unsigned short CreateCRC(unsigned char *mem, unsigned long size) {
  static unsigned short CRCTab[256] = {
      0x0000, 0xC0C1, 0xC181, 0x0140, 0xC301, 0x03C0, 0x0280, 0xC241, 0xC601,
      0x06C0, 0x0780, 0xC741, 0x0500, 0xC5C1, 0xC481, 0x0440, 0xCC01, 0x0CC0,
      0x0D80, 0xCD41, 0x0F00, 0xCFC1, 0xCE81, 0x0E40, 0x0A00, 0xCAC1, 0xCB81,
      0x0B40, 0xC901, 0x09C0, 0x0880, 0xC841, 0xD801, 0x18C0, 0x1980, 0xD941,
      0x1B00, 0xDBC1, 0xDA81, 0x1A40, 0x1E00, 0xDEC1, 0xDF81, 0x1F40, 0xDD01,
      0x1DC0, 0x1C80, 0xDC41, 0x1400, 0xD4C1, 0xD581, 0x1540, 0xD701, 0x17C0,
      0x1680, 0xD641, 0xD201, 0x12C0, 0x1380, 0xD341, 0x1100, 0xD1C1, 0xD081,
      0x1040, 0xF001, 0x30C0, 0x3180, 0xF141, 0x3300, 0xF3C1, 0xF281, 0x3240,
      0x3600, 0xF6C1, 0xF781, 0x3740, 0xF501, 0x35C0, 0x3480, 0xF441, 0x3C00,
      0xFCC1, 0xFD81, 0x3D40, 0xFF01, 0x3FC0, 0x3E80, 0xFE41, 0xFA01, 0x3AC0,
      0x3B80, 0xFB41, 0x3900, 0xF9C1, 0xF881, 0x3840, 0x2800, 0xE8C1, 0xE981,
      0x2940, 0xEB01, 0x2BC0, 0x2A80, 0xEA41, 0xEE01, 0x2EC0, 0x2F80, 0xEF41,
      0x2D00, 0xEDC1, 0xEC81, 0x2C40, 0xE401, 0x24C0, 0x2580, 0xE541, 0x2700,
      0xE7C1, 0xE681, 0x2640, 0x2200, 0xE2C1, 0xE381, 0x2340, 0xE101, 0x21C0,
      0x2080, 0xE041, 0xA001, 0x60C0, 0x6180, 0xA141, 0x6300, 0xA3C1, 0xA281,
      0x6240, 0x6600, 0xA6C1, 0xA781, 0x6740, 0xA501, 0x65C0, 0x6480, 0xA441,
      0x6C00, 0xACC1, 0xAD81, 0x6D40, 0xAF01, 0x6FC0, 0x6E80, 0xAE41, 0xAA01,
      0x6AC0, 0x6B80, 0xAB41, 0x6900, 0xA9C1, 0xA881, 0x6840, 0x7800, 0xB8C1,
      0xB981, 0x7940, 0xBB01, 0x7BC0, 0x7A80, 0xBA41, 0xBE01, 0x7EC0, 0x7F80,
      0xBF41, 0x7D00, 0xBDC1, 0xBC81, 0x7C40, 0xB401, 0x74C0, 0x7580, 0xB541,
      0x7700, 0xB7C1, 0xB681, 0x7640, 0x7200, 0xB2C1, 0xB381, 0x7340, 0xB101,
      0x71C0, 0x7080, 0xB041, 0x5000, 0x90C1, 0x9181, 0x5140, 0x9301, 0x53C0,
      0x5280, 0x9241, 0x9601, 0x56C0, 0x5780, 0x9741, 0x5500, 0x95C1, 0x9481,
      0x5440, 0x9C01, 0x5CC0, 0x5D80, 0x9D41, 0x5F00, 0x9FC1, 0x9E81, 0x5E40,
      0x5A00, 0x9AC1, 0x9B81, 0x5B40, 0x9901, 0x59C0, 0x5880, 0x9841, 0x8801,
      0x48C0, 0x4980, 0x8941, 0x4B00, 0x8BC1, 0x8A81, 0x4A40, 0x4E00, 0x8EC1,
      0x8F81, 0x4F40, 0x8D01, 0x4DC0, 0x4C80, 0x8C41, 0x4400, 0x84C1, 0x8581,
      0x4540, 0x8701, 0x47C0, 0x4680, 0x8641, 0x8201, 0x42C0, 0x4380, 0x8341,
      0x4100, 0x81C1, 0x8081, 0x4040};
  register unsigned short CRC = 0;
  while (size--)
    CRC = (unsigned short)(CRCTab[((CRC ^ *mem++) & 255)] ^ ((CRC >> 8) & 255));
  return CRC;
}


x86 assembly (clang, -O0):
