unsigned int __os_crc32(unsigned int crc_in, const void *buf,
                        unsigned int size) {
  unsigned int table[256] = {
      0x00000000, 0x77073096, 0xEE0E612C, 0x990951BA, 0x076DC419, 0x706AF48F,
      0xE963A535, 0x9E6495A3, 0x0EDB8832, 0x79DCB8A4, 0xE0D5E91E, 0x97D2D988,
      0x09B64C2B, 0x7EB17CBD, 0xE7B82D07, 0x90BF1D91, 0x1DB71064, 0x6AB020F2,
      0xF3B97148, 0x84BE41DE, 0x1ADAD47D, 0x6DDDE4EB, 0xF4D4B551, 0x83D385C7,
      0x136C9856, 0x646BA8C0, 0xFD62F97A, 0x8A65C9EC, 0x14015C4F, 0x63066CD9,
      0xFA0F3D63, 0x8D080DF5, 0x3B6E20C8, 0x4C69105E, 0xD56041E4, 0xA2677172,
      0x3C03E4D1, 0x4B04D447, 0xD20D85FD, 0xA50AB56B, 0x35B5A8FA, 0x42B2986C,
      0xDBBBC9D6, 0xACBCF940, 0x32D86CE3, 0x45DF5C75, 0xDCD60DCF, 0xABD13D59,
      0x26D930AC, 0x51DE003A, 0xC8D75180, 0xBFD06116, 0x21B4F4B5, 0x56B3C423,
      0xCFBA9599, 0xB8BDA50F, 0x2802B89E, 0x5F058808, 0xC60CD9B2, 0xB10BE924,
      0x2F6F7C87, 0x58684C11, 0xC1611DAB, 0xB6662D3D, 0x76DC4190, 0x01DB7106,
      0x98D220BC, 0xEFD5102A, 0x71B18589, 0x06B6B51F, 0x9FBFE4A5, 0xE8B8D433,
      0x7807C9A2, 0x0F00F934, 0x9609A88E, 0xE10E9818, 0x7F6A0DBB, 0x086D3D2D,
      0x91646C97, 0xE6635C01, 0x6B6B51F4, 0x1C6C6162, 0x856530D8, 0xF262004E,
      0x6C0695ED, 0x1B01A57B, 0x8208F4C1, 0xF50FC457, 0x65B0D9C6, 0x12B7E950,
      0x8BBEB8EA, 0xFCB9887C, 0x62DD1DDF, 0x15DA2D49, 0x8CD37CF3, 0xFBD44C65,
      0x4DB26158, 0x3AB551CE, 0xA3BC0074, 0xD4BB30E2, 0x4ADFA541, 0x3DD895D7,
      0xA4D1C46D, 0xD3D6F4FB, 0x4369E96A, 0x346ED9FC, 0xAD678846, 0xDA60B8D0,
      0x44042D73, 0x33031DE5, 0xAA0A4C5F, 0xDD0D7CC9, 0x5005713C, 0x270241AA,
      0xBE0B1010, 0xC90C2086, 0x5768B525, 0x206F85B3, 0xB966D409, 0xCE61E49F,
      0x5EDEF90E, 0x29D9C998, 0xB0D09822, 0xC7D7A8B4, 0x59B33D17, 0x2EB40D81,
      0xB7BD5C3B, 0xC0BA6CAD, 0xEDB88320, 0x9ABFB3B6, 0x03B6E20C, 0x74B1D29A,
      0xEAD54739, 0x9DD277AF, 0x04DB2615, 0x73DC1683, 0xE3630B12, 0x94643B84,
      0x0D6D6A3E, 0x7A6A5AA8, 0xE40ECF0B, 0x9309FF9D, 0x0A00AE27, 0x7D079EB1,
      0xF00F9344, 0x8708A3D2, 0x1E01F268, 0x6906C2FE, 0xF762575D, 0x806567CB,
      0x196C3671, 0x6E6B06E7, 0xFED41B76, 0x89D32BE0, 0x10DA7A5A, 0x67DD4ACC,
      0xF9B9DF6F, 0x8EBEEFF9, 0x17B7BE43, 0x60B08ED5, 0xD6D6A3E8, 0xA1D1937E,
      0x38D8C2C4, 0x4FDFF252, 0xD1BB67F1, 0xA6BC5767, 0x3FB506DD, 0x48B2364B,
      0xD80D2BDA, 0xAF0A1B4C, 0x36034AF6, 0x41047A60, 0xDF60EFC3, 0xA867DF55,
      0x316E8EEF, 0x4669BE79, 0xCB61B38C, 0xBC66831A, 0x256FD2A0, 0x5268E236,
      0xCC0C7795, 0xBB0B4703, 0x220216B9, 0x5505262F, 0xC5BA3BBE, 0xB2BD0B28,
      0x2BB45A92, 0x5CB36A04, 0xC2D7FFA7, 0xB5D0CF31, 0x2CD99E8B, 0x5BDEAE1D,
      0x9B64C2B0, 0xEC63F226, 0x756AA39C, 0x026D930A, 0x9C0906A9, 0xEB0E363F,
      0x72076785, 0x05005713, 0x95BF4A82, 0xE2B87A14, 0x7BB12BAE, 0x0CB61B38,
