[cfe-dev] [RFC][OpenCL] Implement OpenCL builtin functions in Clang
Andrew Savonichev via cfe-dev
cfe-dev at lists.llvm.org
Tue Sep 25 08:02:01 PDT 2018
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), 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 for rationale).
I implemented several builtins as a POC:
- D52457: [OpenCL] Implement OpenCL convert builtin 
- D52458: [OpenCL] Implement OpenCL math builtins: fp -> fp 
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.
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