On 02/07/2012 05:33 PM, Xin Tong wrote: > On Tue, Feb 7, 2012 at 12:09 PM, Andrew Haley <aph@xxxxxxxxxx> 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.
