------- Comment #2 from pinskia at gcc dot gnu dot org 2010-02-17 23:18 ------- This is all documented at http://gcc.gnu.org/onlinedocs/gcc-4.4.3/gcc/Modifiers.html
"Means (in a particular alternative) that this operand is an earlyclobber operand, which is modified before the instruction is finished using the input operands. Therefore, this operand may not lie in a register that is used as an input operand or as part of any memory address. " -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43110