      0x92D28E9B, 0xE5D5BE0D, 0x7CDCEFB7, 0x0BDBDF21, 0x86D3D2D4, 0xF1D4E242,
      0x68DDB3F8, 0x1FDA836E, 0x81BE16CD, 0xF6B9265B, 0x6FB077E1, 0x18B74777,
      0x88085AE6, 0xFF0F6A70, 0x66063BCA, 0x11010B5C, 0x8F659EFF, 0xF862AE69,
      0x616BFFD3, 0x166CCF45, 0xA00AE278, 0xD70DD2EE, 0x4E048354, 0x3903B3C2,
      0xA7672661, 0xD06016F7, 0x4969474D, 0x3E6E77DB, 0xAED16A4A, 0xD9D65ADC,
      0x40DF0B66, 0x37D83BF0, 0xA9BCAE53, 0xDEBB9EC5, 0x47B2CF7F, 0x30B5FFE9,
      0xBDBDF21C, 0xCABAC28A, 0x53B39330, 0x24B4A3A6, 0xBAD03605, 0xCDD70693,
      0x54DE5729, 0x23D967BF, 0xB3667A2E, 0xC4614AB8, 0x5D681B02, 0x2A6F2B94,
      0xB40BBE37, 0xC30C8EA1, 0x5A05DF1B, 0x2D02EF8D};
  unsigned int crc = crc_in ^ 0xFFFFFFFF;
  unsigned char *B = (unsigned char *)buf;
  int i;
  for (i = 0; i < size; i++) {
    crc = (crc >> 8) ^ table[(crc ^ B[i]) & 0xFF];
  }
  return crc ^ 0xFFFFFFFF;
}


x86 assembly (clang, -O0):
<__os_crc32>:
push   %rbp
mov    %rsp,%rbp
sub    $0x440,%rsp
mov    %edi,-0x4(%rbp)
mov    %rsi,-0x10(%rbp)
mov    %edx,-0x14(%rbp)
lea    -0x420(%rbp),%rdi
lea    0xecd(%rip),%rsi
mov    $0x400,%edx
call   1030 <memcpy@plt>
mov    -0x4(%rbp),%eax
xor    $0xffffffff,%eax
mov    %eax,-0x424(%rbp)
mov    -0x10(%rbp),%rax
mov    %rax,-0x430(%rbp)
movl   $0x0,-0x434(%rbp)
mov    -0x434(%rbp),%eax
cmp    -0x14(%rbp),%eax
jae    11b9 <__os_crc32+0xa9>
mov    -0x424(%rbp),%eax
shr    $0x8,%eax
mov    -0x424(%rbp),%ecx
mov    -0x430(%rbp),%rdx
movslq -0x434(%rbp),%rsi
movzbl (%rdx,%rsi,1),%edx
xor    %edx,%ecx
and    $0xff,%ecx
mov    %ecx,%ecx
xor    -0x420(%rbp,%rcx,4),%eax
mov    %eax,-0x424(%rbp)
mov    -0x434(%rbp),%eax
add    $0x1,%eax
mov    %eax,-0x434(%rbp)
jmp    115e <__os_crc32+0x4e>
mov    -0x424(%rbp),%eax
xor    $0xffffffff,%eax
add    $0x440,%rsp
pop    %rbp
ret

x86 assembly (clang, -O2):
<__os_crc32>:
mov    %edi,%eax
not    %eax
test   %edx,%edx
je     1176 <__os_crc32+0x76>
mov    %edx,%r8d
cmp    $0x1,%edx
jne    1114 <__os_crc32+0x14>
xor    %ecx,%ecx
jmp    115a <__os_crc32+0x5a>
mov    %r8d,%r10d
and    $0xfffffffe,%r10d
xor    %ecx,%ecx
lea    0xedc(%rip),%r9
cs nopw 0x0(%rax,%rax,1)
xchg   %ax,%ax
movzbl %al,%edi
shr    $0x8,%eax
movzbl (%rsi,%rcx,1),%edx
xor    %edx,%edi
xor    (%r9,%rdi,4),%eax
movzbl %al,%edx
shr    $0x8,%eax
movzbl 0x1(%rsi,%rcx,1),%edi
xor    %edi,%edx
xor    (%r9,%rdx,4),%eax
add    $0x2,%rcx
cmp    %rcx,%r10
jne    1130 <__os_crc32+0x30>
test   $0x1,%r8b
je     1176 <__os_crc32+0x76>
movzbl %al,%edx
shr    $0x8,%eax
movzbl (%rsi,%rcx,1),%ecx
xor    %ecx,%edx
lea    0xe8d(%rip),%rcx
xor    (%rcx,%rdx,4),%eax
not    %eax
ret

LLVM IR (-O0):
