Extend alloca: Specify address space for allocation

[swaroop-sridhar]

Currently the alloca instruction always allocates in the generic address space (0).
This proposal is to extend the semantics of alloca instruction to allow allocation in any specified address space.

Proposed Syntax

<result> = alloca [inalloca] <type> [, <ty> <NumElements>] [, align <alignment>] [, addrspace <num>] ; yields type addrspace(num)*:result

Motivation

Managed language clients typically use address space 1 to represent GC-pointers. Some managed clients (CLR, in particular) allow construction of GC pointers to stack locations. For example, MSIL instruction ldloca (ECMA 335, p 370) creates a GC pointer to a stack local.

Creating an addrespace(1)* pointer to a stack location currently involves two steps: the alloca, and an addrespacecast. Managed code transformations (ex: RewriteStatepointsForGC) do not handle arbitrary address space casting. So, it is desirable to obtain the addrespace(1)* pointer by construction.

Thanks,
Swaroop.

[swaroop-sridhar]

RFC sent: http://lists.llvm.org/pipermail/llvm-dev/2015-August/089706.html

[swaroop-sridhar]

Discussion: https://docs.google.com/document/d/1H5am1PyY8n8hc1hIAisDQMgbZDmMgnjU8wdN25YqMbQ/edit