Re: How does GCC access global variables on x86

[Andrew Haley <aph at redhat dot com>]

On 02/07/2012 05:33 PM, Xin Tong wrote:
> On Tue, Feb 7, 2012 at 12:09 PM, Andrew Haley <aph@redhat.com> wrote:
>> On 02/07/2012 04:56 PM, Xin Tong wrote:
>>> I am wondering how gcc accesses global variables on x86. from the code
>>> i have seen so far, it seems to use the %RIP as the base register. Is
>>> it always like this?
>>>
>>>   4006f9:       f0 80 0d bf 0b 20 00    lock orb $0x0,0x200bbf(%rip)
>>>      # 6012c0 <t2lockor>
>>>   400700:       00
>>>
>>> t2lockor is a global variables.
>>
>> This is x86_64, I think.  The answer is that it depends on whether you
>> are using PIC, and the model you're using.  Try -mcmodel=large for a
>> variation.  PC-relative loads are convenient for everything except the
>> large memory model.
> 
> What are large memory model and PIC, can you please briefly explain.

For large memory model see -mcmodel in the gcc docs.
PIC means position independent code, as in the gcc command -fpic.

> the PC of instructions are going to change in the linkage stage. so
> the linker patches the offset if rip is used to access the global
> variables.

Yes.

Andrew.