[LLVMdev] Is address space 1 reserved?

[Nick Lewycky]

On 7 January 2015 at 11:25, Owen Anderson <resistor at mac.com> wrote:

> I'm not aware of any such restriction, and I know of several LLVM based
> systems that use address space 1 for something other than that.
>

Oof. It was discussed when the patches to add addrspace were being
considered, and this is why we should've written it down.

It would be nice to have an addrspace that does mean "same as addrspace(0)
except that null may be dereferenceable", and to attach that to
-fno-delete-null-pointer-checks. Any ideas for what that addrspace should
be?


>
> -Owen
>
> On Jan 7, 2015, at 1:18 PM, Philip Reames <listmail at philipreames.com>
> wrote:
>
> On the review for http://reviews.llvm.org/D6808, majnemer
> <http://reviews.llvm.org/p/majnemer/> commented that:
> "Address space 1 has a special meaning in LLVM, it's identical to address
> space 0 except for the fact that "null" may be dereferenced. You might want
> to consider a different address space."
>
> This is the first I've heard of this and I can't find any documentation
> about it being reserved, either in general, or specifically for x86.  Can
> anyone clarify?
>
> The only address spaces with special meanings I know of are:
> - 0 (the normal address space, null is not dereferencable)
> - 256 - TLS, GS relative addressing
> - 257 - FS relative addressing
>
> Philip
>
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