llvm-9-doc 1:9.0.1-6build1 (amd64 binary) in ubuntu focal

 LLVM is a collection of libraries and tools that make it easy to build
 compilers, optimizers, just-in-time code generators, and many other
 compiler-related programs.
 .
 LLVM uses a single, language-independent virtual instruction set both
 as an offline code representation (to communicate code between
 compiler phases and to run-time systems) and as the compiler internal
 representation (to analyze and transform programs). This persistent
 code representation allows a common set of sophisticated compiler
 techniques to be applied at compile-time, link-time, install-time,
 run-time, or "idle-time" (between program runs).
 .
 This package contains all documentation (extensive).