between JVM bytecode and and LLVM bitcode is that JVM instructions are stack-oriented, whereas LLVM bitcode is not. This means that rather than loading values into registers, JVM bytecode loads values onto a stack and computes values from there. I believe that an advantage of this is that the compiler doesn't have to allocate registers, but I'm not sure.
LLVM bitcode is closer to machine-level code, but isn't bound by a particular architecture. For instance, I think that LLVM bitcode can make use of an arbitrary number of logical registers.
And openned the path to this comment "ARC in Swift is much better than Java GC" pointing to this article at Quora: Why doesn't Apple Swift adopt the memory management method of garbage collection like in Java? Which I'll definately read in the next days!