Swift 4.1 introduces new -Osize optimization mode.
In Swift 4.1 the compiler now supports a new optimization mode which enables dedicated optimizations to reduce code size.
The Swift compiler comes with powerful optimizations. When compiling with -O the compiler tries to transform the code so that it executes with maximum performance. However, this improvement in runtime performance can sometimes come with a tradeoff of increased code size. With the new -Osize optimization mode the user has the choice to compile for minimal code size rather than for maximum speed.
To enable the size optimization mode on the command line, use -Osize instead of -O.
Further reading : https://swift.org/blog/osize/