I understand when to use a clobber list (e.g. listing a register which is modified in the assembly so that it doesn't get chosen for use as an input register, etc), but I can't wrap my head around the the earlyclobber constraint &. If you list your outputs, wouldn't that already mean that inputs can't use the selected register (aside from matching digit constraints)?
For example:
asm("movl $1, %0;""addl $3, %0;""addl $4, %1;" // separate bug: modifies input-only operand"addl %1, %0;" : "=g"(num_out) : "g"(num_in) :);Would & even be needed for the output variables? The compiler should know the register that was selected for the output, and thus know not to use it for the input.