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Comment by chrisseaton | Hacker News Reader

In the compiler community, people usually use IR to mean something lower than bytecode, usually used as part of the compilation process itself rather than used for storage or when not actively compiling. IR is usually not intended for direct execution.

(LLVM IR is an unusual exception therefore. Please don't 'well actually' me I'm trying to give a useful practical explanation.)

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I thought IR was just the internal processing format used by a compiler, eg: MIR or LLVM IR, and so is at a level usually “higher” on the abstraction stack than assembly/bytecode, which can be seen as the primary outputs of a compiler

We'd say it is 'lower' not 'higher' as it usually decomposes single higher-level compound operations into multiple simpler operations, and it makes things that were implicit explicit so they can be reasoned about. Usually more verbose therefore. Converting into IR is often called 'lowering'.

IR is usually represented as a sort of linked list of instructions where you can easily move around instructions, add/remove them, and replace higher level instructions with lower level instructions as you go through compiler passes. At the end, the IR is a linked list of machine instructions, which gets written out to a code buffer as the last step.

Bytecode is usually a buffer of high level instructions that is compact and fast to read (for the machine). Good for interpreting and holding the semantics of a program in a format that is more efficient than the source code, but it is usually not a good format for running compiler passes.

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