What is .cfi and .LFE in assembly code produced by GCC from c++ program?

Question

I have the following c++ code

int factorial(int n){      if(n==0){         return 1;     }     return n*factorial(n-1);  }  int main(void){     factorial(5);     return 0; } 

When I create an assembly file using g++ -S factorial.cpp I get the following:

    .file   "tail_call_opt.cpp"     .text .globl _Z9factoriali     .type   _Z9factoriali, @function _Z9factoriali: .LFB0:     .cfi_startproc     .cfi_personality 0x0,__gxx_personality_v0     pushl   %ebp     .cfi_def_cfa_offset 8     movl    %esp, %ebp     .cfi_offset 5, -8     .cfi_def_cfa_register 5     subl    $24, %esp     cmpl    $0, 8(%ebp)     jne .L2     movl    $1, %eax     jmp .L3 .L2:     movl    8(%ebp), %eax     subl    $1, %eax     movl    %eax, (%esp)     call    _Z9factoriali     imull   8(%ebp), %eax .L3:     leave     ret     .cfi_endproc .LFE0:     .size   _Z9factoriali, .-_Z9factoriali .globl main     .type   main, @function main: .LFB1:     .cfi_startproc     .cfi_personality 0x0,__gxx_personality_v0     pushl   %ebp     .cfi_def_cfa_offset 8     movl    %esp, %ebp     .cfi_offset 5, -8     .cfi_def_cfa_register 5     andl    $-16, %esp     subl    $16, %esp     movl    $5, (%esp)     call    _Z9factoriali     movl    $0, %eax     leave     ret     .cfi_endproc .LFE1:     .size   main, .-main     .ident  "GCC: (Ubuntu 4.4.3-4ubuntu5) 4.4.3"     .section    .note.GNU-stack,"",@progbits 

I can follow most of this, but what is the purpose of the .cfi and the .LFE parts? Where can I learn more about the assembly produced by gcc?

Answer 1

These directives tell gas to emit Dwarf Call Frame Information tags which are apparently used to reconstruct a stack backtrace when a frame pointer is missing. In your case the frame pointer is present, so I guess it could be used to perform unwinding during exception handling. Such mechanism has less overhead than the old sjlj (setjump/longjump) one. See here, and also the linked Dwarf spec.

As for .Lxx labels, .L prefix indicates that the label is local to this file and so will not conflict with the same-name labels in other files. GCC generally uses .L for auto-generated labels. In this case most likely "FB" means "function begin" and "FE" means "function end".