How to associate assembly code to exact line in C program?

Question

Here is an example found via an assembly website. This is the C code:

 int main()
 {
     int a = 5;
     int b = a + 6;
     return 0;
 }

Here is the associated assembly code:

    (gdb) disassemble
    Dump of assembler code for function main:
    0x0000000100000f50 <main+0>:    push   %rbp
    0x0000000100000f51 <main+1>:    mov    %rsp,%rbp
    0x0000000100000f54 <main+4>:    mov    $0x0,%eax
    0x0000000100000f59 <main+9>:    movl   $0x0,-0x4(%rbp)
    0x0000000100000f60 <main+16>:   movl   $0x5,-0x8(%rbp)
    0x0000000100000f67 <main+23>:   mov    -0x8(%rbp),%ecx
    0x0000000100000f6a <main+26>:   add    $0x6,%ecx
    0x0000000100000f70 <main+32>:   mov    %ecx,-0xc(%rbp)
    0x0000000100000f73 <main+35>:   pop    %rbp
    0x0000000100000f74 <main+36>:   retq   
    End of assembler dump.

I can safely assume that this line of assembly code:

  0x0000000100000f6a <main+26>:   add    $0x6,%ecx

correlates to this line of C:

     int b = a + 6;

But is there a way to extract which lines of assembly are associated to the specific line of C code?
In this small sample it's not too difficult, but in larger programs and when debugging a larger amount of code it gets a bit cumbersome.

Answer 1

But is there a way to extract which lines of assembly are associated to the specific line of C code?

Yes, in principle - your compiler can probably do it (GCC option -fverbose-asm, for example). Alternatively, objdump -lSd or similar will disassemble a program or object file with source and line number annotations where available.

In general though, for a large optimized program, this can be very hard to follow.

Even with perfect annotation, you'll see the same source line mentioned multiple times as expressions and statements are split up, interleaved and reordered, and some instructions associated with multiple source expressions.

In this case, you just need to think about the relationship between your source and the assembly, but it takes some effort.

Answer 2

One of the best tools I've found for this is Matthew Godbolt's Compiler Explorer.

It features multiple compiler toolchains, auto-recompiles, and it immediately shows the assembly output with colored lines to show the corresponding line of source code.