when would I use addrspacecast?

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1

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?

2

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

3

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].

4

Thanks. Seems like a case where a hardware provider makes such things
available. My development is Windows and Linux on x86 and ARM, don't
think I'll be seeing this.