Manual decompilation of asm snippet - c

Question

I've been trying to decompile the following asm snippet(that's all I have):
55 push %rbp
48 89 e5 mov %rsp,%rbp
48 81 ec d0 00 00 00 sub $0xd0,%rsp
64 48 8b 04 25 28 00 mov %fs:0x28,%rax
00 00
48 89 45 f8 mov %rax,-0x8(%rbp)
31 c0 xor %eax,%eax
48 c7 85 30 ff ff ff movq $0x0,-0xd0(%rbp)
00 00 00 00
48 8d b5 38 ff ff ff lea -0xc8(%rbp),%rsi
b8 00 00 00 00 mov $0x0,%eax
ba 18 00 00 00 mov $0x18,%edx
48 89 f7 mov %rsi,%rdi
48 89 d1 mov %rdx,%rcx
f3 48 ab rep stos %rax,%es:(%rdi)
48 8b 15 19 06 20 00 mov 0x200619(%rip),%rdx
48 8d 85 30 ff ff ff lea -0xd0(%rbp),%rax
be ce 0f 40 00 mov $0x400fce,%esi
48 89 c7 mov %rax,%rdi
b8 00 00 00 00 mov $0x0,%eax
e8 4e fc ff ff callq 4008a0 <sprintf#plt>
Here is my attempt:
char buf[192] = {0};
sprintf(buf, "hello %s", name);
I've compiled this with gcc 4.8.5, and it gave me:
55 push %rbp
48 89 e5 mov %rsp,%rbp
48 81 ec d0 00 00 00 sub $0xd0,%rsp
64 48 8b 04 25 28 00 mov %fs:0x28,%rax
00 00
48 89 45 f8 mov %rax,-0x8(%rbp)
31 c0 xor %eax,%eax
48 8d b5 30 ff ff ff lea -0xd0(%rbp),%rsi
b8 00 00 00 00 mov $0x0,%eax
ba 18 00 00 00 mov $0x18,%edx
48 89 f7 mov %rsi,%rdi
48 89 d1 mov %rdx,%rcx
f3 48 ab rep stos %rax,%es:(%rdi)
48 8b 15 14 14 20 00 mov 0x201414(%rip),%rdx
48 8d 85 30 ff ff ff lea -0xd0(%rbp),%rax
be 2e 10 40 00 mov $0x40102e,%esi
48 89 c7 mov %rax,%rdi
b8 00 00 00 00 mov $0x0,%eax
e8 cb fb ff ff callq 4008a0 <sprintf#plt>
I'm struggling to figure out why this exists:
movq $0x0,-0xd0(%rbp)
and also the subsequent usage of -0xd0(%rbp) as a pointer for the argument to sprintf. I'm puzzled because the rep stos begin at -0xc8(%rbp) and not -0xd0(%rbp).
This is probably compiler specific, but still I'm curious what could possibly be the original code that produced that asm.

Answer 1

I imagine something like:
char buf[192] = {0, 0, 0, 0, 0, 0, 0, 0};
sprintf(buf + 8, "hello %s", name);
... would give you that output.
The movq instruction you refer to stores 0 (an 8-byte quantity) at the beginning of an array. The -0xc8(%rbp) comes from copying a string to an offset within the array.