[cfe-dev] [RFC][OpenCL] Implement OpenCL builtin functions in Clang

[Andrew Savonichev via cfe-dev]

Hello,

OpenCL C language has ~200 builtin functions defined in the
specification: math builtins (sin, cos, tan, etc.), conversion builtins
(float -> half, char2 -> int2), image builtins, and many others.

Currently, all these builtins are declared in `opencl-c.h' header for
each supported combination of types (using `overloadable' attribute), so
~200 builtins turn into ~14000 function declarations.

It takes a significant amount of time to parse these ~14000
declarations, and we have to do it for *every* OpenCL C program. This
leads to bad compile time performance for small OpenCL C programs, and
most of the time is eaten by `opencl-c.h' header.

There were several of attempts to improve this design by pre-compilation
(see D51544: Split opencl-c.h header[1]), but pre-compiled headers are
difficult to manage, and they make overall design more complicated.

Alternatively, we can implement all these ~200 builtins in clang. With a
custom type checking we can handle different argument types. Custom
codegen can generate mangled function calls, so IR should be identical
to what we had before.

In the future, I think we can add a switch to replace mangled functions
with LLVM IR intrinsics, as it could be useful for SPIR-V translation
(see SPIR-V discussion[4] for rationale).

I implemented several builtins as a POC:

  - D52457: [OpenCL] Implement OpenCL convert builtin         [2]
  - D52458: [OpenCL] Implement OpenCL math builtins: fp -> fp [3]

Implementation of the remaining ~150 builtins should be straightforward,
but it is time-consuming, so I'd like to make sure that:

  1) this approach fits the needs of OpenCL community

  2) the rest of the Clang community is OK with adding a few hundred
     builtins to Clang.


  [1]: https://reviews.llvm.org/D51544
  [2]: https://reviews.llvm.org/D52457
  [3]: https://reviews.llvm.org/D52458
  [4]: https://lists.llvm.org/pipermail/llvm-dev/2018-September/125977.html

-- 
Andrew

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