int dec_val(char hex) {
  switch (hex) {
  case '0':
    return 0;
    break;
  case '1':
    return 1;
    break;
  case '2':
    return 2;
    break;
  case '3':
    return 3;
    break;
  case '4':
    return 4;
    break;
  case '5':
    return 5;
    break;
  case '6':
    return 6;
    break;
  case '7':
    return 7;
    break;
  case '8':
    return 8;
    break;
  case '9':
    return 9;
    break;
  case 'a':
    return 10;
    break;
  case 'b':
    return 11;
    break;
  case 'c':
    return 12;
    break;
  case 'd':
    return 13;
    break;
  case 'e':
    return 14;
    break;
  case 'f':
    return 15;
    break;
  }
  return 0;
}


x86 assembly (clang, -O0):
<dec_val>:
push   %rbp
mov    %rsp,%rbp
mov    %dil,%al
mov    %al,-0x5(%rbp)
movsbl -0x5(%rbp),%eax
add    $0xffffffd0,%eax
mov    %eax,%ecx
mov    %rcx,-0x10(%rbp)
sub    $0x36,%eax
ja     11f4 <dec_val+0xf4>
mov    -0x10(%rbp),%rax
lea    0xed5(%rip),%rcx
movslq (%rcx,%rax,4),%rax
add    %rcx,%rax
jmp    *%rax
movl   $0x0,-0x4(%rbp)
jmp    11fb <dec_val+0xfb>
movl   $0x1,-0x4(%rbp)
jmp    11fb <dec_val+0xfb>
movl   $0x2,-0x4(%rbp)
jmp    11fb <dec_val+0xfb>
movl   $0x3,-0x4(%rbp)
jmp    11fb <dec_val+0xfb>
movl   $0x4,-0x4(%rbp)
jmp    11fb <dec_val+0xfb>
movl   $0x5,-0x4(%rbp)
jmp    11fb <dec_val+0xfb>
movl   $0x6,-0x4(%rbp)
jmp    11fb <dec_val+0xfb>
movl   $0x7,-0x4(%rbp)
jmp    11fb <dec_val+0xfb>
movl   $0x8,-0x4(%rbp)
jmp    11fb <dec_val+0xfb>
movl   $0x9,-0x4(%rbp)
jmp    11fb <dec_val+0xfb>
movl   $0xa,-0x4(%rbp)
jmp    11fb <dec_val+0xfb>
movl   $0xb,-0x4(%rbp)
jmp    11fb <dec_val+0xfb>
movl   $0xc,-0x4(%rbp)
jmp    11fb <dec_val+0xfb>
movl   $0xd,-0x4(%rbp)
jmp    11fb <dec_val+0xfb>
movl   $0xe,-0x4(%rbp)
jmp    11fb <dec_val+0xfb>
movl   $0xf,-0x4(%rbp)
jmp    11fb <dec_val+0xfb>
movl   $0x0,-0x4(%rbp)
mov    -0x4(%rbp),%eax
pop    %rbp
ret

x86 assembly (clang, -O2):
<dec_val>:
xor    %eax,%eax
add    $0xffffffcf,%edi
cmp    $0x35,%edi
ja     116d <dec_val+0x6d>
lea    0xeef(%rip),%rcx
movslq (%rcx,%rdi,4),%rdx
add    %rcx,%rdx
jmp    *%rdx
mov    $0x1,%eax
ret
mov    $0x2,%eax
ret
mov    $0x3,%eax
ret
mov    $0x4,%eax
ret
mov    $0x5,%eax
ret
mov    $0x6,%eax
ret
mov    $0x7,%eax
ret
mov    $0x8,%eax
ret
mov    $0xa,%eax
ret
mov    $0xb,%eax
ret
mov    $0xc,%eax
ret
mov    $0xd,%eax
ret
mov    $0xe,%eax
ret
mov    $0xf,%eax
ret
mov    $0x9,%eax
ret

LLVM IR (-O0):
define i32 @dec_val(i8 signext %0) {
%2 = alloca i32
%3 = alloca i8
store i8 %0, i8* %3
%4 = load i8, i8* %3
%5 = sext i8 %4 to i32
switch i32 %5, label %22 [
i32 48, label %6
i32 49, label %7
i32 50, label %8
i32 51, label %9
i32 52, label %10
i32 53, label %11
i32 54, label %12
i32 55, label %13
i32 56, label %14
i32 57, label %15
i32 97, label %16
i32 98, label %17
i32 99, label %18
i32 100, label %19
i32 101, label %20
i32 102, label %21
]
6: ; preds = %1
store i32 0, i32* %2
br label %23
7: ; preds = %1
store i32 1, i32* %2
br label %23
8: ; preds = %1
store i32 2, i32* %2
br label %23
9: ; preds = %1
store i32 3, i32* %2
br label %23
10: ; preds = %1
store i32 4, i32* %2
br label %23
11: ; preds = %1
store i32 5, i32* %2
br label %23
12: ; preds = %1
store i32 6, i32* %2
br label %23
13: ; preds = %1
store i32 7, i32* %2
br label %23
14: ; preds = %1
store i32 8, i32* %2
br label %23
15: ; preds = %1
store i32 9, i32* %2
br label %23
16: ; preds = %1
store i32 10, i32* %2
br label %23
17: ; preds = %1
store i32 11, i32* %2
br label %23
18: ; preds = %1
store i32 12, i32* %2
br label %23
19: ; preds = %1
store i32 13, i32* %2
br label %23
20: ; preds = %1
store i32 14, i32* %2
br label %23
21: ; preds = %1
store i32 15, i32* %2
br label %23
22: ; preds = %1
store i32 0, i32* %2
br label %23
23: ; preds = %22, %21, %20, %19, %18, %17, %16, %15, %14, %13, %12, %11, %10, %9, %8, %7, %6
%24 = load i32, i32* %2
ret i32 %24
}

LLVM IR (-O2):
define i32 @dec_val(i8 signext %0) {
%2 = sext i8 %0 to i32
switch i32 %2, label %18 [
i32 102, label %17
i32 49, label %3
i32 50, label %4
i32 51, label %5
i32 52, label %6
i32 53, label %7
i32 54, label %8
i32 55, label %9
i32 56, label %10
i32 57, label %11
i32 97, label %12
i32 98, label %13
i32 99, label %14
i32 100, label %15
i32 101, label %16
]
3: ; preds = %1
br label %18
4: ; preds = %1
br label %18
5: ; preds = %1
br label %18
6: ; preds = %1
br label %18
7: ; preds = %1
br label %18
8: ; preds = %1
br label %18
9: ; preds = %1
br label %18
10: ; preds = %1
br label %18
11: ; preds = %1
br label %18
12: ; preds = %1
br label %18
13: ; preds = %1
br label %18
14: ; preds = %1
br label %18
15: ; preds = %1
br label %18
16: ; preds = %1
br label %18
17: ; preds = %1
br label %18
18: ; preds = %1, %17, %16, %15, %14, %13, %12, %11, %10, %9, %8, %7, %6, %5, %4, %3
%19 = phi i32 [ 15, %17 ], [ 14, %16 ], [ 13, %15 ], [ 12, %14 ], [ 11, %13 ], [ 10, %12 ], [ 9, %11 ], [ 8, %10 ], [ 7, %9 ], [ 6, %8 ], [ 5, %7 ], [ 4, %6 ], [ 3, %5 ], [ 2, %4 ], [ 1, %3 ], [ 0, %1 ]
ret i32 %19
}