char ascii2bits(char byte) {
  char ret = 0x00;
  switch (byte) {
  case '0':
    ret = 0x00;
    break;
  case '1':
    ret = 0x01;
    break;
  case '2':
    ret = 0x02;
    break;
  case '3':
    ret = 0x03;
    break;
  case '4':
    ret = 0x04;
    break;
  case '5':
    ret = 0x05;
    break;
  case '6':
    ret = 0x06;
    break;
  case '7':
    ret = 0x07;
    break;
  case '8':
    ret = 0x08;
    break;
  case '9':
    ret = 0x09;
    break;
  case 'A':
  case 'a':
    ret = 0x0a;
    break;
  case 'B':
  case 'b':
    ret = 0x0b;
    break;
  case 'C':
  case 'c':
    ret = 0x0c;
    break;
  case 'D':
  case 'd':
    ret = 0x0d;
    break;
  case 'E':
  case 'e':
    ret = 0x0e;
    break;
  case 'F':
  case 'f':
    ret = 0x0f;
    break;
  default:
    ret = 0x00;
    break;
  }
  return ret;
}


x86 assembly (clang, -O0):
<ascii2bits>:
push   %rbp
mov    %rsp,%rbp
mov    %dil,%al
mov    %al,-0x1(%rbp)
movb   $0x0,-0x2(%rbp)
movsbl -0x1(%rbp),%eax
add    $0xffffffd0,%eax
mov    %eax,%ecx
mov    %rcx,-0x10(%rbp)
sub    $0x36,%eax
ja     11c8 <ascii2bits+0xc8>
mov    -0x10(%rbp),%rax
lea    0xed1(%rip),%rcx
movslq (%rcx,%rax,4),%rax
add    %rcx,%rax
jmp    *%rax
movb   $0x0,-0x2(%rbp)
jmp    11cc <ascii2bits+0xcc>
movb   $0x1,-0x2(%rbp)
jmp    11cc <ascii2bits+0xcc>
movb   $0x2,-0x2(%rbp)
jmp    11cc <ascii2bits+0xcc>
movb   $0x3,-0x2(%rbp)
jmp    11cc <ascii2bits+0xcc>
movb   $0x4,-0x2(%rbp)
jmp    11cc <ascii2bits+0xcc>
movb   $0x5,-0x2(%rbp)
jmp    11cc <ascii2bits+0xcc>
movb   $0x6,-0x2(%rbp)
jmp    11cc <ascii2bits+0xcc>
movb   $0x7,-0x2(%rbp)
jmp    11cc <ascii2bits+0xcc>
movb   $0x8,-0x2(%rbp)
jmp    11cc <ascii2bits+0xcc>
movb   $0x9,-0x2(%rbp)
jmp    11cc <ascii2bits+0xcc>
movb   $0xa,-0x2(%rbp)
jmp    11cc <ascii2bits+0xcc>
movb   $0xb,-0x2(%rbp)
jmp    11cc <ascii2bits+0xcc>
movb   $0xc,-0x2(%rbp)
jmp    11cc <ascii2bits+0xcc>
movb   $0xd,-0x2(%rbp)
jmp    11cc <ascii2bits+0xcc>
movb   $0xe,-0x2(%rbp)
jmp    11cc <ascii2bits+0xcc>
movb   $0xf,-0x2(%rbp)
jmp    11cc <ascii2bits+0xcc>
movb   $0x0,-0x2(%rbp)
movsbl -0x2(%rbp),%eax
pop    %rbp
ret

x86 assembly (clang, -O2):
<ascii2bits>:
xor    %eax,%eax
add    $0xffffffcf,%edi
cmp    $0x35,%edi
ja     112b <ascii2bits+0x2b>
lea    0xeef(%rip),%rcx
movslq (%rcx,%rdi,4),%rdx
add    %rcx,%rdx
jmp    *%rdx
mov    $0xa,%al
ret
mov    $0xb,%al
ret
mov    $0xc,%al
ret
mov    $0xd,%al
ret
mov    $0xe,%al
ret
mov    $0xf,%al
ret
mov    $0x1,%al
ret
mov    $0x2,%al
ret
mov    $0x3,%al
ret
mov    $0x4,%al
ret
mov    $0x5,%al
ret
mov    $0x6,%al
ret
mov    $0x7,%al
ret
mov    $0x8,%al
ret
mov    $0x9,%al
ret

LLVM IR (-O0):
define signext i8 @ascii2bits(i8 signext %0) {
%2 = alloca i8
%3 = alloca i8
store i8 %0, i8* %2
store i8 0, i8* %3
%4 = load i8, i8* %2
%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 65, label %16
i32 97, label %16
i32 66, label %17
i32 98, label %17
i32 67, label %18
i32 99, label %18
i32 68, label %19
i32 100, label %19
i32 69, label %20
i32 101, label %20
i32 70, label %21
i32 102, label %21
]
6: ; preds = %1
store i8 0, i8* %3
br label %23
7: ; preds = %1
store i8 1, i8* %3
br label %23
8: ; preds = %1
store i8 2, i8* %3
br label %23
9: ; preds = %1
store i8 3, i8* %3
br label %23
10: ; preds = %1
store i8 4, i8* %3
br label %23
11: ; preds = %1
store i8 5, i8* %3
br label %23
12: ; preds = %1
store i8 6, i8* %3
br label %23
13: ; preds = %1
store i8 7, i8* %3
br label %23
14: ; preds = %1
store i8 8, i8* %3
br label %23
15: ; preds = %1
store i8 9, i8* %3
br label %23
16: ; preds = %1, %1
store i8 10, i8* %3
br label %23
17: ; preds = %1, %1
store i8 11, i8* %3
br label %23
18: ; preds = %1, %1
store i8 12, i8* %3
br label %23
19: ; preds = %1, %1
store i8 13, i8* %3
br label %23
20: ; preds = %1, %1
store i8 14, i8* %3
br label %23
21: ; preds = %1, %1
store i8 15, i8* %3
br label %23
22: ; preds = %1
store i8 0, i8* %3
br label %23
23: ; preds = %22, %21, %20, %19, %18, %17, %16, %15, %14, %13, %12, %11, %10, %9, %8, %7, %6
%24 = load i8, i8* %3
ret i8 %24
}

LLVM IR (-O2):
define signext i8 @ascii2bits(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 65, label %12
i32 97, label %12
i32 66, label %13
i32 98, label %13
i32 67, label %14
i32 99, label %14
i32 68, label %15
i32 100, label %15
i32 69, label %16
i32 101, label %16
i32 70, label %17
]
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, %1
br label %18
13: ; preds = %1, %1
br label %18
14: ; preds = %1, %1
br label %18
15: ; preds = %1, %1
br label %18
16: ; preds = %1, %1
br label %18
17: ; preds = %1, %1
br label %18
18: ; preds = %1, %17, %16, %15, %14, %13, %12, %11, %10, %9, %8, %7, %6, %5, %4, %3
%19 = phi i8 [ 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 i8 %19
}