int letti(char let) {
  switch (let) {
  case 'a':
    return 0;
    break;
  case 'b':
    return 1;
    break;
  case 'c':
    return 2;
    break;
  case 'd':
    return 3;
    break;
  case 'e':
    return 4;
    break;
  case 'f':
    return 5;
    break;
  case 'g':
    return 6;
    break;
  case 'h':
    return 7;
    break;
  case 'i':
    return 8;
    break;
  case 'j':
    return 9;
    break;
  case 'k':
    return 10;
    break;
  case 'l':
    return 11;
    break;
  case 'm':
    return 12;
    break;
  case 'n':
    return 13;
    break;
  case 'o':
    return 14;
    break;
  case 'p':
    return 15;
    break;
  case 'q':
    return 16;
    break;
  case 'r':
    return 17;
    break;
  case 's':
    return 18;
    break;
  case 't':
    return 19;
    break;
  case 'u':
    return 20;
    break;
  case 'v':
    return 21;
    break;
  case 'w':
    return 22;
    break;
  case 'x':
    return 23;
    break;
  case 'y':
    return 24;
    break;
  case 'z':
    return 25;
    break;
  }
  return -1;
}


x86 assembly (clang, -O0):
<letti>:
push   %rbp
mov    %rsp,%rbp
mov    %dil,%al
mov    %al,-0x5(%rbp)
movsbl -0x5(%rbp),%eax
add    $0xffffff9f,%eax
mov    %eax,%ecx
mov    %rcx,-0x10(%rbp)
sub    $0x19,%eax
ja     126c <letti+0x16c>
mov    -0x10(%rbp),%rax
lea    0xed5(%rip),%rcx
movslq (%rcx,%rax,4),%rax
add    %rcx,%rax
jmp    *%rax
movl   $0x0,-0x4(%rbp)
jmp    1273 <letti+0x173>
movl   $0x1,-0x4(%rbp)
jmp    1273 <letti+0x173>
movl   $0x2,-0x4(%rbp)
jmp    1273 <letti+0x173>
movl   $0x3,-0x4(%rbp)
jmp    1273 <letti+0x173>
movl   $0x4,-0x4(%rbp)
jmp    1273 <letti+0x173>
movl   $0x5,-0x4(%rbp)
jmp    1273 <letti+0x173>
movl   $0x6,-0x4(%rbp)
jmp    1273 <letti+0x173>
movl   $0x7,-0x4(%rbp)
jmp    1273 <letti+0x173>
movl   $0x8,-0x4(%rbp)
jmp    1273 <letti+0x173>
movl   $0x9,-0x4(%rbp)
jmp    1273 <letti+0x173>
movl   $0xa,-0x4(%rbp)
jmp    1273 <letti+0x173>
movl   $0xb,-0x4(%rbp)
jmp    1273 <letti+0x173>
movl   $0xc,-0x4(%rbp)
jmp    1273 <letti+0x173>
movl   $0xd,-0x4(%rbp)
jmp    1273 <letti+0x173>
movl   $0xe,-0x4(%rbp)
jmp    1273 <letti+0x173>
movl   $0xf,-0x4(%rbp)
jmp    1273 <letti+0x173>
movl   $0x10,-0x4(%rbp)
jmp    1273 <letti+0x173>
movl   $0x11,-0x4(%rbp)
jmp    1273 <letti+0x173>
movl   $0x12,-0x4(%rbp)
jmp    1273 <letti+0x173>
movl   $0x13,-0x4(%rbp)
jmp    1273 <letti+0x173>
movl   $0x14,-0x4(%rbp)
jmp    1273 <letti+0x173>
movl   $0x15,-0x4(%rbp)
jmp    1273 <letti+0x173>
movl   $0x16,-0x4(%rbp)
jmp    1273 <letti+0x173>
movl   $0x17,-0x4(%rbp)
jmp    1273 <letti+0x173>
movl   $0x18,-0x4(%rbp)
jmp    1273 <letti+0x173>
movl   $0x19,-0x4(%rbp)
jmp    1273 <letti+0x173>
movl   $0xffffffff,-0x4(%rbp)
mov    -0x4(%rbp),%eax
pop    %rbp
ret

x86 assembly (clang, -O2):
<letti>:
lea    -0x61(%rdi),%ecx
cmp    $0x1a,%ecx
mov    $0xffffffff,%eax
cmovb  %ecx,%eax
ret

LLVM IR (-O0):
define i32 @letti(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 %32 [
i32 97, label %6
i32 98, label %7
i32 99, label %8
i32 100, label %9
i32 101, label %10
i32 102, label %11
i32 103, label %12
i32 104, label %13
i32 105, label %14
i32 106, label %15
i32 107, label %16
i32 108, label %17
i32 109, label %18
i32 110, label %19
i32 111, label %20
i32 112, label %21
i32 113, label %22
i32 114, label %23
i32 115, label %24
i32 116, label %25
i32 117, label %26
i32 118, label %27
i32 119, label %28
i32 120, label %29
i32 121, label %30
i32 122, label %31
]
6: ; preds = %1
store i32 0, i32* %2
br label %33
7: ; preds = %1
store i32 1, i32* %2
br label %33
8: ; preds = %1
store i32 2, i32* %2
br label %33
9: ; preds = %1
store i32 3, i32* %2
br label %33
10: ; preds = %1
store i32 4, i32* %2
br label %33
11: ; preds = %1
store i32 5, i32* %2
br label %33
12: ; preds = %1
store i32 6, i32* %2
br label %33
13: ; preds = %1
store i32 7, i32* %2
br label %33
14: ; preds = %1
store i32 8, i32* %2
br label %33
15: ; preds = %1
store i32 9, i32* %2
br label %33
16: ; preds = %1
store i32 10, i32* %2
br label %33
17: ; preds = %1
store i32 11, i32* %2
br label %33
18: ; preds = %1
store i32 12, i32* %2
br label %33
19: ; preds = %1
store i32 13, i32* %2
br label %33
20: ; preds = %1
store i32 14, i32* %2
br label %33
21: ; preds = %1
store i32 15, i32* %2
br label %33
22: ; preds = %1
store i32 16, i32* %2
br label %33
23: ; preds = %1
store i32 17, i32* %2
br label %33
24: ; preds = %1
store i32 18, i32* %2
br label %33
25: ; preds = %1
store i32 19, i32* %2
br label %33
26: ; preds = %1
store i32 20, i32* %2
br label %33
27: ; preds = %1
store i32 21, i32* %2
br label %33
28: ; preds = %1
store i32 22, i32* %2
br label %33
29: ; preds = %1
store i32 23, i32* %2
br label %33
30: ; preds = %1
store i32 24, i32* %2
br label %33
31: ; preds = %1
store i32 25, i32* %2
br label %33
32: ; preds = %1
store i32 -1, i32* %2
br label %33
33: ; preds = %32, %31, %30, %29, %28, %27, %26, %25, %24, %23, %22, %21, %20, %19, %18, %17, %16, %15, %14, %13, %12, %11, %10, %9, %8, %7, %6
%34 = load i32, i32* %2
ret i32 %34
}

LLVM IR (-O2):
define i32 @letti(i8 signext %0) {
%2 = sext i8 %0 to i32
%3 = add nsw i32 %2, -97
%4 = icmp ult i32 %3, 26
%5 = select i1 %4, i32 %3, i32 -1
ret i32 %5
}