What does %P1 mean in gcc inline assembly?

Hi In /arch/x86/boot/main.c (http://lxr.free-electrons.com/source/arch/x86/boot/main.c) , In the function init_heap, asm("leal %P1(%%esp),%0" 122 <http://lxr.free-electrons.com/source/arch/x86/boot/source/arch/x86/boot/main...> : "=r" (stack_end) : "i" (-STACK_SIZE <http://lxr.free-electrons.com/source/arch/x86/boot/ident?i=STACK_SIZE>)); What does the ‘%P1’ mean in the inline assembly above ? Thanks !