[cfe-dev] [LLVMdev] Function-level metadata for OpenCL (was Re: OpenCL support)

[Villmow, Micah]

Sorry for the late reply, as I have been on vacation for awhile.

One method which I haven't seen mentioned is to separate out the kernel semantics from the function definition.

All the kernel attribute does is specify that this function is an entry point to the device from the host. So, why not just create a separate entry point that is only callable by the host and everything from the device goes to the original entry point.

For example, you have two functions and one calls the other:

kernel foo() {
}
kernel bar() {
  foo();
}

If you separate kernel function from the function body, then handling this becomes easy.

You end up with four functions:

kernel foo_kernel() {
 foo();
}

foo() {
}

kernel bar_kernel() {
 bar();
}

bar(){
 foo();
}

Then the issue is no longer a compilation problem, but just an entry point runtime issue. Instead of calling foo(), the runtime just calls foo_kernel() which handles all of the kernel setup issues and then calls the function body itself.

This removes the need to have any metadata nodes in the IR and allows the kernel function to handle any setup issues for the specific device such as __local's, id/group calculations, memory offsets, etc... without having to impact the performance of a kernel calling another kernel. 

Micah

> -----Original Message-----
> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu]
> On Behalf Of Peter Collingbourne
> Sent: Friday, December 24, 2010 3:50 PM
> To: Anton Lokhmotov
> Cc: cfe-dev at cs.uiuc.edu; llvmdev at cs.uiuc.edu
> Subject: Re: [LLVMdev] Function-level metadata for OpenCL (was Re:
> OpenCL support)
> 
> On Tue, Dec 21, 2010 at 07:17:40PM -0000, Anton Lokhmotov wrote:
> > > From: Peter Collingbourne [mailto:peter at pcc.me.uk]
> > > Sent: 20 December 2010 20:11
> > > As with __local variables, it may be that "kernelness" cannot be
> > > represented in a standard form in LLVM.  For example on a CPU a
> > > kernel function may have an additional parameter which is a pointer
> to
> > > __local memory space, which would not be necessary on GPUs.  Then
> in
> > > fact you would use a standard calling convention on a CPU.
> > >
> > > But for GPUs, I think using the calling convention is appropriate.
> > > If we standardise the calling convention number, this can be the
> > > default behaviour.
> > I don't think we want LLVM-IR coming from an OpenCL C frontend to be
> > different for GPU and CPU targets. In my view, the frontend should be
> > parameterised by only two (more or less) parameters: bitness (32/64)
> and
> > endianness (little/big).
> 
> Not only sizes but alignment requirements will change between
> platforms.  Also, what about __local on CPU?
> 
> > How one can even guarantee e.g. that a calling
> > convention for NVIDIA GPUs is appropriate for ATI GPUs?
> 
> We have full control over the target code generators.  There's nothing
> stopping us defining a specific constant representing the 'kernel'
> calling convention and harmonising the GPU targets to use that
> calling convention.
> 
> Thanks,
> --
> Peter
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