[discuss] When is RBX used for base pointer?

[Michael Matz]

Hi,

On Wed, 13 Feb 2008, H.J. Lu wrote:

> We need a callee-saved register for stack alignment.

Can you expand on why?

> In 64bit, our choices are rbx, and r12-r15. r12-r15 need the REX byte 
> and r12 also needs the SIB byte. So I'd like to use rbx. x86-64 psABI 
> says rbx is callee-saved and optionally used as base pointer. However, I 
> couldn't find an example where rbx is used as base pointer. Is this a 
> carryover from i386 psABI where ebx is used to hold GOT?

In the large memory model you need a base pointer for the GOT.  Of course 
we could do the same as in recent i386 and chose an arbitrary register as 
GOT pointer.


Ciao,
Michael.