c++ - Tracing call stack in disassembled code

I am trying to debug a tricky core dump (from an -O2 optimized binary).

// Caller Function 

void caller(Container* c)
{
  std::list < Message*> msgs;
  if(!decoder.called(c->buf_, msgs))
  {
   ....
   .....
  }
// Called Function
bool

Decoder::called(Buffer* buf, list < Message*>& msgs)
{
   add_data(buf); // Inlined code to append buf to decoders buf chain
   while(m_data_in && m_data_in->length() > 0)
   {
     .....
   }
}

In both caller and the callee, the first argument is optimized out, that means it must be somewhere in the register.

Caller Disassembly:

push   %r15
mov    %rdi,%r15
push   %r14
push   %r13
push   %r12
push   %rbp
push   %rbx

sub    $0x68,%rsp
test   %rsi,%rsi
je     0x8ccd62 
cmpq   $0x0,(%rsi)
je     0x8ccd62 
lea    0x40(%rsp),%rax
lea    0x1b8(%rdi),%rdi
mov    %rax,(%rsp)
mov    %rax,0x40(%rsp)
mov    %rax,%rdx

mov    %rax,0x48(%rsp)
mov    (%rsi),%rsi
callq  0x8cc820 

Caller Register Info:

rax            0x7fbfffc7e0 548682057696
rbx            0x2a97905ba0 182931446688

rcx            0x0  0
rdx            0x2  2
rsi            0x1  1
rdi            0x7fbfffc7e2 548682057698
rbp            0x4f 0x4f
rsp            0x7fbfffc870 0x7fbfffc870
r8             0x40 64
r9             0x20 32
r10            0x7fbfffc7e0 548682057696
r11            0x2abe466600 183580911104

r12            0x7fbfffd910 548682062096 // THIS IS HOLDING buf_
r13            0x7fbfffdec0 548682063552
r14            0x5dc    1500
r15            0x2a97905ba0 182931446688
rip            0x8cca89 0x8cca89 
eflags         0x206    [ PF IF ]
cs             0x33 51
ss             0x2b 43
ds             0x0  0
es             0x0  0

fs             0x0  0
gs             0x0  0

Called function Disassembly:

push   %r14
push   %r13
mov    %rdx,%r13

push   %r12
mov    %rdi,%r12
push   %rbp
push   %rbx
sub    $0x10,%rsp
mov    0x8(%rdi),%rdx
test   %rdx,%rdx
jne    0x8cc843 
jmpq   0x8cc9cb 
mov    %rax,%rdx

mov    0x8(%rdx),%rax
test   %rax,%rax
mov    %rsi,0x8(%rdx)
mov    0x8(%r12),%rax
test   %rax,%rax
xor    %edx,%edx
add    0x4(%rax),%edx
mov    0x8(%rax),%rax
lea    0x8(%rsp),%rsi
mov    %r12,%rdi

movq   $0x0,0x8(%rsp)

Called function Register Info
:

rax            0x7fbfffc7e0     548682057696
rbx            0x2abc49f9c0     183547591104
rcx            0x0      0
rdx            0x2      2

rsi            0x1      1
rdi            0x7fbfffc7e2     548682057698
rbp            0xffffffff       0xffffffff
rsp            0x7fbfffc830     0x7fbfffc830
r8             0x40     64
r9             0x20     32
r10            0x7fbfffc7e0     548682057696
r11            0x2abe466600     183580911104
r12            0x2a97905d58     182931447128
r13            0x7fbfffc8b0     548682057904

r14            0x5dc    1500
r15            0x2a97905ba0     182931446688
rip            0x8cc88a 0x8cc88a 
eflags         0x206    [ PF IF ]
cs             0x33     51
ss             0x2b     43
ds             0x0      0
es             0x0      0
fs             0x0      0
gs             0x0      0

The issue is, in the called function, it appears that "add_data" function achieved nothing.
So, wanted to know whether in disassembly of called function, do we see the "buf_" pointer being used anywhere (Register r12 in callee function).

I do understand assembly to some level, but all those code inlining has left me confused.
Would appreciate some help in demistifying called function disassembly.

UPDATE:
add_data does below:

if (m_data_in) {
    m_data_in->next = data;
} else {
    m_data_in = data;
}