[cfe-dev] [RFC][OpenCL] Pass alignment of arguments in local addr space for device-side enqueued kernel to __enqueue_kernel functions

Thu Jan 11 10:47:49 PST 2018

We will implement our proposal under cl_khr_device_enqueue_local_arg_types extension. Thanks.

Sam

From: Anastasia Stulova [mailto:Anastasia.Stulova at arm.com]
Sent: Thursday, January 11, 2018 12:44 PM
To: Sumner, Brian <Brian.Sumner at amd.com>; Liu, Yaxun (Sam) <Yaxun.Liu at amd.com>; cfe-dev (cfe-dev at lists.llvm.org) <cfe-dev at lists.llvm.org>; Bader, Alexey (alexey.bader at intel.com) <alexey.bader at intel.com>
Cc: nd <nd at arm.com>
Subject: Re: [RFC][OpenCL] Pass alignment of arguments in local addr space for device-side enqueued kernel to __enqueue_kernel functions

We don't have this extension unfortunately implemented upstream. But if AMD is willing to add, it would allow to do the optimization that Sam was suggesting earlier.

Anastasia
________________________________
From: Sumner, Brian <Brian.Sumner at amd.com<mailto:Brian.Sumner at amd.com>>
Sent: 11 January 2018 14:00
To: Anastasia Stulova; Liu, Yaxun (Sam); cfe-dev (cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>); Bader, Alexey (alexey.bader at intel.com<mailto:alexey.bader at intel.com>)
Cc: nd
Subject: RE: [RFC][OpenCL] Pass alignment of arguments in local addr space for device-side enqueued kernel to __enqueue_kernel functions

Hi Anastasia,

There is a 2.0 extension, cl_khr_device_enqueue_local_arg_types, that we requested when we first encountered this problem.  Clang should implement this if it hasn't already, and ideally this would eventually become the default in the spec.

Thanks,

Brian

From: Anastasia Stulova [mailto:Anastasia.Stulova at arm.com]
Sent: Thursday, January 11, 2018 4:02 AM
To: Sumner, Brian; Liu, Yaxun (Sam); cfe-dev (cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>); Bader, Alexey (alexey.bader at intel.com<mailto:alexey.bader at intel.com>)
Cc: nd
Subject: Re: [RFC][OpenCL] Pass alignment of arguments in local addr space for device-side enqueued kernel to __enqueue_kernel functions

Hi Brian,

Considering the current implementation there is no reason we couldn't generate code with arbitrary pointer types instead of void. This is anyways implemented as a custom check. I don't know though if there might be limitations if using different compilation toolchains or so. Although I can imagine this will require custom implementation anywhere. Should we clarify this in spec?

Cheers,

Anastasia

________________________________

From: Sumner, Brian <Brian.Sumner at amd.com<mailto:Brian.Sumner at amd.com>>
Sent: 10 January 2018 18:26:44
To: Anastasia Stulova; Liu, Yaxun (Sam); cfe-dev (cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>); Bader, Alexey (alexey.bader at intel.com<mailto:alexey.bader at intel.com>)
Cc: nd
Subject: RE: [RFC][OpenCL] Pass alignment of arguments in local addr space for device-side enqueued kernel to __enqueue_kernel functions

>From my perspective, this restriction is nonsense.  OpenCL kernel local* arguments are not required to point to void.  Why must block local* arguments point to void?  They have to be cast to actually be useful; this is an unnecessary extra step.  And unless the actual type is available, the kernel enqueue mechanism has no choice to align the storage to 128 bytes since any local void * could actually be a local ulong16 *.

Thanks,

Brian

From: Anastasia Stulova [mailto:Anastasia.Stulova at arm.com]
Sent: Wednesday, January 10, 2018 9:55 AM
To: Liu, Yaxun (Sam); cfe-dev (cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>); Bader, Alexey (alexey.bader at intel.com<mailto:alexey.bader at intel.com>)
Cc: Sumner, Brian; nd
Subject: Re: [RFC][OpenCL] Pass alignment of arguments in local addr space for device-side enqueued kernel to __enqueue_kernel functions

Hi Sam,

There is a restriction in OpenCL spec I have referenced in my previous email - s6.13.17.2, which is implemented by Clang. If you look in the file test/SemaOpenCL/cl20-device-side-enqueue.cl around line 117, you will see that block_B is rejected to be passed into enqueue_kernel because it has a parameter which isn't "local void*". If you think this is wrong perhaps it would make sense to revisit this bit and understand whether the current spec should be changed to allow more optimal implementations to exist. But as for the current state, I don't think we can implement what you are suggesting because we can only have one block argument type for a block in enqueue.

Cheer,

Anastasia

________________________________

From: Liu, Yaxun (Sam) <Yaxun.Liu at amd.com<mailto:Yaxun.Liu at amd.com>>
Sent: 08 January 2018 22:25
To: Anastasia Stulova; cfe-dev (cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>); Bader, Alexey (alexey.bader at intel.com<mailto:alexey.bader at intel.com>)
Cc: Sumner, Brian; nd
Subject: RE: [RFC][OpenCL] Pass alignment of arguments in local addr space for device-side enqueued kernel to __enqueue_kernel functions

My comments are below.

From: Anastasia Stulova [mailto:Anastasia.Stulova at arm.com]
Sent: Tuesday, December 19, 2017 10:21 AM
To: Liu, Yaxun (Sam) <Yaxun.Liu at amd.com<mailto:Yaxun.Liu at amd.com>>; cfe-dev (cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>) <cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>>; Bader, Alexey (alexey.bader at intel.com<mailto:alexey.bader at intel.com>) <alexey.bader at intel.com<mailto:alexey.bader at intel.com>>
Cc: Sumner, Brian <Brian.Sumner at amd.com<mailto:Brian.Sumner at amd.com>>; nd <nd at arm.com<mailto:nd at arm.com>>
Subject: Re: [RFC][OpenCL] Pass alignment of arguments in local addr space for device-side enqueued kernel to __enqueue_kernel functions

> For example, if a block kernel has argument local int4*. Its alignment should be 16 bytes.

