user: 请将这段汇编代码片段转换成等效的C代码：

```asm
<Concatenate2>:
  endbr64
  push   %rbp
  mov    %rsp,%rbp
  push   %r15
  push   %r14
  push   %r13
  mov    %rdx,%r13
  push   %r12
  mov    %rsi,%r12
  push   %rbx
  mov    %ecx,%ebx
  sub    $0x28,%rsp
  mov    %rdi,-0x48(%rbp)
  mov    %rsi,%rdi
  mov    %fs:0x28,%rax
  mov    %rax,-0x38(%rbp)
  xor    %eax,%eax
  call   <GetStringLength@plt>
  mov    %r13,%rdi
  mov    %eax,%r15d
  call   <GetStringLength@plt>
  mov    %eax,%r14d
  mov    %r15d,%eax
  or     %r14d,%eax
  js     L138
  cmp    $0xffffffff,%ebx
  je     L120
  cmp    %ebx,%r15d
  jl     L138
  test   %ebx,%ebx
  js     L138
L77:
  lea    (%r14,%rbx,1),%eax
  mov    $0x0,%edx
  lea    D8(%rip),%rsi
  sub    %r15d,%eax
  cmovs  %edx,%eax
  add    %eax,%r15d
  mov    %r15d,%edi
  call   <GetStackSize@plt>
  mov    %rsp,%rcx
  cltq
  add    $0xf,%rax
  mov    %rax,%rdx
  and    $0xfffffffffffff000,%rax
  sub    %rax,%rcx
  and    $0xfffffffffffffff0,%rdx
  cmp    %rcx,%rsp
  je     L50
L86:
  sub    $0x1000,%rsp
  orq    $0x0,0xff8(%rsp)
  cmp    %rcx,%rsp
  jne    L86
L50:
  and    $0xfff,%edx
  sub    %rdx,%rsp
  test   %rdx,%rdx
  je     L15
  orq    $0x0,-0x8(%rsp,%rdx,1)
L15:
  mov    %rsp,%r9
  xor    %esi,%esi
  test   %r15d,%r15d
  je     L115
  movslq %r15d,%rsi
  xor    %eax,%eax
  mov    $0xffffffff,%edx
  jmp    L132
  xchg   %ax,%ax
L129:
  movzbl 0x0(%r13),%ecx
  mov    $0x1,%edx
L18:
  mov    %cl,(%r9,%rax,1)
  add    $0x1,%rax
  cmp    %rsi,%rax
  je     L115
L132:
  cmp    $0xffffffff,%edx
  sete   %cl
  cmp    %eax,%ebx
  jne    L81
  test   %cl,%cl
  jne    L129
L81:
  cmp    %edx,%r14d
  je     L39
  test   %cl,%cl
  jne    L39
  movslq %edx,%rcx
  add    $0x1,%edx
  movzbl 0x0(%r13,%rcx,1),%ecx
  jmp    L18
L138:
  mov    -0x48(%rbp),%rdi
  call   <ReturnNullMallocStack@plt>
L124:
  mov    -0x38(%rbp),%rdx
  sub    %fs:0x28,%rdx
  jne    L55
  lea    -0x28(%rbp),%rsp
  pop    %rbx
  pop    %r12
  pop    %r13
  pop    %r14
  pop    %r15
  pop    %rbp
  ret
  nopw   0x0(%rax,%rax,1)
L120:
  mov    %r15d,%ebx
  jmp    L77
  nopl   0x0(%rax,%rax,1)
L39:
  movzbl (%r12,%rax,1),%edx
  mov    %dl,(%r9,%rax,1)
  add    $0x1,%rax
  mov    $0xffffffff,%edx
  cmp    %rsi,%rax
  jne    L132
L115:
  movb   $0x0,(%r9,%rsi,1)
  mov    -0x48(%rbp),%rdi
  mov    %r15d,%edx
  mov    %r9,%rsi
  call   <ReturnOutputMallocStack@plt>
  jmp    L124
L55:
  call   <__stack_chk_fail@plt>
```

tool_calls: name: extract_symbolic_data
arguments: label: D8
type: string

tool: D8: char

assistant: ```c
char *Concatenate2(char *output, char *first, char *second, const int startI) {
  int length1 = GetStringLength(first);
  int length2 = GetStringLength(second);
  if (length1 < 0 || length2 < 0) {
    return ReturnNullMallocStack(output);
  }
  int startI2 = startI;
  if (startI2 == -1)
    startI2 = length1;
  if (startI2 > length1 || startI2 < 0) {
    return ReturnNullMallocStack(output);
  }
  int extra = startI2 + length2 - length1;
  if (extra < 0)
    extra = 0;
  int length0 = length1 + (extra < 0 ? length2 : extra);
  char output2[GetStackSize(length0, "char")];
  int i = 0;
  int i2 = -1;
  for (i = 0; i < length0; i++) {
    if (i2 == -1 && i == startI2)
      i2 = 0;
    else if (i2 == length2)
      i2 = -1;
    output2[i] = (i2 == -1 ? first[i] : second[i2]);
    if (i2 != -1)
      i2++;
  }
  output2[i] = '\0';
  return ReturnOutputMallocStack(output, output2, i);
}
```