[nix-shell:~/dev/zig/build-release]$ gdb ./test
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Reading symbols from ./test...
(gdb) break _start
Breakpoint 1 at 0x800000b: file test.zig, line 2.
(gdb) run
Starting program: /home/andy/dev/zig/build-release/test 

Breakpoint 1, _start () at test.zig:2
2	    assert(add(3, 4) == 7);
(gdb) step
add () at test.zig:9
9	    var x: u32 = undefined;
(gdb) 
10	    x = 0;
(gdb) 
11	    x += a;
(gdb) p x
No symbol "x" in current context.
(gdb) step
12	    x += b;
(gdb) 
13	    return x;
(gdb) 
assert () at test.zig:17
17	    if (!ok) unreachable; // assertion failure
(gdb) 
_start () at test.zig:3
3	    assert(add(20, 10) == 30);
(gdb) 
add () at test.zig:9
9	    var x: u32 = undefined;
(gdb) 
10	    x = 0;
(gdb) 
11	    x += a;
(gdb) 
12	    x += b;
(gdb) 
13	    return x;
(gdb) 
assert () at test.zig:17
17	    if (!ok) unreachable; // assertion failure
(gdb) 
_start () at test.zig:5
5	    exit();
(gdb) 
exit () at test.zig:21
21	    asm volatile ("syscall"
(gdb) 
[Inferior 1 (process 423) exited normally]
(gdb) quit