RE: [PATCH v1 0/4] [RFC] Implement Trampoline File Descriptor

David Laight
2020-08-03T16:57:52Z

From: Madhavan T. Venkataraman
> Sent: 03 August 2020 17:03
>
> On 8/3/20 3:27 AM, David Laight wrote:
> > From: Mark Rutland
> >> Sent: 31 July 2020 19:32
> > ...
> >>> It requires PC-relative data references. I have not worked on all architectures.
> >>> So, I need to study this. But do all ISAs support PC-relative data references?
> >> Not all do, but pretty much any recent ISA will as it's a practical
> >> necessity for fast position-independent code.
> > i386 has neither PC-relative addressing nor moves from %pc.
> > The cpu architecture knows that the sequence:
> > call 1f
> > 1: pop %reg
> > is used to get the %pc value so is treated specially so that
> > it doesn't 'trash' the return stack.
> >
> > So PIC code isn't too bad, but you have to use the correct
> > sequence.
>
> Is that true only for 32-bit systems only? I thought RIP-relative addressing was
> introduced in 64-bit mode. Please confirm.

I said i386 not amd64 or x86-64.

So yes, 64bit code has PC-relative addressing.
But I'm pretty sure it has no other way to get the PC itself
except using call - certainly nothing in the 'usual' instructions.

David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)