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When is RBX used for base pointer?
- From: "H.J. Lu" <hjl dot tools at gmail dot com>
- To: discuss at x86-64 dot org, gcc at gcc dot gnu dot org
- Date: Wed, 13 Feb 2008 13:43:20 -0800
- Subject: When is RBX used for base pointer?
Hi,
We need a callee-saved register for stack alignment. 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?
Thanks.
H.J.