unsigned char reverse_byte(unsigned char x) {
  static const unsigned char table[] = {
      0x00, 0x80, 0x40, 0xc0, 0x20, 0xa0, 0x60, 0xe0, 0x10, 0x90, 0x50, 0xd0,
      0x30, 0xb0, 0x70, 0xf0, 0x08, 0x88, 0x48, 0xc8, 0x28, 0xa8, 0x68, 0xe8,
      0x18, 0x98, 0x58, 0xd8, 0x38, 0xb8, 0x78, 0xf8, 0x04, 0x84, 0x44, 0xc4,
      0x24, 0xa4, 0x64, 0xe4, 0x14, 0x94, 0x54, 0xd4, 0x34, 0xb4, 0x74, 0xf4,
      0x0c, 0x8c, 0x4c, 0xcc, 0x2c, 0xac, 0x6c, 0xec, 0x1c, 0x9c, 0x5c, 0xdc,
      0x3c, 0xbc, 0x7c, 0xfc, 0x02, 0x82, 0x42, 0xc2, 0x22, 0xa2, 0x62, 0xe2,
      0x12, 0x92, 0x52, 0xd2, 0x32, 0xb2, 0x72, 0xf2, 0x0a, 0x8a, 0x4a, 0xca,
      0x2a, 0xaa, 0x6a, 0xea, 0x1a, 0x9a, 0x5a, 0xda, 0x3a, 0xba, 0x7a, 0xfa,
      0x06, 0x86, 0x46, 0xc6, 0x26, 0xa6, 0x66, 0xe6, 0x16, 0x96, 0x56, 0xd6,
      0x36, 0xb6, 0x76, 0xf6, 0x0e, 0x8e, 0x4e, 0xce, 0x2e, 0xae, 0x6e, 0xee,
      0x1e, 0x9e, 0x5e, 0xde, 0x3e, 0xbe, 0x7e, 0xfe, 0x01, 0x81, 0x41, 0xc1,
      0x21, 0xa1, 0x61, 0xe1, 0x11, 0x91, 0x51, 0xd1, 0x31, 0xb1, 0x71, 0xf1,
      0x09, 0x89, 0x49, 0xc9, 0x29, 0xa9, 0x69, 0xe9, 0x19, 0x99, 0x59, 0xd9,
      0x39, 0xb9, 0x79, 0xf9, 0x05, 0x85, 0x45, 0xc5, 0x25, 0xa5, 0x65, 0xe5,
      0x15, 0x95, 0x55, 0xd5, 0x35, 0xb5, 0x75, 0xf5, 0x0d, 0x8d, 0x4d, 0xcd,
      0x2d, 0xad, 0x6d, 0xed, 0x1d, 0x9d, 0x5d, 0xdd, 0x3d, 0xbd, 0x7d, 0xfd,
      0x03, 0x83, 0x43, 0xc3, 0x23, 0xa3, 0x63, 0xe3, 0x13, 0x93, 0x53, 0xd3,
      0x33, 0xb3, 0x73, 0xf3, 0x0b, 0x8b, 0x4b, 0xcb, 0x2b, 0xab, 0x6b, 0xeb,
      0x1b, 0x9b, 0x5b, 0xdb, 0x3b, 0xbb, 0x7b, 0xfb, 0x07, 0x87, 0x47, 0xc7,
      0x27, 0xa7, 0x67, 0xe7, 0x17, 0x97, 0x57, 0xd7, 0x37, 0xb7, 0x77, 0xf7,
      0x0f, 0x8f, 0x4f, 0xcf, 0x2f, 0xaf, 0x6f, 0xef, 0x1f, 0x9f, 0x5f, 0xdf,
      0x3f, 0xbf, 0x7f, 0xff,
  };
  return table[x];
}


x86 assembly (clang, -O0):
<reverse_byte>:
push   %rbp
mov    %rsp,%rbp
mov    %dil,%al
mov    %al,-0x1(%rbp)
movzbl -0x1(%rbp),%eax
mov    %eax,%ecx
lea    0xee9(%rip),%rax
movzbl (%rax,%rcx,1),%eax
pop    %rbp
ret

x86 assembly (clang, -O2):
<reverse_byte>:
mov    %edi,%eax
lea    0xef7(%rip),%rcx
mov    (%rax,%rcx,1),%al
ret