<CreateCRC>:
push   %rbp
mov    %rsp,%rbp
mov    %rdi,-0x8(%rbp)
mov    %rsi,-0x10(%rbp)
movw   $0x0,-0x12(%rbp)
mov    -0x10(%rbp),%rax
mov    %rax,%rcx
add    $0xffffffffffffffff,%rcx
mov    %rcx,-0x10(%rbp)
cmp    $0x0,%rax
je     116e <CreateCRC+0x6e>
movzwl -0x12(%rbp),%eax
mov    -0x8(%rbp),%rcx
mov    %rcx,%rdx
add    $0x1,%rdx
mov    %rdx,-0x8(%rbp)
movzbl (%rcx),%ecx
xor    %ecx,%eax
and    $0xff,%eax
movslq %eax,%rcx
lea    0x2ede(%rip),%rax
movzwl (%rax,%rcx,2),%eax
movzwl -0x12(%rbp),%ecx
sar    $0x8,%ecx
and    $0xff,%ecx
xor    %ecx,%eax
mov    %ax,-0x12(%rbp)
jmp    1112 <CreateCRC+0x12>
movzwl -0x12(%rbp),%eax
pop    %rbp
ret

x86 assembly (clang, -O2):
<CreateCRC>:
test   %rsi,%rsi
je     110f <CreateCRC+0xf>
cmp    $0x1,%rsi
jne    1112 <CreateCRC+0x12>
xor    %eax,%eax
jmp    1164 <CreateCRC+0x64>
xor    %eax,%eax
ret
mov    %rsi,%rcx
and    $0xfffffffffffffffe,%rcx
xor    %eax,%eax
lea    0xede(%rip),%r8
cs nopw 0x0(%rax,%rax,1)
nopl   0x0(%rax)
movzbl (%rdi),%r9d
movzwl %ax,%edx
movzbl %dl,%eax
xor    %r9,%rax
shr    $0x8,%edx
movzbl 0x1(%rdi),%r9d
add    $0x2,%rdi
movzwl (%r8,%rax,2),%eax
xor    %eax,%edx
movzbl %dl,%edx
xor    %r9,%rdx
shr    $0x8,%eax
xor    (%r8,%rdx,2),%ax
add    $0xfffffffffffffffe,%rcx
jne    1130 <CreateCRC+0x30>
test   $0x1,%sil
je     1184 <CreateCRC+0x84>
movzbl (%rdi),%ecx
movzwl %ax,%eax
movzbl %al,%edx
xor    %rcx,%rdx
lea    0xe83(%rip),%rcx
shr    $0x8,%eax
xor    (%rcx,%rdx,2),%ax
ret

LLVM IR (-O0):
@CreateCRC.CRCTab = internal global [256 x i16] [i16 0, i16 -16191, i16 -15999, i16 320, i16 -15615, i16 960, i16 640, i16 -15807, i16 -14847, i16 1728, i16 1920, i16 -14527, i16 1280, i16 -14911, i16 -15231, i16 1088, i16 -13311, i16 3264, i16 3456, i16 -12991, i16 3840, i16 -12351, i16 -12671, i16 3648, i16 2560, i16 -13631, i16 -13439, i16 2880, i16 -14079, i16 2496, i16 2176, i16 -14271, i16 -10239, i16 6336, i16 6528, i16 -9919, i16 6912, i16 -9279, i16 -9599, i16 6720, i16 7680, i16 -8511, i16 -8319, i16 8000, i16 -8959, i16 7616, i16 7296, i16 -9151, i16 5120, i16 -11071, i16 -10879, i16 5440, i16 -10495, i16 6080, i16 5760, i16 -10687, i16 -11775, i16 4800, i16 4992, i16 -11455, i16 4352, i16 -11839, i16 -12159, i16 4160, i16 -4095, i16 12480, i16 12672, i16 -3775, i16 13056, i16 -3135, i16 -3455, i16 12864, i16 13824, i16 -2367, i16 -2175, i16 14144, i16 -2815, i16 13760, i16 13440, i16 -3007, i16 15360, i16 -831, i16 -639, i16 15680, i16 -255, i16 16320, i16 16000, i16 -447, i16 -1535, i16 