The header file compiler_defs.h and the definitions it provides are deprecated, and have been removed from the latest SDK of the Silicon Lab IDE (5.50). The header has been replaced by si_toolchain.h. All of the example code provided with the Silicon Labs IDE now uses si_toolchain.h and its definitions.
The main difference between the two headers is the name of some of the defines. For example, SI_SBIT replaces SBIT, uint16_t replaces U16, and SI_INTERRUPT replaces INTERRUPT.
In order to migrate a project from one header to another:
For more information on why this change was made, see this KB article: https://www.silabs.com/community/mcu/8-bit/knowledge-base.entry.html/2017/03/31/software_licensinga-qzLc