char *tcp_optkind(unsigned char id) {
  switch (id) {
  case 0:
    return ("End of Options");
  case 1:
    return ("No Operation");
  case 2:
    return ("Maximum segment size");
  case 3:
    return ("Window scale");
  case 4:
    return ("SACK permitted");
  case 5:
    return ("Selective ACKnowledgement");
  case 8:
    return ("Timestamp");
  case 14:
    return ("TCP Alternate Checksum Request");
  case 15:
    return ("TCP Alternate Checksum Data");
  }
  return ("UNKNOWN");
}


x86 assembly (clang, -O0):
<tcp_optkind>:
push   %rbp
mov    %rsp,%rbp
mov    %dil,%al
mov    %al,-0x9(%rbp)
movzbl -0x9(%rbp),%eax
mov    %rax,-0x18(%rbp)
sub    $0xf,%rax
ja     11c0 <tcp_optkind+0xc0>
mov    -0x18(%rbp),%rax
lea    0xed9(%rip),%rcx
movslq (%rcx,%rax,4),%rax
add    %rcx,%rax
jmp    *%rax
lea    0xf09(%rip),%rax
mov    %rax,-0x8(%rbp)
jmp    11cb <tcp_optkind+0xcb>
lea    0xf08(%rip),%rax
mov    %rax,-0x8(%rbp)
jmp    11cb <tcp_optkind+0xcb>
lea    0xf05(%rip),%rax
mov    %rax,-0x8(%rbp)
jmp    11cb <tcp_optkind+0xcb>
lea    0xf0a(%rip),%rax
mov    %rax,-0x8(%rbp)
jmp    11cb <tcp_optkind+0xcb>
lea    0xf07(%rip),%rax
mov    %rax,-0x8(%rbp)
jmp    11cb <tcp_optkind+0xcb>
lea    0xf06(%rip),%rax
mov    %rax,-0x8(%rbp)
jmp    11cb <tcp_optkind+0xcb>
lea    0xf10(%rip),%rax
mov    %rax,-0x8(%rbp)
jmp    11cb <tcp_optkind+0xcb>
lea    0xf0a(%rip),%rax
mov    %rax,-0x8(%rbp)
jmp    11cb <tcp_optkind+0xcb>
lea    0xf19(%rip),%rax
mov    %rax,-0x8(%rbp)
jmp    11cb <tcp_optkind+0xcb>
lea    0xf25(%rip),%rax
mov    %rax,-0x8(%rbp)
mov    -0x8(%rbp),%rax
pop    %rbp
ret

x86 assembly (clang, -O2):
<tcp_optkind>:
cmp    $0xf,%dil
ja     1119 <tcp_optkind+0x19>
movsbq %dil,%rax
lea    0xfa3(%rip),%rcx
movslq (%rcx,%rax,4),%rax
add    %rcx,%rax
ret
lea    0xf8c(%rip),%rax
ret

LLVM IR (-O0):
@.str = [15 x i8] c"End of Options\00"
@.str.1 = [13 x i8] c"No Operation\00"
@.str.2 = [21 x i8] c"Maximum segment size\00"
@.str.3 = [13 x i8] c"Window scale\00"
@.str.4 = [15 x i8] c"SACK permitted\00"
@.str.5 = [26 x i8] c"Selective ACKnowledgement\00"
@.str.6 = [10 x i8] c"Timestamp\00"
@.str.7 = [31 x i8] c"TCP Alternate Checksum Request\00"
@.str.8 = [28 x i8] c"TCP Alternate Checksum Data\00"
@.str.9 = [8 x i8] c"UNKNOWN\00"
define i8* @tcp_optkind(i8 zeroext %0) {
%2 = alloca i8*
%3 = alloca i8
store i8 %0, i8* %3
%4 = load i8, i8* %3
%5 = zext i8 %4 to i32
switch i32 %5, label %15 [
i32 0, label %6
i32 1, label %7
i32 2, label %8
i32 3, label %9
i32 4, label %10
i32 5, label %11
i32 8, label %12
i32 14, label %13
i32 15, label %14
]
6: ; preds = %1
store i8* getelementptr inbounds ([15 x i8], [15 x i8]* @.str, i64 0, i64 0), i8** %2
br label %16
7: ; preds = %1
store i8* getelementptr inbounds ([13 x i8], [13 x i8]* @.str.1, i64 0, i64 0), i8** %2
br label %16
8: ; preds = %1
store i8* getelementptr inbounds ([21 x i8], [21 x i8]* @.str.2, i64 0, i64 0), i8** %2
br label %16
9: ; preds = %1
store i8* getelementptr inbounds ([13 x i8], [13 x i8]* @.str.3, i64 0, i64 0), i8** %2
br label %16
10: ; preds = %1
store i8* getelementptr inbounds ([15 x i8], [15 x i8]* @.str.4, i64 0, i64 0), i8** %2
br label %16
11: ; preds = %1
store i8* getelementptr inbounds ([26 x i8], [26 x i8]* @.str.5, i64 0, i64 0), i8** %2
br label %16
12: ; preds = %1
store i8* getelementptr inbounds ([10 x i8], [10 x i8]* @.str.6, i64 0, i64 0), i8** %2
br label %16
13: ; preds = %1
store i8* getelementptr inbounds ([31 x i8], [31 x i8]* @.str.7, i64 0, i64 0), i8** %2
br label %16
14: ; preds = %1
store i8* getelementptr inbounds ([28 x i8], [28 x i8]* @.str.8, i64 0, i64 0), i8** %2
br label %16
15: ; preds = %1
store i8* getelementptr inbounds ([8 x i8], [8 x i8]* @.str.9, i64 0, i64 0), i8** %2
br label %16
16: ; preds = %15, %14, %13, %12, %11, %10, %9, %8, %7, %6
%17 = load i8*, i8** %2
ret i8* %17
}

