llvm dev - Oct 2015 - when would I use addrspacecast?

I was reading the LangRef for semantics of an instruction and came
across addrspacecast. I've never needed it, so I suppose I don't care
about it.

But, why does it exist? What problem is it trying to solve?

On 6 Oct 2015, at 06:39, Hayden Livingston via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> I was reading the LangRef for semantics of an instruction and came
> across addrspacecast. I've never needed it, so I suppose I don't
care
> about it.
You use it when you have a pointer in one address space and you wish to have a
pointer in another address space.
> But, why does it exist? What problem is it trying to solve?
Casts between address spaces may not be simple bitcasts.  For example, you may
have one address space that uses 32-bit integers to cover all of physical memory
and 16-bit pointers in some fast scratchpad memory.  It may or may not be
defined to cast a 16-bit pointer to a 32-bit one (it definitely won’t be a
simple sign / zero extension, but it may be a masking operation if the fast
memory is mapped into the larger address space).

David

Some languages, such as OpenCL, has address spaces that are not part of
standard C. This may mean "add some offset to the pointer [if it's not
NULL]" (the offset may be in a hardware register and depend on the current
thread or some such too).

I expect one could use this in C to support Thread Local Storage, for
example. I don't know if Clang actually does that, or has some separate
mechanism for solving this. In x86, the TLS is supported by using the
segment register to hold the base-address into the TLS memory for a given
thread, so a addrspacecast to "global" address space would then be a
LEA
RDX, FS:[RAX] to convert from offset RAX in TLS into a global pointer in
RDX [it's a few years since I worked on x86-64 assembler, so syntax may be
off].

--
Mats

On 6 October 2015 at 09:31, David Chisnall via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> On 6 Oct 2015, at 06:39, Hayden Livingston via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >
> > I was reading the LangRef for semantics of an instruction and came
> > across addrspacecast. I've never needed it, so I suppose I
don't care
> > about it.
>
> You use it when you have a pointer in one address space and you wish to
> have a pointer in another address space.
>
> > But, why does it exist? What problem is it trying to solve?
>
> Casts between address spaces may not be simple bitcasts.  For example, you
> may have one address space that uses 32-bit integers to cover all of
> physical memory and 16-bit pointers in some fast scratchpad memory.  It may
> or may not be defined to cast a 16-bit pointer to a 32-bit one (it
> definitely won’t be a simple sign / zero extension, but it may be a masking
> operation if the fast memory is mapped into the larger address space).
>
> David
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
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>
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