[cfe-dev] [RFC] Handling implementation limits

[Mark de Wever via cfe-dev]

This RFC asks the community for its interest in centralizing Clang's
implementation limits and how you feel about the proposed approach.

Abstract
========

At the moment the implementation limits of the Clang compiler are hidden
in magic numbers in the code. These numbers sometimes differ between the
frontend and the backend. They are not always properly guarded by compiler
warnings, instead they trigger assertion failures.

This proposal suggests a TableGen based solution to better maintain these
limits. This solution will be able to generate a header and documentation
of these limits. The latter is required by the C++ standard [implimits].


The problem
===========

The proposal tries to solve 2 issues:
* Implementation limits are not always clear and duplicated in the source.
* Document C++ Implementation quantities  is required by the C++ standard
  [implimits].


Unclear limits
--------------

The compiler has several limitations often 'hidden' as the width of a
bit-field in a structure. This makes it hard to find these limits. Not
only for our users but also for developers. While looking at a proper
limit for the maximum width of a bit-field in the frontend I discovered
it didn't matter what the frontend picked, the backend already had a
limit [D71142]. To fix this issue the frontend and backend should have
the same limit. To avoid duplicating the value it should be in a header
available to the frontend and backend.

Since the values are often stored in a bit-field there is no standard way
to get the maximum value. This means the limit needs a small helper
function to get the value, for example [D63975] uses
`getMaxFunctionScopeDepth()`.


Standard conformance
--------------------

This is rather simple the C++ standard Annex B states:
   Because computers are finite, C++ implementations are inevitably
   limited in the size of the programs they can successfully process.
   Every implementation shall document those limitations where known.

Currently this documentation is not available.


The proposed solution
=====================

In order to solve this issue I created a proof-of-concept solution
[D72053]. It contains a new file
clang/include/clang/Basic/ImplementationQuantities.td with two TableGen
generators which create:
* An inc file with a set of constants containing the limits. This file
  $BUILD_DIR/tools/clang/include/clang/Basic/ImplementationQuantities.inc
  is included in the header
  clang/include/clang/Basic/ImplementationQuantities.h.
* The document clang/docs/ImplementationQuantities.rst
  This document documents the limits of Clang. These limits include, but
  are not limited to the quantities in [implimits].

The quantity limit has the following possible types:
* The quantity is limited by the number of bits in a bit-field. TableGen
  generates two constants:
  * FieldNameBits, the number of bits in the bit-field.
  * MaxFieldName, the maximum value of the bit-field. (This assumes all
    bit-fields can be stored in an unsigned.)
* The quantity is limited by a value. TableGen generates one constant:
  * MaxFieldName, the maximum value of the field.
* The quantity's limit is determined by a compiler flag. TableGen
  generates one constant:
  * FieldNameDefault, the default value of the compiler flag.
* The quantity's limit cannot be expressed in a number. For example it
  depends on the stack size or available memory of the system. In this
  case TableGen generates no constant, only documentation.

For all types documentation is generated. The documentation shows the
description of the limit and the limit implemented in Clang. If the
recommended value is not 0 it is a value described in the C++ Standard. In
this case the recommended value is shown in the documentation.


Questions
=========

* If this proposal is accepted how to we make sure the document is updated
  before releasing a new version of clang?
* The compiler flag limits are also documented in the UserManual. This means
  these limits are still duplicated. Do we want to let the UserManual also be
  generated and use the values here or just keep the duplication?


Bikeshed
========

I picked IQ for the namespace of the quantities since it's short and not
often used in the codebase so it's easy greppable.


[implimits] http://eel.is/c++draft/implimits
[D63975] https://reviews.llvm.org/D63975
[D71142] https://reviews.llvm.org/D71142
[D72053] https://reviews.llvm.org/D72053


Kind regards,
Mark de Wever