[llvm-dev] [EXTERNAL] Re: Are AMDGPU intrinsics available in LLVM IR ?

Frank Winter via llvm-dev llvm-dev at lists.llvm.org
Mon Apr 13 10:19:23 PDT 2020

On 4/13/20 11:43 AM, Matt Arsenault wrote:
>> On Apr 13, 2020, at 11:37, Frank Winter via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>> Hi!
>> I'm trying to figure out how to access the workgroup id from within
>> the LLVM IR language when lowering with the AMDGPU backend.
>> Looking at the 'llvm/include/llvm/IR/IntrinsicsAMDGPU.td' file there
>> are intrinsics defined to access the workitem index (thread index),
>> but this file lives in 'llvm/include':
>> //===----------------------------------------------------------------------===//
>> // ABI Special Intrinsics
>> //===----------------------------------------------------------------------===//
>> defm int_amdgcn_workitem_id : AMDGPUReadPreloadRegisterIntrinsic_xyz;
>> defm int_amdgcn_workgroup_id : AMDGPUReadPreloadRegisterIntrinsic_xyz_named
>> <"__builtin_amdgcn_workgroup_id">;
>> There is no new definition of any intrinsics within the target
>> AMDGPU. I was working before with the NVPTX backend and that target
>> has the special registers are associated with strings for the LLVM IR,
>> e.g., PTX_READ_SREG_R32<"tid.x", int_nvvm_read_ptx_sreg_tid_x>;
>> Maybe I'm missing something but does this mean at the moment it is not
>> possible to write a function in the LLVM IR language which accesses
>> the workgroup id?
>> Frank
> These are the definitions, and the standard place for targets to define intrinsics. NVPTX is odd in defining intrinsics in the backend. These are usable like any other IR intrinsic, through the generated Intrinsic::amdgcn_workitem_id_* enums.
> -Matt

Thanks for the quick response!

How do these definitions translate into LLVM IR? I tried a very simple
function, e.g. simple_amd.ll:

define i32 @simple() {
   ret i32 %__builtin_amdgcn_workgroup_id_x

llc -march=amdgcn -mcpu=gfx906 < simple_amd.ll

error: use of undefined value '%__builtin_amdgcn_workgroup_id_x'
   ret i32 %__builtin_amdgcn_workgroup_id_x

llc --version
LLVM (http://llvm.org/):
   LLVM version 10.0.0
   DEBUG build with assertions.
   Default target: x86_64-unknown-linux-gnu
   Host CPU: skylake-avx512

   Registered Targets:
     amdgcn - AMD GCN GPUs
     r600   - AMD GPUs HD2XXX-HD6XXX


More information about the llvm-dev mailing list