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

[David Rector via cfe-dev]

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> 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 <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 <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 <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 <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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