[eckzow]

Thumb-2 immediate encoding is even more gleeful--in addition to allowing rotation, it also allows for spaced repetition of any 8-bit pattern (common in low level hack patterns, like from [1]) to be encoded in single instructions.

For those interested, check out page 122 of the ARMv7-M architecture reference manual[2]:

  // ThumbExpandImm_C()
  // ==================
  (bits(32), bit) ThumbExpandImm_C(bits(12) imm12, bit carry_in)
    if imm12<11:10> == ’00’ then
      case imm12<9:8> of
        when ’00’
          imm32 = ZeroExtend(imm12<7:0>, 32);
        when ’01’
          if imm12<7:0> == ’00000000’ then UNPREDICTABLE;
          imm32 = ’00000000’ : imm12<7:0> : ’00000000’ : imm12<7:0>;
        when ’10’
          if imm12<7:0> == ’00000000’ then UNPREDICTABLE;
          imm32 = imm12<7:0> : ’00000000’ : imm12<7:0> : ’00000000’;
        when ’11’
          if imm12<7:0> == ’00000000’ then UNPREDICTABLE;
          imm32 = imm12<7:0> : imm12<7:0> : imm12<7:0> : imm12<7:0>;
      carry_out = carry_in;
  else
    unrotated_value = ZeroExtend(’1’:imm12<6:0>, 32);
    (imm32, carry_out) = ROR_C(unrotated_value, UInt(imm12<11:7>));
  return (imm32, carry_out)

[1] http://graphics.stanford.edu/~seander/bithacks.html (worth a read on its own if you're into this kind of thing)

[2] http://web.eecs.umich.edu/~prabal/teaching/eecs373-f10/readi... (no-registration link)