Reversing 64-bits using Inline Assembly

Cookbook
1

This is some code to reverse a 64-bit integer, translated from an objdump on a clang intrinsic. movabs can be replaced by a mov.q.

bit_reverse64  :: (x: u64) -> u64 #expand {
  // Modified from clang objdump
  rdi: u64 = x;
  rax, rcx, rdx: u64;
  #asm {
    bswap.q   rdi;
    mov.q     rax, 1085102592571150095;
    and.q     rax, rdi;
    shl.q     rax, 4;
    mov.q     rcx, -1085102592571150096;
    and.q     rcx, rdi;
    shr.q     rcx, 4;
    or.q      rcx, rax;
    mov.q     rax, 3689348814741910323;
    and.q     rax, rcx;
    mov.q     rdx, -3689348814741910324;
    and.q     rdx, rcx;
    shr.q     rdx, 2;
    lea.q     rax, [rdx + rax*4];
    mov.q     rcx, 6148914691236517205;
    and.q     rcx, rax;
    mov.q     rdx, -6148914691236517206;
    and.q     rdx, rax;
    shr.q     rdx;
    lea.q     rax, [rdx + rcx*2];
  }
  return rax;
}

Here is the output of reversing a 64-bit integer using this function:

x := 0xA;
y := bit_reverse64(x);
print("%\n", y); // prints out 010100...0, since we are reversing x.
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