@__const.__os_crc32.table = [256 x i32] [i32 0, i32 1996959894, i32 -301047508, i32 -1727442502, i32 124634137, i32 1886057615, i32 -379345611, i32 -1637575261, i32 249268274, i32 2044508324, i32 -522852066, i32 -1747789432, i32 162941995, i32 2125561021, i32 -407360249, i32 -1866523247, i32 498536548, i32 1789927666, i32 -205950648, i32 -2067906082, i32 450548861, i32 1843258603, i32 -187386543, i32 -2083289657, i32 325883990, i32 1684777152, i32 -43845254, i32 -1973040660, i32 335633487, i32 1661365465, i32 -99664541, i32 -1928851979, i32 997073096, i32 1281953886, i32 -715111964, i32 -1570279054, i32 1006888145, i32 1258607687, i32 -770865667, i32 -1526024853, i32 901097722, i32 1119000684, i32 -608450090, i32 -1396901568, i32 853044451, i32 1172266101, i32 -589951537, i32 -1412350631, i32 651767980, i32 1373503546, i32 -925412992, i32 -1076862698, i32 565507253, i32 1454621731, i32 -809855591, i32 -1195530993, i32 671266974, i32 1594198024, i32 -972236366, i32 -1324619484, i32 795835527, i32 1483230225, i32 -1050600021, i32 -1234817731, i32 1994146192, i32 31158534, i32 -1731059524, i32 -271249366, i32 1907459465, i32 112637215, i32 -1614814043, i32 -390540237, i32 2013776290, i32 251722036, i32 -1777751922, i32 -519137256, i32 2137656763, i32 141376813, i32 -1855689577, i32 -429695999, i32 1802195444, i32 476864866, i32 -2056965928, i32 -228458418, i32 1812370925, i32 453092731, i32 -2113342271, i32 -183516073, i32 1706088902, i32 314042704, i32 -1950435094, i32 -54949764, i32 1658658271, i32 366619977, i32 -1932296973, i32 -69972891, i32 1303535960, i32 984961486, i32 -1547960204, i32 -725929758, i32 1256170817, i32 1037604311, i32 -1529756563, i32 -740887301, i32 1131014506, i32 879679996, i32 -1385723834, i32 -631195440, i32 1141124467, i32 855842277, i32 -1442165665, i32 -586318647, i32 1342533948, i32 654459306, i32 -1106571248, i32 -921952122, i32 1466479909, i32 544179635, i32 -1184443383, i32 -832445281, i32 1591671054, i32 702138776, i32 -1328506846, i32 -942167884, i32 1504918807, i32 783551873, i32 -1212326853, i32 -1061524307, i32 -306674912, i32 -1698712650, i32 62317068, i32 1957810842, i32 -355121351, i32 -1647151185, i32 81470997, i32 1943803523, i32 -480048366, i32 -1805370492, i32 225274430, i32 2053790376, i32 -468791541, i32 -1828061283, i32 167816743, i32 2097651377, i32 -267414716, i32 -2029476910, i32 503444072, i32 1762050814, i32 -144550051, i32 -2140837941, i32 426522225, i32 1852507879, i32 -19653770, i32 -1982649376, i32 282753626, i32 1742555852, i32 -105259153, i32 -1900089351, i32 397917763, i32 1622183637, i32 -690576408, i32 -1580100738, i32 953729732, i32 1340076626, i32 -776247311, i32 -1497606297, i32 1068828381, i32 1219638859, i32 -670225446, i32 -1358292148, i32 906185462, i32 1090812512, i32 -547295293, i32 -1469587627, i32 829329135, i32 1181335161, i32 -882789492, i32 -1134132454, i32 628085408, i32 1382605366, i32 -871598187, i32 -1156888829, i32 570562233, i32 1426400815, i32 -977650754, i32 -1296233688, i32 