Perhaps I am missing something but I still don't see anything in the spec that requires pointers themselves to take alignment from the pointee type. In your example int4* should be aligned to the pointer size (either 4 or 8 bites) while int4 should be 16 byte aligned. Clang will set the alignment of load and store operations correctly according to their data types specified in the source code (which is mainly inherited from C implementation apart from some special data types like vectors). The arguments passed to kernels are allocated elsewhere and OpenCL compiler has no control over this.

  Sam: The spec (v2.0s6.1.5) requires "the pointee is always appropriately aligned as required by the data type", which means the pointee of the kernel argument of int4* type should be aligned at 16 bytes.

Regarding enqueued kernels as far as I understand you suggest to add block argument alignment info into builtin? Even though it shouldn't be strictly necessary I believe some implementation can indeed be done more efficiently using this. So I don't see any problem adding this. However, spec (s6.13.17.2) mandates that the enqueued block function only has void* types as parameters: "Each argument must be declared to be a void pointer to local memory."  So could you elaborate please where exactly do you plan to get the optimal alignment from?

  Sam: The block function is passed to the builtin. The argument of the block function has the proper data type instead of void* type. Clang can deduce the alignment of the pointee of the kernel argument from the block function type.

Thanks,
Anastasia

________________________________

From: Liu, Yaxun (Sam) <Yaxun.Liu at amd.com<mailto:Yaxun.Liu at amd.com>>
Sent: 15 December 2017 19:08
To: Anastasia Stulova; cfe-dev (cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>); Bader, Alexey (alexey.bader at intel.com<mailto:alexey.bader at intel.com>)
Cc: Sumner, Brian; nd
Subject: RE: [RFC][OpenCL] Pass alignment of arguments in local addr space for device-side enqueued kernel to __enqueue_kernel functions

Spec reference:

OpenCL v2.0 s6.1.5

The OpenCL compiler is responsible for aligning data items to the appropriate alignment as required by the data type. For arguments to a __kernel function declared to be a pointer to a data type, the OpenCL compiler can assume that the pointee is always appropriately aligned as required by the data type. The behavior of an unaligned load or store is undefined, except for the vloadn, vload_halfn, vstoren, and vstore_halfn functions defined in section 6.13.7.

s6.2.5

Casting a pointer to a new type represents an unchecked assertion that the address is correctly aligned.

The C Standard, 6.3.2.3, paragraph 7 [ISO/IEC 9899:2011], states

A pointer to an object or incomplete type may be converted to a pointer to a different object or incomplete type. If the resulting pointer is not correctly aligned for the referenced type, the behavior is undefined.

For example, if a block kernel has argument local int4*. Its alignment should be 16 bytes. Passing a pointer aligned to 1 byte may result in undefined behavior. Most hardware can still load from the unaligned memory but will a performance hit. If runtime wants to avoid the performance hit, it has to allocate the buffer at maximum possible alignment e.g. 32 bytes, which will result in wasted memory.

Sam

From: Anastasia Stulova [mailto:Anastasia.Stulova at arm.com]
Sent: Friday, December 15, 2017 10:40 AM
To: Liu, Yaxun (Sam) <Yaxun.Liu at amd.com<mailto:Yaxun.Liu at amd.com>>; cfe-dev (cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>) <cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>>; Bader, Alexey (alexey.bader at intel.com<mailto:alexey.bader at intel.com>) <alexey.bader at intel.com<mailto:alexey.bader at intel.com>>
Cc: Sumner, Brian <Brian.Sumner at amd.com<mailto:Brian.Sumner at amd.com>>; nd <nd at arm.com<mailto:nd at arm.com>>
Subject: Re: [RFC][OpenCL] Pass alignment of arguments in local addr space for device-side enqueued kernel to __enqueue_kernel functions

> OpenCL spec requires that a pointer should be aligned to at least the pointee type.

So a pointer to int16 would be 64 byte aligned? Seems strange though. Can you give me the spec reference?

> Otherwise, __enqueue_kernel has to either allocate unaligned local buffer, which degrades performance, or allocates local buffer with extra alignment therefore wasted memory space.

Can you explain in more details here, please.

Cheer,
Anastasia

________________________________

From: Liu, Yaxun (Sam) <Yaxun.Liu at amd.com<mailto:Yaxun.Liu at amd.com>>
Sent: 01 December 2017 19:45
To: Anastasia Stulova; cfe-dev (cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>); Bader, Alexey (alexey.bader at intel.com<mailto:alexey.bader at intel.com>)
Cc: Sumner, Brian
Subject: [RFC][OpenCL] Pass alignment of arguments in local addr space for device-side enqueued kernel to __enqueue_kernel functions

Hi,

OpenCL spec requires that a pointer should be aligned to at least the pointee type. Therefore, if a device-side enqueued kernel has a local int* argument, it should be aligned to 4 bytes.

Since these buffers in local addr space are allocated by __enqueue_kernel, it needs to know the alignment of these buffers, not just their sizes.

Although such information is not passed to the original OpenCL builtin function enqueue_kernel, it can be obtained by checking the prototype of the block invoke function at compile time.

I would like to create a patch to pass this information to  __enqueue_kernel. Otherwise, __enqueue_kernel has to either allocate unaligned local buffer, which degrades performance, or allocates local buffer with extra alignment therefore wasted memory space.

Any comments?

Thanks.

Sam
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