Function merging is an important technique for reducing code size by combining identical or similar functions into a single function. This technique has been extensively researched in both industry and academia. However, the existing methodologies have not been evaluated with -Oz, which includes aggressive outlinings, and the linker's identical code folding, which can already fold identical pieces of code. Additionally, none of these methodologies suggest a sound approach that works globally with ThinLTO, which is crucial when building large apps. In this talk, we propose our global merge function (GMF), which utilizes global merge information obtained from a prior codegen run and optimistically creates merging instances within each module context independently. Our evaluation showed that GMF can reduce code size in real-world iOS apps by up to 3.5% on top of state-of-the-art outliners that are fully enabled with ThinLTO.

Elvina Yakubova