[cfe-dev] [RFC][OpenCL] Add builtin types and functions from the standard headers implicitly in the driver

Thu Feb 11 08:11:41 PST 2021

Thanks to everyone for the feedback. FYI I have created the following review:
https://reviews.llvm.org/D96515  that enabled the Tablegen based declarations
by default for OpenCL and adds an extra flag to disable those explicitly.

Cheers,
Anastasia

________________________________
From: cfe-dev <cfe-dev-bounces at lists.llvm.org> on behalf of Anastasia Stulova via cfe-dev <cfe-dev at lists.llvm.org>
Sent: 09 February 2021 17:48
To: David Rector <davrecthreads at gmail.com>; Reid Kleckner <rnk at google.com>; James Y Knight <jyknight at google.com>; clang developer list <cfe-dev at lists.llvm.org>
Cc: nd <nd at arm.com>; Sven Van Haastregt <Sven.VanHaastregt at arm.com>
Subject: Re: [cfe-dev] [RFC][OpenCL] Add builtin types and functions from the standard headers implicitly in the driver

OpenCL Tablegen builtin implementation is fairly generic but right now it only
supports function declarations.

The X86 header seems to contain the function definition instead. However
the definitions seem to follow regular patterns, so it feels like using a
similar Tablegen solution should work, but some infrastructural changes will
be needed first. However, if there are no requirements for the memory size
to store the headers perhaps the modules could be good enough.
________________________________
From: David Rector <davrecthreads at gmail.com>
Sent: 09 February 2021 00:21
To: Reid Kleckner <rnk at google.com>
Cc: James Y Knight <jyknight at google.com>; nd <nd at arm.com>; clang developer list <cfe-dev at lists.llvm.org>; Sven Van Haastregt <Sven.VanHaastregt at arm.com>; Anastasia Stulova <Anastasia.Stulova at arm.com>
Subject: Re: [cfe-dev] [RFC][OpenCL] Add builtin types and functions from the standard headers implicitly in the driver

To me the tablegen proposal seems a clever and generalizable means of basically "instantiating" non-templatable builtins and related declarations only as they are needed, avoiding not only parsing as a module/PCH would, but also storage of unused AST nodes.

I have little familiarity with x86intrin.h et al, but why couldn't the same approach be fruitful there?  To the extent their content is not manually written/maintained but rather is generated from much smaller data via some program (is it? At a glance seems possible), couldn’t tablegen be that program, generating only the bare-bones data needed to create all the necessary declarations, and leaving it to the compiler to actually create those declarations as needed, just as Anastasia proposes for OpenCL?

Modules need not be the ne plus ultra solution for large, universally-#included headers, given the speed + storage improvements the OpenCL folks have apparently realized with this alternative approach.

On Feb 8, 2021, at 3:34 PM, Reid Kleckner via cfe-dev <cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>> wrote:

My short, not carefully considered, input is that the suggested approach of tablegen + a header seems reasonable.

The modules / PCH approach has been considered for the ever-growing immintrin.h on x86, but I don't believe it got much traction:
https://lists.llvm.org/pipermail/cfe-dev/2016-September/050980.html

On Fri, Feb 5, 2021 at 2:13 PM James Y Knight via cfe-dev <cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>> wrote:
The whole tablegen thing seems like a sad path to have gone down, although I can certainly see the practical benefits. Substantially the same problem of compilation-speed exists for <x86intrin.h> (and friends), and I really don't think we want to start defining all of those with a tablegen rule.

It would be really nice to instead somehow take advantage of the modules infrastructure to fix this problem -- I'd really love it if we could somehow start shipping a pre-built module artifact for our giant intrinsics headers, included with the compiler distribution. And then use that by default, regardless of whether users are otherwise enabling modules. If we got that to work, we could use the same solution for both X86 and opencl.

However, independent of that discussion -- and more to the immediate issue you're trying to raise -- your proposal seems like it's resulting in a very complex set of options, and I'm not sure what the purpose is.

