6.6.11.1 -fcall-saved-reg

Treat the register named reg as an allocatable register saved by functions. It may be allocated even for temporaries or variables that live across a call. Functions compiled this way will save and restore the register reg if they use it.

It is an error to use this flag with the Frame Pointer or Stack Pointer. Use of this flag for other registers that have fixed pervasive roles in the machine’s execution model will produce disastrous results.

A different sort of disaster will result from the use of this flag for a register in which function values may be returned.

This flag should be used consistently through all modules.