Need help understanding GCC assembly code - c

Question

For my homework assignment I am supposed to convert this C code
#define UPPER 15
const int lower = 12;
int sum = 0;
int main(void) {
int i;
for (i = lower; i < UPPER; i++) {
sum += i;
}
return sum;
}
into gcc assembly. I already compiled it to first study the code before doing it per hand (obviously translating by hand is going to look much differently). This is the assembler code I received:
.file "upper.c"
.globl lower
.section .rodata
.align 4
.type lower, #object
.size lower, 4
lower:
.long 12
.globl sum
.bss
.align 4
.type sum, #object
.size sum, 4
sum:
.zero 4
.text
.globl main
.type main, #function
main:
.LFB0:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
movl $12, -4(%rbp)
jmp .L2
.L3:
movl sum(%rip), %edx
movl -4(%rbp), %eax
addl %edx, %eax
movl %eax, sum(%rip)
addl $1, -4(%rbp)
.L2:
cmpl $14, -4(%rbp)
jle .L3
movl sum(%rip), %eax
popq %rbp
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE0:
.size main, .-main
.ident "GCC: (SUSE Linux) 4.8.1 20130909 [gcc-4_8-branch revision 202388]"
.section .note.GNU-stack,"",#progbits
Now I was wondering if someone could give me a few examples like
where the constructors i, lower, upper and sum are located it in code
where some of the expressions i = lower or i < UPPER are located
where the for-loop starts
and such things so I can then get an idea of how the assembler code is constructed. Thank you!

Answer 1

If I understood correctly you question, here is the answers:
Q: where the constructors i, lower, upper and sum are located it in code?
lower is located inside .rodata section (readonly data section). It's value is initialized by linux loader during program loading stage to the value .long 12. lower constructor is a linux loader. It just loads lower value from binary image.
.globl lower
.section .rodata
.align 4
.type lower, #object
.size lower, 4
lower:
.long 12
sum is located inside .bss section (data segment containing statically-allocated variables). It's value is initialized by _init function what gets called when program execution begins. It's value is zero (.zero 4). Every variable located inside .bss section has zero as initial value (link to wiki's article for .bss).
.globl sum
.bss
.align 4
.type sum, #object
.size sum, 4
sum:
.zero 4
upper is a constant. The compiler did not put it's declaration into assembly. There is a reference to upper-1 (as $14) here:
.L2:
cmpl $14, -4(%rbp)
i is a on stack temporary variable. It's value is accessed using addresses relative %rbp (%rbp is a pointer to current function stack frame). The is no explicit declaration of i into assembly. There is no explicit stack reservation for i (no instruction like sub $0x8,%rsp at main preamble), I think, because main doesn't call another functions. Here is code for i initialization (note compiler knows that lower initial value is $12 and removes access to lower during i initialization):
movl $12, -4(%rbp)
Q: where some of the expressions i = lower or i < UPPER are located
i = lower:
movl $12, -4(%rbp)
jmp .L2
i < UPPER:
.L2:
cmpl $14, -4(%rbp)
jle .L3
i++:
addl $1, -4(%rbp)
sum += i;:
movl sum(%rip), %edx
movl -4(%rbp), %eax
addl %edx, %eax
movl %eax, sum(%rip)
return sum; (%eax register is used to hold function return value - more about this: X86 calling conventions):
jle .L3
movl sum(%rip), %eax
popq %rbp
.cfi_def_cfa 7, 8
ret
Q: where the for-loop starts
it start here:
movl $12, -4(%rbp)
jmp .L2