# In-Class Exercise 10: Unwind Information

Consider the following assembly code generated by GCC:

    func1:
      /* 0*/  push    rbp
      /* 1*/  movsx   rax, edi
      /* 4*/  mov     rbp, rsp
      /* 7*/  push    r12
      /* 9*/  push    rbx
      /* a*/  mov     rbx, rax
      /* d*/  lea     rax, [15+rax*4]
      /*15*/  and     rax, -16
      /*19*/  sub     rsp, rax
      /*1c*/  lea     rdi, [rsp+3]
      /*21*/  mov     r12, rdi
      /*24*/  and     rdi, -4
      /*28*/  call    ext2
      /*2d*/  shr     r12, 2
      /*31*/  mov     eax, DWORD PTR [0+r12*4]
      /*39*/  lea     rsp, [rbp-16]
      /*3d*/  add     eax, ebx
      /*3f*/  pop     rbx
      /*40*/  pop     r12
      /*42*/  pop     rbp
      /*43*/  ret

    func2:
      /* 0*/  push    rbp
      /* 1*/  mov     rbp, rsp
      /* 4*/  push    r15
      /* 6*/  push    r14
      /* 8*/  push    r13
      /* a*/  mov     r13d, edi
      /* d*/  push    r12
      /* f*/  mov     r12d, esi
      /*12*/  push    rbx
      /*13*/  mov     ebx, edi
      /*15*/  sub     rsp, 8
      /*19*/  cmp     edi, esi
      /*1b*/  jge     .L5
    .L11:
      /*1d*/  mov     eax, ebx
      /*1f*/  and     eax, 3
      /*22*/  je      .L6
      /*24*/  cmp     eax, 2
      /*27*/  je      .L7
      /*29*/  movsx   rax, ebx
      /*2c*/  mov     r14, rsp
      /*2f*/  lea     rax, [15+rax*4]
      /*37*/  and     rax, -16
      /*3b*/  sub     rsp, rax
      /*3e*/  lea     rdi, [rsp+3]
      /*43*/  mov     r15, rdi
      /*46*/  and     rdi, -4
      /*4a*/  call    ext2
      /*4f*/  shr     r15, 2
      /*53*/  add     ebx, DWORD PTR [0+r15*4]
      /*5b*/  mov     rsp, r14
    .L9:
      /*5e*/  cmp     r12d, ebx
      /*61*/  jg      .L11
    .L5:
      /*63*/  lea     rsp, [rbp-40]
      /*67*/  mov     eax, r12d
      /*6a*/  imul    eax, ebx
      /*6d*/  pop     rbx
      /*6e*/  pop     r12
      /*70*/  pop     r13
      /*72*/  pop     r14
      /*74*/  pop     r15
      /*76*/  pop     rbp
      /*77*/  ret
        .p2align 4,,10
        .p2align 3
    .L6:
      /*80*/  mov     edi, ebx
      /*82*/  mov     esi, r12d
      /*85*/  call    ext1
      /*8a*/  lea     ebx, [rax+r13]
      /*8e*/  jmp     .L9
        .p2align 4,,10
        .p2align 3
    .L7:
      /*90*/  mov     esi, ebx
      /*92*/  mov     edi, r12d
      /*95*/  call    ext1
      /*9a*/  lea     rsp, [rbp-40]
      /*9e*/  pop     rbx
      /*9f*/  add     eax, r13d
      /*a2*/  pop     r12
      /*a4*/  pop     r13
      /*a6*/  pop     r14
      /*a8*/  pop     r15
      /*aa*/  pop     rbp
      /*ab*/  ret

1. What does a typical CIE on x86-64 look like? Use `llvm-dwarfdump --eh-frame` on some binary (`/bin/false` should do).

2. Construct the call frame information (CFI) table for both functions.

3. Encode the CFI tables with DWARF CFI bytecode instructions. Make sure unwind info is correct at every instruction boundary. Refer to the lecture slides and/or the DWARF5 standard (Sec. 6.4.2) and look for viable instructions.

4. Can you reduce the size of the bytecode by changing or omitting instructions while maintaining correctness?

5. Can you optimize for the case that the unwind info only has to be correct at function calls?


## Solution (1–3)

```
# C input
base64 -d <<EOF | gunzip
H4sIAAAAAAAAA1WQwQ6CMBBE73zFnAwFDog30C8xHlosoRGJWUowMfy77VYt9rDJzM6+dmtGC/20
+9SMtoAroklM8CrvZU6z0c1jyylIgVcCd7yw98dZXhrWPOMMESRpO9MIiZxTpUutkVUFFt8JtUUS
TpABsfRm0EgJxxjxZ1qMbXvf2OGwbbRy0ihrZvBaVPjJ3AGhSMtb8x+t6u8zOa0KUEj/YlfdyXmw
ARk+gURsr0msHxIhg/K7vgEyV5KzXAEAAA==
EOF

# Compiled with: gcc -masm=intel -O2 -c -o obj.o src.c

# llvm-dwarfdump --eh-frame obj.o
base64 -d <<EOF | gunzip
H4sIAAAAAAAAA8VU226jMBB9Xr7C6msWhMEmLFIecilS3ypW2lZaVcgx4xSJSwSkqvbr11wK4Ral
rbbLC9hnZo7PcMZFfNRS55sII0AizWJWIIiERdRX21ItoihaAPvTwRcZiwHxNCkgKXJHkQA8j3f1
5kH1CxPU7mzvbhWE3IrDQYNn97D2XNOQAb8gy8M0GUZgCa1Ph1jysKKP3/zxbiS6TQNALAoPSRmE
BONFmjlV4o4VbAJSbYl5UJyyBLEgyCDPpZboFMvq2BoQokAWqVnxRimLPvhbd+0HIHwuJOL9vEcL
uwNSIXKQSr27+5qp2U/SozNYlad31ytZYWFXCavfcq3aT28NxXbTR6PtLEfu7hbxEFZth4+8/dY0
rf4gZNT0rtfNGVjwwhIOfpTypl89aa2SRdWUobyNlHcO9KpZ42oZHMK8gKxKnUkzJ3iwgVSDTB3g
Eamzcujyil81/Uv0V92Z+y8lijsUW5WaGsbW0/dR8LIJ3lwTzIbBjzVsGmVwPxUb9cogE4WI2Vfw
oUKNB03eWM988yDRL3iQdh4U/L960PyYB6d4MJ3xoLzoeh7sABOpRJ8z54zNiT2TMGd/Oq1dOhTr
Z+QZxBDvIfNzeaeBc8Vk9Nh/nFfK5RUKo0L90447f/3s0cvDR981fZS8Z/zoaP76g2LWK6JXK3I+
UphemEYqLtdtBpDYn2KxzJnbg1rzQ/9JTr6cvmf+IWWgf71Msf9Smcpf1eThVZ0JAAA=
EOF
```