15040, i16 15232, i16 -1215, i16 14592, i16 -1599, i16 -1919, i16 14400, i16 10240, i16 -5951, i16 -5759, i16 10560, i16 -5375, i16 11200, i16 10880, i16 -5567, i16 -4607, i16 11968, i16 12160, i16 -4287, i16 11520, i16 -4671, i16 -4991, i16 11328, i16 -7167, i16 9408, i16 9600, i16 -6847, i16 9984, i16 -6207, i16 -6527, i16 9792, i16 8704, i16 -7487, i16 -7295, i16 9024, i16 -7935, i16 8640, i16 8320, i16 -8127, i16 -24575, i16 24768, i16 24960, i16 -24255, i16 25344, i16 -23615, i16 -23935, i16 25152, i16 26112, i16 -22847, i16 -22655, i16 26432, i16 -23295, i16 26048, i16 25728, i16 -23487, i16 27648, i16 -21311, i16 -21119, i16 27968, i16 -20735, i16 28608, i16 28288, i16 -20927, i16 -22015, i16 27328, i16 27520, i16 -21695, i16 26880, i16 -22079, i16 -22399, i16 26688, i16 30720, i16 -18239, i16 -18047, i16 31040, i16 -17663, i16 31680, i16 31360, i16 -17855, i16 -16895, i16 32448, i16 32640, i16 -16575, i16 32000, i16 -16959, i16 -17279, i16 31808, i16 -19455, i16 29888, i16 30080, i16 -19135, i16 30464, i16 -18495, i16 -18815, i16 30272, i16 29184, i16 -19775, i16 -19583, i16 29504, i16 -20223, i16 29120, i16 28800, i16 -20415, i16 20480, i16 -28479, i16 -28287, i16 20800, i16 -27903, i16 21440, i16 21120, i16 -28095, i16 -27135, i16 22208, i16 22400, i16 -26815, i16 21760, i16 -27199, i16 -27519, i16 21568, i16 -25599, i16 23744, i16 23936, i16 -25279, i16 24320, i16 -24639, i16 -24959, i16 24128, i16 23040, i16 -25919, i16 -25727, i16 23360, i16 -26367, i16 22976, i16 22656, i16 -26559, i16 -30719, i16 18624, i16 18816, i16 -30399, i16 19200, i16 -29759, i16 -30079, i16 19008, i16 19968, i16 -28991, i16 -28799, i16 20288, i16 -29439, i16 19904, i16 19584, i16 -29631, i16 17408, i16 -31551, i16 -31359, i16 17728, i16 -30975, i16 18368, i16 18048, i16 -31167, i16 -32255, i16 17088, i16 17280, i16 -31935, i16 16640, i16 -32319, i16 -32639, i16 16448]
define zeroext i16 @CreateCRC(i8* %0, i64 %1) {
%3 = alloca i8*
%4 = alloca i64
%5 = alloca i16
store i8* %0, i8** %3
store i64 %1, i64* %4
store i16 0, i16* %5
br label %6
6: ; preds = %10, %2
%7 = load i64, i64* %4
%8 = add i64 %7, -1
store i64 %8, i64* %4
%9 = icmp ne i64 %7, 0
br i1 %9, label %10, label %29
10: ; preds = %6
%11 = load i16, i16* %5
%12 = zext i16 %11 to i32
%13 = load i8*, i8** %3
%14 = getelementptr inbounds i8, i8* %13, i32 1
store i8* %14, i8** %3
%15 = load i8, i8* %13
%16 = zext i8 %15 to i32
%17 = xor i32 %12, %16
%18 = and i32 %17, 255
%19 = sext i32 %18 to i64
