This document describes the LLVM bitstream file format and the encoding of the LLVM IR into it.

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, like 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.

This document first describes the LLVM bitstream format, then describes the record structure used by LLVM IR files.

The bitstream format is literally a stream of bits, with a very simple structure. This structure consists of the following concepts:

• A magic number that identifies the stream.
• Encoding primitives like variable bit-rate integers.
• Blocks, which define nested content.
• Data Records, which describe entities within the file.
• Abbreviations, which specify compression optimizations for the file.

Note that the llvm-bcanalyzer tool can be used to dump and inspect arbitrary bitstreams, which is very useful for understanding the encoding.

LLVM

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