LLVM IR (-O2):
@.str = [15 x i8] c"End of Options\00"
@.str.1 = [13 x i8] c"No Operation\00"
@.str.2 = [21 x i8] c"Maximum segment size\00"
@.str.3 = [13 x i8] c"Window scale\00"
@.str.4 = [15 x i8] c"SACK permitted\00"
@.str.5 = [26 x i8] c"Selective ACKnowledgement\00"
@.str.6 = [10 x i8] c"Timestamp\00"
@.str.7 = [31 x i8] c"TCP Alternate Checksum Request\00"
@.str.8 = [28 x i8] c"TCP Alternate Checksum Data\00"
@.str.9 = [8 x i8] c"UNKNOWN\00"
@reltable.tcp_optkind = [16 x i32] [i32 trunc (i64 sub (i64 ptrtoint ([15 x i8]* @.str to i64), i64 ptrtoint ([16 x i32]* @reltable.tcp_optkind to i64)) to i32), i32 trunc (i64 sub (i64 ptrtoint ([13 x i8]* @.str.1 to i64), i64 ptrtoint ([16 x i32]* @reltable.tcp_optkind to i64)) to i32), i32 trunc (i64 sub (i64 ptrtoint ([21 x i8]* @.str.2 to i64), i64 ptrtoint ([16 x i32]* @reltable.tcp_optkind to i64)) to i32), i32 trunc (i64 sub (i64 ptrtoint ([13 x i8]* @.str.3 to i64), i64 ptrtoint ([16 x i32]* @reltable.tcp_optkind to i64)) to i32), i32 trunc (i64 sub (i64 ptrtoint ([15 x i8]* @.str.4 to i64), i64 ptrtoint ([16 x i32]* @reltable.tcp_optkind to i64)) to i32), i32 trunc (i64 sub (i64 ptrtoint ([26 x i8]* @.str.5 to i64), i64 ptrtoint ([16 x i32]* @reltable.tcp_optkind to i64)) to i32), i32 trunc (i64 sub (i64 ptrtoint ([8 x i8]* @.str.9 to i64), i64 ptrtoint ([16 x i32]* @reltable.tcp_optkind to i64)) to i32), i32 trunc (i64 sub (i64 ptrtoint ([8 x i8]* @.str.9 to i64), i64 ptrtoint ([16 x i32]* @reltable.tcp_optkind to i64)) to i32), i32 trunc (i64 sub (i64 ptrtoint ([10 x i8]* @.str.6 to i64), i64 ptrtoint ([16 x i32]* @reltable.tcp_optkind to i64)) to i32), i32 trunc (i64 sub (i64 ptrtoint ([8 x i8]* @.str.9 to i64), i64 ptrtoint ([16 x i32]* @reltable.tcp_optkind to i64)) to i32), i32 trunc (i64 sub (i64 ptrtoint ([8 x i8]* @.str.9 to i64), i64 ptrtoint ([16 x i32]* @reltable.tcp_optkind to i64)) to i32), i32 trunc (i64 sub (i64 ptrtoint ([8 x i8]* @.str.9 to i64), i64 ptrtoint ([16 x i32]* @reltable.tcp_optkind to i64)) to i32), i32 trunc (i64 sub (i64 ptrtoint ([8 x i8]* @.str.9 to i64), i64 ptrtoint ([16 x i32]* @reltable.tcp_optkind to i64)) to i32), i32 trunc (i64 sub (i64 ptrtoint ([8 x i8]* @.str.9 to i64), i64 ptrtoint ([16 x i32]* @reltable.tcp_optkind to i64)) to i32), i32 trunc (i64 sub (i64 ptrtoint ([31 x i8]* @.str.7 to i64), i64 ptrtoint ([16 x i32]* @reltable.tcp_optkind to i64)) to i32), i32 trunc (i64 sub (i64 ptrtoint ([28 x i8]* @.str.8 to i64), i64 ptrtoint ([16 x i32]* @reltable.tcp_optkind to i64)) to i32)]
define i8* @tcp_optkind(i8 zeroext %0) {
%2 = icmp ult i8 %0, 16
br i1 %2, label %3, label %7
3: ; preds = %1
%4 = sext i8 %0 to i64
%5 = shl i64 %4, 2
%6 = call i8* @llvm.load.relative.i64(i8* bitcast ([16 x i32]* @reltable.tcp_optkind to i8*), i64 %5)
br label %7
7: ; preds = %1, %3
%8 = phi i8* [ %6, %3 ], [ getelementptr inbounds ([8 x i8], [8 x i8]* @.str.9, i64 0, i64 0), %1 ]
ret i8* %8
}