%20 = getelementptr inbounds [256 x i16], [256 x i16]* @CreateCRC.CRCTab, i64 0, i64 %19
%21 = load i16, i16* %20
%22 = zext i16 %21 to i32
%23 = load i16, i16* %5
%24 = zext i16 %23 to i32
%25 = ashr i32 %24, 8
%26 = and i32 %25, 255
%27 = xor i32 %22, %26
%28 = trunc i32 %27 to i16
store i16 %28, i16* %5
br label %6
29: ; preds = %6
%30 = load i16, i16* %5
ret i16 %30
}

LLVM IR (-O2):
@CreateCRC.CRCTab = internal unnamed_addr constant [256 x i16] [i16 0, i16 -16191, i16 -15999, i16 320, i16 -15615, i16 960, i16 640, i16 -15807, i16 -14847, i16 1728, i16 1920, i16 -14527, i16 1280, i16 -14911, i16 -15231, i16 1088, i16 -13311, i16 3264, i16 3456, i16 -12991, i16 3840, i16 -12351, i16 -12671, i16 3648, i16 2560, i16 -13631, i16 -13439, i16 2880, i16 -14079, i16 2496, i16 2176, i16 -14271, i16 -10239, i16 6336, i16 6528, i16 -9919, i16 6912, i16 -9279, i16 -9599, i16 6720, i16 7680, i16 -8511, i16 -8319, i16 8000, i16 -8959, i16 7616, i16 7296, i16 -9151, i16 5120, i16 -11071, i16 -10879, i16 5440, i16 -10495, i16 6080, i16 5760, i16 -10687, i16 -11775, i16 4800, i16 4992, i16 -11455, i16 4352, i16 -11839, i16 -12159, i16 4160, i16 -4095, i16 12480, i16 12672, i16 -3775, i16 13056, i16 -3135, i16 -3455, i16 12864, i16 13824, i16 -2367, i16 -2175, i16 14144, i16 -2815, i16 13760, i16 13440, i16 -3007, i16 15360, i16 -831, i16 -639, i16 15680, i16 -255, i16 16320, i16 16000, i16 -447, i16 -1535, i16 15040, i16 15232, i16 -1215, i16 14592, i16 -1599, i16 -1919, i16 14400, i16 10240, i16 -5951, i16 -5759, i16 10560, i16 -5375, i16 11200, i16 10880, i16 -5567, i16 -4607, i16 11968, i16 12160, i16 -4287, i16 11520, i16 -4671, i16 -4991, i16 11328, i16 -7167, i16 9408, i16 9600, i16 -6847, i16 9984, i16 -6207, i16 -6527, i16 9792, i16 8704, i16 -7487, i16 -7295, i16 9024, i16 -7935, i16 8640, i16 8320, i16 -8127, i16 -24575, i16 24768, i16 24960, i16 -24255, i16 25344, i16 -23615, i16 -23935, i16 25152, i16 26112, i16 -22847, i16 -22655, i16 26432, i16 -23295, i16 26048, i16 25728, i16 -23487, i16 27648, i16 -21311, i16 -21119, i16 27968, i16 -20735, i16 28608, i16 28288, i16 -20927, i16 -22015, i16 27328, i16 27520, i16 -21695, i16 26880, i16 -22079, i16 -22399, i16 26688, i16 30720, i16 -18239, i16 -18047, i16 31040, i16 -17663, i16 31680, i16 31360, i16 -17855, i16 -16895, i16 32448, i16 32640, i16 -16575, i16 32000, i16 -16959, i16 -17279, i16 31808, i16 -19455, i16 29888, i16 30080, i16 -19135, i16 30464, i16 -18495, i16 -18815, i16 30272, i16 29184, i16 -19775, i16 -19583, i16 