[Openmp-dev] Preferred alternative to a C++ dialect for device library functions

[Finkel, Hal J. via Openmp-dev]

On 1/14/20 10:37 AM, Jon Chesterfield via Openmp-dev wrote:
Hello OpenMP dev,

A motivating example is atomicInc for amdgcn. There is ISA support for this so a good implementation folds to a single instruction. There is no corresponding clang intrinsic, though there is an llvm intrinsic.


Do you mean a target-specific intrinsic, or a target-independent intrinsic?

 -Hal


I see the following options:
- Implement it in IR, linked into deviceRTL
- Inline assembly
- Delay implementation until the intrinsic can be added to clang
- Implement in terms of CAS
- Your suggestion here

Adding atomicInc.ll to the source tree is the easy short term fix. It has drawbacks in terms of future ABI change, build complexity and limited precedent - libclc does this, but nowhere else.

Inline assembly works (modulo getting the syntax right) and hits the right instruction.

Implementing in terms of CAS means one can stay in HIP or OpenCL, but performance suffers.

What would the you prefer out of these options?

Thanks,

Jon



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--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory
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