Reducing code size with LLVM Machine Outliner on 32-bit Arm targets
Yvan Roux | 9 mins read
With the upcoming release of LLVM 12.0.0, 32-bit Arm targets have gained the full support of the Machine Outliner code size optimization for Arm and Thumb-2 instruction sets. The expected code size gain provided by this optimization is around 5% on average (you can jump straight to the results part for more details). It is not turned on by default (see How to use it section) but our goal is to have it enabled under -Oz for all Arm cores inside LLVM 13.0.0.
Function outlining is a compilation process which consists of replacing a chunk of consecutive statements with a call to a new function containing those statements. In a nutshell, it is the inverse of the well known inlining optimization. It is used in different areas of compilation to achieve various goals such as code refactoring or kernel extraction in source to source compilers, shrinking large functions to reduce compile time in