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

StackOverflow https://stackoverflow.com/questions/2529185

  •  22-09-2019
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Question

There seem to be a .CFI directive after every line and also there are wide varities of these ex.,.cfi_startproc , .cfi_endproc etc.. more here. 

    .file   "temp.c"
    .text
.globl main
    .type   main, @function
main:
.LFB0:
    .cfi_startproc
    pushq   %rbp
    .cfi_def_cfa_offset 16
    movq    %rsp, %rbp
    .cfi_offset 6, -16
    .cfi_def_cfa_register 6
    movl    $0, %eax
    leave
    ret
    .cfi_endproc
.LFE0:
    .size   main, .-main
.globl func
    .type   func, @function
func:
.LFB1:
    .cfi_startproc
    pushq   %rbp
    .cfi_def_cfa_offset 16
    movq    %rsp, %rbp
    .cfi_offset 6, -16
    .cfi_def_cfa_register 6
    movl    %edi, -4(%rbp)
    movl    %esi, %eax
    movb    %al, -8(%rbp)
    leave
    ret
    .cfi_endproc
.LFE1:
    .size   func, .-func
    .ident  "GCC: (Ubuntu 4.4.1-4ubuntu9) 4.4.1"
    .section    .note.GNU-stack,"",@progbits

I didn't get the purpose of these.

Was it helpful?

Solution

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.

OTHER TIPS

To disable these, use the gcc option

-fno-asynchronous-unwind-tables

Note, I know this is a really old thread, but this is the top result on google for cfi_startproc, so many people probably come here to disable that output.

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

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)

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