[Pocl-devel] Kernel library portability

[Carlos S. de La L. <car...@ur...>]

Hi all,

I have been thinking about how to implement the kernel library for
different devices, and some related issues.

Right now, pocl flow goes like this:

   Compilation: .cl to .bc (bytecode)
                   |
                   V
      Linking with kerneĺ library
                   |
                   V
           Fully inlining
                   |
                   V
Workgroup creation (replicate workitems)
                   |
                   V
        Device-dependant driver

Workgroup creation needs to detect barriers, thats why it needs to be
done after fully inlining (there can be barriers in a function called by
the kernel, not in the kernel itself).

One desirable thing is bytecode to be device independent as long as
possible, until device driver if possible, so we do not have to store
several binaries in the host (there might be some unavoidable
dependencies, but I think given OpenCL restricted C support those will
be minor). Then there are two possibilities:

1) Make kernel library runtime compatible with all devices. This was the
planned approach, it can be done by selection the implementation for a
device using runtime conditional (C-level ifs) instead of preprocessor
ones (#if/#ifdefs). LLVM should then eliminate dead code when generating
the final binary.

2) Perform inlining and replication before linking. Only a minor part of
the kernel library (get_xxx_id() and friends) need to be linked before
WG creation, and those are going to be common for all device because
they depend on replication passes. But the big "functional" kernel
runtime library could be linked later, even in device-dependant binary
form instead of bytecode form, allowing the use of different kernel
libraries for different devices. This would have the additional
advantage of smaller bytecode and faster code generation.

Thoughs?

Carlos