What are CFI directives in Gnu Assembler (GAS) used for?

To disable these, use the gcc option

-fno-asynchronous-unwind-tables

-fno-dwarf2-cfi-asm may be needed also.

I've got a feeling it stands for Call Frame Information and is a GNU AS extension to manage call frames. From DeveloperWorks:

On some architectures, exception handling must be managed with Call Frame Information directives. These directives are used in the assembly to direct exception handling. These directives are available on Linux on POWER, if, for any reason (portability of the code base, for example), the GCC generated exception handling information is not sufficient.

It looks like these are generated on some platforms depending on need for exception handling.

If you are looking to disable these, please see David's answer.

The CFI directives are used for debugging. It allows the debugger to unwind a stack. For example: if procedure A calls procedure B which then calls a common procedure C. Procedure C fails. You now want to know who actually called C and then you may want to know who called B.

A debugger can unwind this stack by using the stack pointer (%rsp) and register %rbp, however it needs to know how to find them. That is where the CFI directives come in.

movq    %rsp, %rbp
.cfi_def_cfa_register 6

so the last line here tell it that the "Call frame address" is now in register 6 (%rbp)

To disable these, g++ needs -fno-exceptions along with the previously mentioned -fno-asynchronous-unwind-tables, provided that you don't use exceptions.