733239954, i32 1555261956, i32 -1026031705, i32 -1244606671, i32 752459403, i32 1541320221, i32 -1687895376, i32 -328994266, i32 1969922972, i32 40735498, i32 -1677130071, i32 -351390145, i32 1913087877, i32 83908371, i32 -1782625662, i32 -491226604, i32 2075208622, i32 213261112, i32 -1831694693, i32 -438977011, i32 2094854071, i32 198958881, i32 -2032938284, i32 -237706686, i32 1759359992, i32 534414190, i32 -2118248755, i32 -155638181, i32 1873836001, i32 414664567, i32 -2012718362, i32 -15766928, i32 1711684554, i32 285281116, i32 -1889165569, i32 -127750551, i32 1634467795, i32 376229701, i32 -1609899400, i32 -686959890, i32 1308918612, i32 956543938, i32 -1486412191, i32 -799009033, i32 1231636301, i32 1047427035, i32 -1362007478, i32 -640263460, i32 1088359270, i32 936918000, i32 -1447252397, i32 -558129467, i32 1202900863, i32 817233897, i32 -1111625188, i32 -893730166, i32 1404277552, i32 615818150, i32 -1160759803, i32 -841546093, i32 1423857449, i32 601450431, i32 -1285129682, i32 -1000256840, i32 1567103746, i32 711928724, i32 -1274298825, i32 -1022587231, i32 1510334235, i32 755167117]
define i32 @__os_crc32(i32 %0, i8* %1, i32 %2) {
%4 = alloca i32
%5 = alloca i8*
%6 = alloca i32
%7 = alloca [256 x i32]
%8 = alloca i32
%9 = alloca i8*
%10 = alloca i32
store i32 %0, i32* %4
store i8* %1, i8** %5
store i32 %2, i32* %6
%11 = bitcast [256 x i32]* %7 to i8*
%12 = load i32, i32* %4
%13 = xor i32 %12, -1
store i32 %13, i32* %8
%14 = load i8*, i8** %5
store i8* %14, i8** %9
store i32 0, i32* %10
br label %15
15: ; preds = %35, %3
%16 = load i32, i32* %10
%17 = load i32, i32* %6
%18 = icmp ult i32 %16, %17
br i1 %18, label %19, label %38
19: ; preds = %15
%20 = load i32, i32* %8
%21 = lshr i32 %20, 8
%22 = load i32, i32* %8
%23 = load i8*, i8** %9
%24 = load i32, i32* %10
%25 = sext i32 %24 to i64
%26 = getelementptr inbounds i8, i8* %23, i64 %25
%27 = load i8, i8* %26
%28 = zext i8 %27 to i32
%29 = xor i32 %22, %28
%30 = and i32 %29, 255
%31 = zext i32 %30 to i64
%32 = getelementptr inbounds [256 x i32], [256 x i32]* %7, i64 0, i64 %31
%33 = load i32, i32* %32
%34 = xor i32 %21, %33
store i32 %34, i32* %8
br label %35
35: ; preds = %19
%36 = load i32, i32* %10
%37 = add nsw i32 %36, 1
store i32 %37, i32* %10
br label %15
38: ; preds = %15
%39 = load i32, i32* %8
%40 = xor i32 %39, -1
ret i32 %40
}

LLVM IR (-O2):
@__const.__os_crc32.table = [256 x i32] [i32 0, i32 1996959894, i32 -301047508, i32 -1727442502, i32 124634137, i32 1886057615, i32 -379345611, i32 -1637575261, i32 249268274, i32 2044508324, i32 -522852066, i32 -1747789432, i32 162941995, i32 2125561021, i32 -407360249, i32 -1866523247, i32 498536548, i32 1789927666, i32 -205950648, i32 -2067906082, i32 450548861, i32 1843258603, i32 -187386543, i32 -2083289657, i32 325883990, i32 