Re: PCREL preinit/init/fini arrays?

[Alan Modra <amodra at gmail dot com>]

On Mon, Aug 18, 2014 at 01:47:28PM -0700, Matt Thomas wrote:
> 
> I'm adding support for a few new architectures to NetBSD I'm planning on using
> the preinit/init/fini arrays rather than .init/.fini/.ctors/.dtors.
> 
> Rather than emitting relocation entries for the arrays, I'm thinking about making
> each entry PC-relative to itself.  This would allow them to move to the .text
> PT_LOAD program section since they could be read-only.

Only if the pointers in .init_array etc. are always local.  Usually
the case, but I don't think anything in the ELF gABI precludes
.init_array from holding pointers to global functions, which would
need dynamic relocations in a shared library.

In any case, .init_array is already read-only after relocation, so
moving to a fully read-only section doesn't gain you much.

>  Having to compute the
> address of each entry shouldn't be any more expensive than doing the R_RELATIVE 
> relocation that the PCREL eliminates.

Right, it's likely to be quite a lot cheaper than a dynamic relocation.

> I can't think of any downsides, except for getting binutils to support it, but
> maybe I'm overlooking something.  Am I? :)

The biggest downside is that the ELF gABI says .init_array
etc. contain arrays of function pointers.  So you're talking about
modifying the gABI if you want to use the same section names, or using
different section names.

-- 
Alan Modra
Australia Development Lab, IBM