relation between address spaces and physical memory locations

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1

Hi,

Do address spaces in llvm corespond to different memory locations? For example, Shared and Global refer to RAM while Local refers to registers?

I guess that this may be true in GPU programming. So, I would like to know about CPUs.

Thanks.

Best,

Mohammad

2

Hi,

Address spaces in LLVM are an abstract concept and LLVM attaches no internal meaning to address spaces, apart from:

  • Location 0 in address space 0 is ‘nullptr’ and a pointer to this cannot be dereferenced in a well formed program.

  • pointers in different address spaces cannot alias.

Different backends attach different meanings. So for example an OpenCL backend might interpret address space 1 as local memory, 2 as private memory etc (in fact for OpenCL, these mappings are defined in the Clang frontend, if I recall correctly)

Cheers,

James

3

This is also target specific. In our back end, the two address spaces that we use can alias (we use address spaces for indicating different kinds of pointer, rather than different kinds of underlying memory).

If address spaces could never alias, then there would be no need for an address space cast instruction, as the result would always be an invalid value.

David

4

Thanks for the reply.

Hi,

Address spaces in LLVM are an abstract concept and LLVM attaches no internal meaning to address spaces, apart from:

- Location 0 in address space 0 is 'nullptr' and a pointer to this cannot be dereferenced in a well formed program.

- pointers in different address spaces cannot alias.

Different backends attach different meanings. So for example an OpenCL backend might interpret address space 1 as local memory, 2 as private memory etc (in fact for OpenCL, these mappings are defined in the Clang frontend, if I recall correctly)

When are the meanings attached? After or before register allocation?

The meanings are target specific. They are assigned by the targets. Note that the C address space and the LLVM IR address space do not always correspond - Clang’s OpenCL mode, for example, maps OpenCL address spaces to different IR address spaces depending on the target.

This is important as register allocation can use this information to allocate registers to variables. For example, in a parallel code we have shared and private variables. Shared variables should not go into registers. This alleviates register pressure.

I think that you are talking about something different to what I understand register allocation to mean. For most targets, values must be in registers (or, at least, one of a pair of values must be) to be used by instructions. If they reach the back end without a canonical memory location then they are not referenced by address and so may end up solely in registers, or on the stack. If they are referenced by address, then they must be loaded before being computed on and assignments will require a store.

For parallel code, this may make a difference to instruction selection (for example, you don’t need atomic operations on private variables), but shouldn’t make a different to register allocation.

David

5

Hi,

Is it feasible to assign different DataLayout to different address space in the same LLVM IR module?

Thanks
Hongbin

6

DataLayout is per module and describes, among other things, the different size and alignment requirements of pointers to different address spaces.

David