title: Unknown use of a instruction mnemonic without a size suffix (x86_64)
body: <p>I am getting this error after writing some inline assembly in an llvm pass module:</p>
  
  <pre><code>&lt;inline asm&gt;:1:2: error: unkown use of instruction mnemonic without a size suffix
          test %rsi, $1
          ^
  </code></pre>
  
  <p>My understanding is that this error means I am trying to use an operand size that the instruction doesn't support, or that the size is ambiguous. I carefully reviewed the Intel x86_64 Software Development Manual and I am fairly certain I am unambiguously specifying a "REX.W + F7 /0 id" or "TEST r/m64, imm32" operation. Do I need to be more explicit that I want the REX version? How?</p>