Re: What does %P1 mean in gcc inline assembly?

[张云 <zyunone@xxxxxxx>]

On Mar 4, 2016, at 11:59 PM, Dave Tian <dave.jing.tian@xxxxxxxxx> wrote:

This ‘P’ is used to make gcc happy and work.

Without ‘P’, this inline would be interpreted as:

leal $-512(%esp), %eax

With ‘P’, this inline is the thing we really want:

leal -512(%esp), %eax

Eventually, my gcc 4.9.2 does not compile with ‘P’ is missing. I am not sure if this is still the case for newer gcc (5/6). But you get the point.

-daveti

Thank you for your detailed answer !

By the way， If someone have the problems alike, I suggest them to write some inline assembly and check the compiler’s assembly output.

On Mar 4, 2016, at 7:36 AM, 张云 <zyunone@xxxxxxx> wrote:

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"
                     : "=r" (stack_end) : "i" (-STACK_SIZE));

 What does the ‘%P1’ mean in the inline assembly above ?

Thanks !

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