[SOLVED] Understanding x86 gcc assembly output

Issue

Pasted below is unoptimized GCC assembly output for "int main(){}". I'm relatively good with x86 assembly, but some of this is unfamiliar. Could someone please do a line-by-line walk-through of what's going on here?

    .text
.globl _main
_main:
LFB2:
    pushq   %rbp
LCFI0:
    movq    %rsp, %rbp
LCFI1:
    leave
    ret
LFE2:
    .section __TEXT,__eh_frame,coalesced,no_toc+strip_static_syms+live_support
EH_frame1:
    .set L$set$0,LECIE1-LSCIE1
    .long L$set$0
LSCIE1:
    .long   0x0
    .byte   0x1
    .ascii "zR\0"
    .byte   0x1
    .byte   0x78
    .byte   0x10
    .byte   0x1
    .byte   0x10
    .byte   0xc
    .byte   0x7
    .byte   0x8
    .byte   0x90
    .byte   0x1
    .align 3
LECIE1:
.globl _main.eh
_main.eh:
LSFDE1:
    .set L$set$1,LEFDE1-LASFDE1
    .long L$set$1
LASFDE1:
    .long   LASFDE1-EH_frame1
    .quad   LFB2-.
    .set L$set$2,LFE2-LFB2
    .quad L$set$2
    .byte   0x0
    .byte   0x4
    .set L$set$3,LCFI0-LFB2
    .long L$set$3
    .byte   0xe
    .byte   0x10
    .byte   0x86
    .byte   0x2
    .byte   0x4
    .set L$set$4,LCFI1-LCFI0
    .long L$set$4
    .byte   0xd
    .byte   0x6
    .align 3
LEFDE1:
    .subsections_via_symbols

Solution

Tell the linker to put this into the executable's .text section:

    .text

Export main as a external symbol:

.globl _main

Define the main function itself:

_main:
LFB2:

Save the previous frame pointer:

    pushq   %rbp
LCFI0:

Set up a new frame pointer:

    movq    %rsp, %rbp
LCFI1:

Restore the old frame pointer and return to caller:

    leave
    ret

The following directives are setting up an .eh_frame section, containing information required by the C++ runtime for exception handling.

LFE2:
    .section __TEXT,__eh_frame,coalesced,no_toc+strip_static_syms+live_support

This is the Common Information Entry table:

EH_frame1:

It starts with a length, calculated from the difference of the LSCIE1 and LECIE1 labels:

    .set L$set$0,LECIE1-LSCIE1
    .long L$set$0

(The .long, .byte, .ascii and .quad cause a value of the appropriate type to be directly emitted by the assembler). Then follows the CIE table itself:

LSCIE1:
    .long   0x0
    .byte   0x1
    .ascii "zR\0"
    .byte   0x1
    .byte   0x78
    .byte   0x10
    .byte   0x1
    .byte   0x10
    .byte   0xc
    .byte   0x7
    .byte   0x8
    .byte   0x90
    .byte   0x1
    .align 3
LECIE1:

Another external symbol, this one for the main function's Frame Description Entry (still part of the exception handling information):

.globl _main.eh
_main.eh:

Again, the FDE starts with a length:

LSFDE1:
    .set L$set$1,LEFDE1-LASFDE1
    .long L$set$1

..and continues with the rest of the FDE table.

LASFDE1:
    .long   LASFDE1-EH_frame1
    .quad   LFB2-.
    .set L$set$2,LFE2-LFB2
    .quad L$set$2
    .byte   0x0
    .byte   0x4
    .set L$set$3,LCFI0-LFB2
    .long L$set$3
    .byte   0xe
    .byte   0x10
    .byte   0x86
    .byte   0x2
    .byte   0x4
    .set L$set$4,LCFI1-LCFI0
    .long L$set$4
    .byte   0xd
    .byte   0x6
    .align 3
LEFDE1:
    .subsections_via_symbols

Those exception handling tables are mostly uninteresting, but if you want to know then information on the format is available here.

Answered By - caf

Answer Checked By - Mildred Charles (WPSolving Admin)