// Zbb contains rev8 but not general grevi
// Zbkb contains rev8 and brev8 (a.k.a. rev.b) but not general grevi
int shamt = SHAMT;
require(((shamt == xlen - 8) && (p->extension_enabled(EXT_ZBB) || p->extension_enabled(EXT_ZBKB))) //rev8
  || ((shamt == 7) && p->extension_enabled(EXT_ZBKB)) // rev8.b
  || ((shamt == 8) && p->extension_enabled(EXT_ZPN)) // rev8.h
  || ((shamt == xlen - 1) && p->extension_enabled(EXT_ZPN)) // rev
  || p->extension_enabled(EXT_XZBP));
require(shamt < xlen);
reg_t x = RS1;
if (shamt &  1) x = ((x & 0x5555555555555555LL) <<  1) | ((x & 0xAAAAAAAAAAAAAAAALL) >>  1);
if (shamt &  2) x = ((x & 0x3333333333333333LL) <<  2) | ((x & 0xCCCCCCCCCCCCCCCCLL) >>  2);
if (shamt &  4) x = ((x & 0x0F0F0F0F0F0F0F0FLL) <<  4) | ((x & 0xF0F0F0F0F0F0F0F0LL) >>  4);
if (shamt &  8) x = ((x & 0x00FF00FF00FF00FFLL) <<  8) | ((x & 0xFF00FF00FF00FF00LL) >>  8);
if (shamt & 16) x = ((x & 0x0000FFFF0000FFFFLL) << 16) | ((x & 0xFFFF0000FFFF0000LL) >> 16);
if (shamt & 32) x = ((x & 0x00000000FFFFFFFFLL) << 32) | ((x & 0xFFFFFFFF00000000LL) >> 32);
WRITE_RD(sext_xlen(x));