29504, i16 -20223, i16 29120, i16 28800, i16 -20415, i16 20480, i16 -28479, i16 -28287, i16 20800, i16 -27903, i16 21440, i16 21120, i16 -28095, i16 -27135, i16 22208, i16 22400, i16 -26815, i16 21760, i16 -27199, i16 -27519, i16 21568, i16 -25599, i16 23744, i16 23936, i16 -25279, i16 24320, i16 -24639, i16 -24959, i16 24128, i16 23040, i16 -25919, i16 -25727, i16 23360, i16 -26367, i16 22976, i16 22656, i16 -26559, i16 -30719, i16 18624, i16 18816, i16 -30399, i16 19200, i16 -29759, i16 -30079, i16 19008, i16 19968, i16 -28991, i16 -28799, i16 20288, i16 -29439, i16 19904, i16 19584, i16 -29631, i16 17408, i16 -31551, i16 -31359, i16 17728, i16 -30975, i16 18368, i16 18048, i16 -31167, i16 -32255, i16 17088, i16 17280, i16 -31935, i16 16640, i16 -32319, i16 -32639, i16 16448]
define zeroext i16 @CreateCRC(i8* nocapture %0, i64 %1) {
%3 = icmp eq i64 %1, 0
br i1 %3, label %50, label %4
4: ; preds = %2
%5 = and i64 %1, 1
%6 = icmp eq i64 %1, 1
br i1 %6, label %35, label %7
7: ; preds = %4
%8 = and i64 %1, -2
br label %9
9: ; preds = %9, %7
%10 = phi i16 [ 0, %7 ], [ %32, %9 ]
%11 = phi i8* [ %0, %7 ], [ %22, %9 ]
%12 = phi i64 [ 0, %7 ], [ %33, %9 ]
%13 = getelementptr inbounds i8, i8* %11, i64 1
%14 = load i8, i8* %11
%15 = zext i8 %14 to i64
%16 = and i16 %10, 255
%17 = zext i16 %16 to i64
%18 = xor i64 %15, %17
%19 = getelementptr inbounds [256 x i16], [256 x i16]* @CreateCRC.CRCTab, i64 0, i64 %18
%20 = load i16, i16* %19
%21 = lshr i16 %10, 8
%22 = getelementptr inbounds i8, i8* %11, i64 2
%23 = load i8, i8* %13
%24 = zext i8 %23 to i64
%25 = and i16 %20, 255
%26 = xor i16 %25, %21
%27 = zext i16 %26 to i64
%28 = xor i64 %24, %27
%29 = getelementptr inbounds [256 x i16], [256 x i16]* @CreateCRC.CRCTab, i64 0, i64 %28
%30 = load i16, i16* %29
%31 = lshr i16 %20, 8
%32 = xor i16 %30, %31
%33 = add i64 %12, 2
%34 = icmp eq i64 %33, %8
br i1 %34, label %35, label %9
35: ; preds = %9, %4
%36 = phi i16 [ undef, %4 ], [ %32, %9 ]
%37 = phi i16 [ 0, %4 ], [ %32, %9 ]
%38 = phi i8* [ %0, %4 ], [ %22, %9 ]
%39 = icmp eq i64 %5, 0
br i1 %39, label %50, label %40
40: ; preds = %35
%41 = load i8, i8* %38
%42 = zext i8 %41 to i64
%43 = and i16 %37, 255
%44 = zext i16 %43 to i64
%45 = xor i64 %42, %44
%46 = getelementptr inbounds [256 x i16], [256 x i16]* @CreateCRC.CRCTab, i64 0, i64 %45
%47 = load i16, i16* %46
%48 = lshr i16 %37, 8
%49 = xor i16 %47, %48
br label %50
50: ; preds = %40, %35, %2
%51 = phi i16 [ 0, %2 ], [ %36, %35 ], [ %49, %40 ]
ret i16 %51
}