Forcing integer constant to be quadword

Question

Writing a compiler for class, and no one in the class could figure out exactly why we couldn't do the straight-forward thing.

cmpq %r13, %r10
movq $0, %r10
cmovne $1, %r10

My best guess is that since cmovXX doesn't explicitly define the size of its arguments like movq or movl, $1 doesn't know how big to be, and therefore, throws a type mismatch tantrum.

My question is, how does one force an integer constant to be a quadword? $1q didn't work, so I'm out of guesses.

Thanks!

Answer 1

Not really. cmov is simply not available (neither Intel, nor AMD created such an encoding of this particular instruction) with an immediate operand. It operates only on registers and memory locations.

Forcing a particular size of an instruction in AT&T syntax is done by appending one of the size prefixes to the instruction's mnemonic - just the way you have done it.

The only instruction in the x86-64 instruction set that can accept a quadword (64-bit) immediate is the mov instruction with a 64-bit register. However, doing movq $0, %rax will give you the ordinary encoding with a 32-bit immediate. In order to force the assembler to emit a 64-bit immediate, you have to use movabs $0, %rax.