1684777152, i32 -43845254, i32 -1973040660, i32 335633487, i32 1661365465, i32 -99664541, i32 -1928851979, i32 997073096, i32 1281953886, i32 -715111964, i32 -1570279054, i32 1006888145, i32 1258607687, i32 -770865667, i32 -1526024853, i32 901097722, i32 1119000684, i32 -608450090, i32 -1396901568, i32 853044451, i32 1172266101, i32 -589951537, i32 -1412350631, i32 651767980, i32 1373503546, i32 -925412992, i32 -1076862698, i32 565507253, i32 1454621731, i32 -809855591, i32 -1195530993, i32 671266974, i32 1594198024, i32 -972236366, i32 -1324619484, i32 795835527, i32 1483230225, i32 -1050600021, i32 -1234817731, i32 1994146192, i32 31158534, i32 -1731059524, i32 -271249366, i32 1907459465, i32 112637215, i32 -1614814043, i32 -390540237, i32 2013776290, i32 251722036, i32 -1777751922, i32 -519137256, i32 2137656763, i32 141376813, i32 -1855689577, i32 -429695999, i32 1802195444, i32 476864866, i32 -2056965928, i32 -228458418, i32 1812370925, i32 453092731, i32 -2113342271, i32 -183516073, i32 1706088902, i32 314042704, i32 -1950435094, i32 -54949764, i32 1658658271, i32 366619977, i32 -1932296973, i32 -69972891, i32 1303535960, i32 984961486, i32 -1547960204, i32 -725929758, i32 1256170817, i32 1037604311, i32 -1529756563, i32 -740887301, i32 1131014506, i32 879679996, i32 -1385723834, i32 -631195440, i32 1141124467, i32 855842277, i32 -1442165665, i32 -586318647, i32 1342533948, i32 654459306, i32 -1106571248, i32 -921952122, i32 1466479909, i32 544179635, i32 -1184443383, i32 -832445281, i32 1591671054, i32 702138776, i32 -1328506846, i32 -942167884, i32 1504918807, i32 783551873, i32 -1212326853, i32 -1061524307, i32 -306674912, i32 -1698712650, i32 62317068, i32 1957810842, i32 -355121351, i32 -1647151185, i32 81470997, i32 1943803523, i32 -480048366, i32 -1805370492, i32 225274430, i32 2053790376, i32 -468791541, i32 -1828061283, i32 167816743, i32 2097651377, i32 -267414716, i32 -2029476910, i32 503444072, i32 1762050814, i32 -144550051, i32 -2140837941, i32 426522225, i32 1852507879, i32 -19653770, i32 -1982649376, i32 282753626, i32 1742555852, i32 -105259153, i32 -1900089351, i32 397917763, i32 1622183637, i32 -690576408, i32 -1580100738, i32 953729732, i32 1340076626, i32 -776247311, i32 -1497606297, i32 1068828381, i32 1219638859, i32 -670225446, i32 -1358292148, i32 906185462, i32 1090812512, i32 -547295293, i32 -1469587627, i32 829329135, i32 1181335161, i32 -882789492, i32 -1134132454, i32 628085408, i32 1382605366, i32 -871598187, i32 -1156888829, i32 570562233, i32 1426400815, i32 -977650754, i32 -1296233688, i32 733239954, i32 1555261956, i32 -1026031705, i32 -1244606671, i32 752459403, i32 1541320221, i32 -1687895376, i32 -328994266, i32 1969922972, i32 40735498, i32 -1677130071, i32 -351390145, i32 1913087877, i32 83908371, i32 -1782625662, i32 -491226604, i32 2075208622, i32 213261112, i32 -1831694693, i32 -438977011, i32 2094854071, i32 198958881, i32 -2032938284, i32 -237706686, i32 1759359992, i32 534414190, i32 -2118248755, i32 -155638181, i32 1873836001, i32 414664567, i32 -2012718362, i32 -15766928, i32 1711684554, i32 285281116, i32 -1889165569, i32 -127750551, i32 1634467795, i32 376229701, i32 -1609899400, i32 -686959890, i32 1308918612, i32 956543938, i32 -1486412191, i32 -799009033, i32 1231636301, i32 1047427035, i32 -1362007478, i32 -640263460, i32 1088359270, i32 936918000, i32 -1447252397, i32 -558129467, i32 1202900863, i32 817233897, i32 -1111625188, i32 -893730166, i32 1404277552, i32 615818150, i32 -1160759803, i32 -841546093, i32 1423857449, i32 601450431, i32 -1285129682, i32 -1000256840, i32 1567103746, i32 711928724, i32 -1274298825, i32 -1022587231, i32 1510334235, i32 755167117]
define i32 @__os_crc32(i32 %0, i8* nocapture %1, i32 %2) {
%4 = xor i32 %0, -1
%5 = icmp eq i32 %2, 0
br i1 %5, label %56, label %6
6: ; preds = %3
%7 = zext i32 %2 to i64
%8 = and i64 %7, 1
%9 = icmp eq i32 %2, 1
br i1 %9, label %40, label %10
10: ; preds = %6
%11 = and i64 %7, 4294967294
br label %12
12: ; preds = %12, %10
%13 = phi i64 [ 0, %10 ], [ %37, %12 ]
%14 = phi i32 [ %4, %10 ], [ %36, %12 ]
%15 = phi i64 [ 0, %10 ], [ %38, %12 ]
%16 = lshr i32 %14, 8
%17 = getelementptr inbounds i8, i8* %1, i64 %13
%18 = load i8, i8* %17
%19 = zext i8 %18 to i32
%20 = and i32 %14, 255
%21 = xor i32 %20, %19
%22 = zext i32 %21 to i64
%23 = getelementptr inbounds [256 x i32], [256 x i32]* @__const.__os_crc32.table, i64 0, i64 %22
%24 = load i32, i32* %23
%25 = xor i32 %24, %16
%26 = or i64 %13, 1
%27 = lshr i32 %25, 8
%28 = getelementptr inbounds i8, i8* %1, i64 %26
%29 = load i8, i8* %28
%30 = zext i8 %29 to i32
%31 = and i32 %25, 255
%32 = xor i32 %31, %30
%33 = zext i32 %32 to i64
%34 = getelementptr inbounds [256 x i32], [256 x i32]* @__const.__os_crc32.table, i64 0, i64 %33
%35 = load i32, i32* %34
%36 = xor i32 %35, %27
%37 = add nuw nsw i64 %13, 2
%38 = add i64 %15, 2
%39 = icmp eq i64 %38, %11
br i1 %39, label %40, label %12
40: ; preds = %12, %6
%41 = phi i32 [ undef, %6 ], [ %36, %12 ]
%42 = phi i64 [ 0, %6 ], [ %37, %12 ]
%43 = phi i32 [ %4, %6 ], [ %36, %12 ]
%44 = icmp eq i64 %8, 0
br i1 %44, label %56, label %45
45: ; preds = %40
%46 = lshr i32 %43, 8
%47 = getelementptr inbounds i8, i8* %1, i64 %42
%48 = load i8, i8* %47
%49 = zext i8 %48 to i32
%50 = and i32 %43, 255
%51 = xor i32 %50, %49
%52 = zext i32 %51 to i64
%53 = getelementptr inbounds [256 x i32], [256 x i32]* @__const.__os_crc32.table, i64 0, i64 %52
%54 = load i32, i32* %53
%55 = xor i32 %54, %46
br label %56
56: ; preds = %45, %40, %3
%57 = phi i32 [ %4, %3 ], [ %41, %40 ], [ %55, %45 ]
%58 = xor i32 %57, -1
ret i32 %58
}