LLVM IR (-O0):
@reverse_byte.table = internal constant [256 x i8] c"\00\80@\C0 \A0`\E0\10\90P\D00\B0p\F0\08\88H\C8(\A8h\E8\18\98X\D88\B8x\F8\04\84D\C4$\A4d\E4\14\94T\D44\B4t\F4\0C\8CL\CC,\ACl\EC\1C\9C\\\DC<\BC|\FC\02\82B\C2\22\A2b\E2\12\92R\D22\B2r\F2\0A\8AJ\CA*\AAj\EA\1A\9AZ\DA:\BAz\FA\06\86F\C6&\A6f\E6\16\96V\D66\B6v\F6\0E\8EN\CE.\AEn\EE\1E\9E^\DE>\BE~\FE\01\81A\C1!\A1a\E1\11\91Q\D11\B1q\F1\09\89I\C9)\A9i\E9\19\99Y\D99\B9y\F9\05\85E\C5%\A5e\E5\15\95U\D55\B5u\F5\0D\8DM\CD-\ADm\ED\1D\9D]\DD=\BD}\FD\03\83C\C3#\A3c\E3\13\93S\D33\B3s\F3\0B\8BK\CB+\ABk\EB\1B\9B[\DB;\BB{\FB\07\87G\C7'\A7g\E7\17\97W\D77\B7w\F7\0F\8FO\CF/\AFo\EF\1F\9F_\DF?\BF\7F\FF"
define zeroext i8 @reverse_byte(i8 zeroext %0) {
%2 = alloca i8
store i8 %0, i8* %2
%3 = load i8, i8* %2
%4 = zext i8 %3 to i64
%5 = getelementptr inbounds [256 x i8], [256 x i8]* @reverse_byte.table, i64 0, i64 %4
%6 = load i8, i8* %5
ret i8 %6
}

LLVM IR (-O2):
@reverse_byte.table = internal unnamed_addr constant [256 x i8] c"\00\80@\C0 \A0`\E0\10\90P\D00\B0p\F0\08\88H\C8(\A8h\E8\18\98X\D88\B8x\F8\04\84D\C4$\A4d\E4\14\94T\D44\B4t\F4\0C\8CL\CC,\ACl\EC\1C\9C\\\DC<\BC|\FC\02\82B\C2\22\A2b\E2\12\92R\D22\B2r\F2\0A\8AJ\CA*\AAj\EA\1A\9AZ\DA:\BAz\FA\06\86F\C6&\A6f\E6\16\96V\D66\B6v\F6\0E\8EN\CE.\AEn\EE\1E\9E^\DE>\BE~\FE\01\81A\C1!\A1a\E1\11\91Q\D11\B1q\F1\09\89I\C9)\A9i\E9\19\99Y\D99\B9y\F9\05\85E\C5%\A5e\E5\15\95U\D55\B5u\F5\0D\8DM\CD-\ADm\ED\1D\9D]\DD=\BD}\FD\03\83C\C3#\A3c\E3\13\93S\D33\B3s\F3\0B\8BK\CB+\ABk\EB\1B\9B[\DB;\BB{\FB\07\87G\C7'\A7g\E7\17\97W\D77\B7w\F7\0F\8FO\CF/\AFo\EF\1F\9F_\DF?\BF\7F\FF"
define zeroext i8 @reverse_byte(i8 zeroext %0) {
%2 = zext i8 %0 to i64
%3 = getelementptr inbounds [256 x i8], [256 x i8]* @reverse_byte.table, i64 0, i64 %2
%4 = load i8, i8* %3
ret i8 %4
}