IIUC, the overall desire is to have, by default, these tens-of-thousands of prototypes available to all OpenCL compilations. But, I don't see any reason why users should care exactly HOW these are provided. I'd expect that a given prototype should be provided either by the tablegen-based builtin code, OR an auto-included header file -- not both, and not neither.

What difference does it make if the builtin-tablegen code doesn't provide 100% of the declarations, so long as the remainder are provided by an automatically-included header? Why do you want to make users choose between getting a half-baked set of function prototypes (tablegen version) and slow compilation (auto-including giant header), when you can have the fully correct set of functions AND nearly-as-fast compilation, by simply supplementing tablegen with an auto-included header providing the remainder?

And then you need just a single user-visible option: to disable the automatic declarations (via both tablegen + autoinclude).

On Wed, Feb 3, 2021 at 11:58 AM Anastasia Stulova via cfe-dev <cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>> wrote:
Hello,

I would like to check if there is any feedback to the following proposal for
improving the interface of standard type and function includes.

Background

Currently the default clang driver provides incomplete functionality for OpenCL
because the headers with builtin function declarations are not included by
default. The header can only be added using frontend flags requiring ‘-cc1’ or
other frontend forwarding options
(https://clang.llvm.org/docs/UsersManual.html#opencl-header). This means it
is inaccessible to users in a conventional way.

We propose to add the implicit header by default when a source is compiled in
OpenCL mode. A review for this has been uploaded by Matt a few months ago:
https://reviews.llvm.org/D78979. Note that the standard library functionality is
added by default in OpenCL C without using include directives in the compiling
sources.  This means all builtin function declarations (there are 17000 of
them) are to be parsed every time the source is compiled because which builtins
are used by the kernels is not known beforehand. This impacts the compilation
speed. For example, parsing a simple kernel with the builtin function
declarations is 138 times slower in a Debug build and 13 times slower in a
Release build than parsing the same kernel without those declarations.

To mitigate the overhead of parsing the full header, a fast Tablegen based
solution has been developed
(https://llvm.org/devmtg/2019-10/talk-abstracts.html#lit5). The parsing speed with
this mechanism for builtin function declarations is only 1.3 times slower in a Debug
build and 1.05 times slower in a Release build compared to clang without the
builtins. While this mechanism covers most of OpenCL standard
functions it lacks two main classes of builtins: builtins defined by
vendor extensions and builtins with enum arguments.

Proposal

We propose the following changes in the clang driver interface for OpenCL:
- Enable the fast Tablegen based builtin function declaration mechanism by
default in the clang driver. This makes the majority of OpenCL builtin
functions available.
- In addition, include the small header opencl-c-base.h by default in the clang
driver. This header provides basic types and constants.
No frontend or driver flags will be needed to allow using the majority of OpenCL
types and functions from the standard, at a very low parsing speed increase.

Since the Tablegen mechanism has some small overhead and it is not fully
complete, we propose to add the following additional clang driver flags:
1. Add a new clang driver flag -cl-no-stdinc (*) that disables such extra includes to
minimize further compilation speed or for the use cases that don’t require
standard libraries. The majority of OpenCL clang tests will use this option.
2. [Optionally, if there is enough interest] Add a new clang driver flag
-cl-all-stdinc (*) that will include the full header instead of using the Tablegen
mechanism, at the cost of a significant increase in parsing time.

At present we propose no change to the ‘cc1’ interface, but in the future it is
expected that the functionality will be aligned between driver and frontend
command line interfaces for the OpenCL headers.

(*) The exact spelling of command line options is to be discussed.

Summary

This proposal enhances the clang driver with full functionality of OpenCL C by
adding builtin function declarations implicitly without the need for any
frontend flags to be given in the command line.

The default clang behavior proposed is not expected to negatively impact users
of clang as the parsing speed difference remains within the same order of
magnitude. While the fast header mechanism matures, a fallback mechanism will
be provided if needed that would allow switching to the slow header with the
full functionality using a new driver flag. For backward compatibility, another
flag is provided to disable all OpenCL declarations that are not builtin into
the compiler.

The solution proposed improves the driver interface and reduces risks of
forcing the OpenCL community to update their use of clang due to significant
regression of the compilation speed.
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